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# Units and Conversions
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This document covers Math.js's comprehensive unit system, including unit creation, conversion, arithmetic with units, and the extensive built-in unit library covering physical quantities, measurements, and conversions.
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## Import
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```typescript
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import {
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  // Core unit functions
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  unit, to, createUnit, splitUnit, toBest,
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  // Unit-related utilities
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  isUnit, hasNumericValue,
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  // Mathematical operations (work with units)
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  add, subtract, multiply, divide, pow, sqrt,
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  // Constants with units (physical constants)
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  speedOfLight, planckConstant, electronMass, avogadro
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} from 'mathjs'
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```
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## Basic Unit Creation and Usage
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### Creating Units
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```typescript
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unit(value?: string | number, unit?: string): Unit
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```
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{ .api }
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```typescript
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// Create units from strings
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unit('5 meter') // 5 meter
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unit('10 cm') // 10 cm  
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unit('3.5 kg') // 3.5 kg
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unit('25 celsius') // 25 celsius
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// Create dimensionless units
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unit('5') // Just the number 5
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unit() // Dimensionless 1
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// Create units with separate value and unit
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unit(5, 'meter') // 5 meter
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unit(3.14159, 'radian') // π radians
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// Complex values with units  
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unit(complex(3, 4), 'meter') // (3+4i) meter
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unit(fraction(1, 3), 'hour') // 1/3 hour
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// From expressions
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unit('5 * 3 meter') // 15 meter
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unit('sqrt(16) kg') // 4 kg
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```
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### Unit Conversion
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```typescript
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to(value: Unit, unit: string | Unit): Unit
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```
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{ .api }
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```typescript
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// Basic conversions
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const distance = unit('5 km')
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to(distance, 'meter') // 5000 meter
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to(distance, 'mile') // ~3.10686 mile
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to(distance, 'inch') // ~196850 inch
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// Temperature conversions
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const temp = unit('100 celsius')
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to(temp, 'fahrenheit') // 212 fahrenheit  
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to(temp, 'kelvin') // 373.15 kelvin
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// Time conversions
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const time = unit('2 hours')
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to(time, 'minute') // 120 minute
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to(time, 'second') // 7200 second
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// Area conversions
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const area = unit('1 m^2')
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to(area, 'cm^2') // 10000 cm^2
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to(area, 'ft^2') // ~10.7639 ft^2
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// Volume conversions
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const volume = unit('1 liter')
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to(volume, 'gallon') // ~0.264172 gallon
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to(volume, 'm^3') // 0.001 m^3
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// Compound units
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const speed = unit('60 mile/hour')
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to(speed, 'km/h') // ~96.5606 km/h
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to(speed, 'm/s') // ~26.8224 m/s
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```
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### Automatic Best Unit Selection
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```typescript
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toBest(value: Unit, options?: ToBestOptions): Unit
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```
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{ .api }
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```typescript
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interface ToBestOptions {
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  system?: 'si' | 'us' | 'imperial' | 'metric'
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}
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```
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```typescript
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// Automatically choose the most readable unit
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toBest(unit('0.001 m')) // 1 mm
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toBest(unit('1000 m')) // 1 km
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toBest(unit('3600 second')) // 1 hour
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// With system preference
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toBest(unit('1000 meter'), { system: 'us' }) // ~0.621371 mile
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toBest(unit('1 kg'), { system: 'us' }) // ~2.20462 lb
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// For very large or small quantities
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toBest(unit('0.000001 meter')) // 1 micrometer  
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toBest(unit('1000000 gram')) // 1 tonne
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```
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## Unit Arithmetic
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### Basic Operations with Units
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```typescript
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// Addition and subtraction (units must be compatible)
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add(unit('5 meter'), unit('3 meter')) // 8 meter
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add(unit('2 km'), unit('500 m')) // 2.5 km (automatic conversion)
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subtract(unit('10 inch'), unit('2 inch')) // 8 inch
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// Multiplication and division
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multiply(unit('5 meter'), unit('3 meter')) // 15 meter^2
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multiply(unit('60 km'), unit('2 hour')) // Error: incompatible for this operation
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divide(unit('60 km'), unit('2 hour')) // 30 km/hour
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// With scalars
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multiply(unit('5 meter'), 3) // 15 meter
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divide(unit('10 kg'), 2) // 5 kg
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// Powers and roots
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pow(unit('5 meter'), 2) // 25 meter^2  
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pow(unit('8 m^3'), 1/3) // 2 meter
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sqrt(unit('16 meter^2')) // 4 meter
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```
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### Complex Unit Operations
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```typescript
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// Force calculation: F = ma
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const mass = unit('5 kg')
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const acceleration = unit('10 m/s^2')  
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const force = multiply(mass, acceleration) // 50 N (automatically converts to Newtons)
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// Energy calculation: E = 0.5 * m * v^2
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const velocity = unit('20 m/s')
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const kineticEnergy = multiply(0.5, multiply(mass, pow(velocity, 2))) // 1000 J
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// Pressure calculation: P = F/A
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const area = unit('2 m^2')
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const pressure = divide(force, area) // 25 Pa
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// Electrical calculations
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const voltage = unit('12 V')
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const current = unit('2 A')  
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const power = multiply(voltage, current) // 24 W
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const resistance = divide(voltage, current) // 6 ohm
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```
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## Creating Custom Units
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### Simple Custom Units
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```typescript
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createUnit(name: string, definition?: string | Unit | UnitDefinition, options?: CreateUnitOptions): Unit
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```
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{ .api }
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```typescript
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interface CreateUnitOptions {
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  aliases?: string[]
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  prefixes?: string
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  offset?: number
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  override?: boolean
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}
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```
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```typescript
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// Create simple unit based on existing unit
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createUnit('byte') // Base unit for digital storage
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createUnit('kilobyte', '1024 byte')
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createUnit('megabyte', '1024 kilobyte') 
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createUnit('gigabyte', '1024 megabyte')
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// Use new units
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const storage = unit('2.5 gigabyte')
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to(storage, 'megabyte') // 2560 megabyte
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to(storage, 'byte') // 2684354560 byte
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// Create unit with aliases
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createUnit('dollar', { aliases: ['usd', '$'] })
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unit('100 dollar') // 100 dollar
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unit('50 usd') // 50 usd (same unit)
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// Create derived unit
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createUnit('mph', 'mile/hour')
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unit('60 mph') // 60 mph
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to(unit('60 mph'), 'm/s') // ~26.82 m/s
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```
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### Advanced Custom Units
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```typescript
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// Create unit with offset (like temperature scales)
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createUnit('rankine', { definition: '(5/9) kelvin', offset: 0 })
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createUnit('reaumur', { definition: '(5/4) celsius', offset: 0 })
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// Create unit system
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createUnit('furlong', '220 yard')
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createUnit('fortnight', '14 day')  
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createUnit('speed_unusual', 'furlong/fortnight')
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const unusualSpeed = unit('1 furlong/fortnight')
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to(unusualSpeed, 'meter/second') // ~0.00000166 m/s
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// Create currency units (relative values)
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createUnit('euro', '1.1 dollar') // Exchange rate example
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createUnit('pound', '1.3 dollar')
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const price = unit('100 euro')
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to(price, 'dollar') // 110 dollar (using defined rate)
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```
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## Built-in Unit Categories
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### Length Units
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```typescript
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// Metric
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unit('1 mm') // millimeter  
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unit('1 cm') // centimeter
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unit('1 dm') // decimeter
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unit('1 m') // meter
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unit('1 km') // kilometer
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// Imperial/US
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unit('1 inch') // inch
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unit('1 ft') // foot  
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unit('1 yard') // yard
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unit('1 mile') // mile
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// Other
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unit('1 au') // astronomical unit
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unit('1 lightyear') // light year
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unit('1 parsec') // parsec
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unit('1 angstrom') // angstrom
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// Conversions
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to(unit('1 inch'), 'cm') // 2.54 cm
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to(unit('1 mile'), 'km') // ~1.609 km
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to(unit('1 lightyear'), 'm') // ~9.461e15 m
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```
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### Mass and Weight Units  
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```typescript
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// Metric mass
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unit('1 mg') // milligram
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unit('1 g') // gram
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unit('1 kg') // kilogram  
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unit('1 tonne') // metric ton
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// Imperial/US mass  
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unit('1 grain') // grain
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unit('1 ounce') // ounce
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unit('1 lb') // pound
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unit('1 ton') // short ton (US)
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unit('1 longton') // long ton (UK)
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// Other
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unit('1 carat') // carat (gemstones)
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unit('1 atomicmass') // atomic mass unit
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unit('1 electronmass') // electron mass
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// Weight (force) units
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unit('1 N') // newton
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unit('1 lbf') // pound-force
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unit('1 kgf') // kilogram-force
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// Conversions
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to(unit('1 lb'), 'kg') // ~0.453592 kg
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to(unit('1 ounce'), 'g') // ~28.3495 g
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```
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### Time Units
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```typescript
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// Basic time
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unit('1 ns') // nanosecond
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unit('1 microsecond') // microsecond  
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unit('1 ms') // millisecond
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unit('1 s') // second
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unit('1 minute') // minute
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unit('1 hour') // hour
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unit('1 day') // day
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unit('1 week') // week
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unit('1 month') // month (30.4375 days)
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unit('1 year') // year (365.25 days)
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// Specialized
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unit('1 decade') // 10 years
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unit('1 century') // 100 years  
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unit('1 millennium') // 1000 years
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// Conversions
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to(unit('2 hours'), 'second') // 7200 second
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to(unit('30 day'), 'year') // ~0.082 year
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```
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### Area Units
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```typescript
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// Metric area
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unit('1 mm^2') // square millimeter
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unit('1 cm^2') // square centimeter  
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unit('1 m^2') // square meter
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unit('1 km^2') // square kilometer
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unit('1 hectare') // hectare (10000 m^2)
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// Imperial/US area  
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unit('1 in^2') // square inch
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unit('1 ft^2') // square foot
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unit('1 yard^2') // square yard
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unit('1 mile^2') // square mile  
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unit('1 acre') // acre
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// Conversions
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to(unit('1 acre'), 'm^2') // ~4047 m^2
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to(unit('1 hectare'), 'acre') // ~2.471 acre
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```
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### Volume Units
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```typescript
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// Metric volume
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unit('1 mm^3') // cubic millimeter
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unit('1 cm^3') // cubic centimeter
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unit('1 dm^3') // cubic decimeter  
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unit('1 m^3') // cubic meter
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unit('1 liter') // liter (dm^3)
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unit('1 ml') // milliliter (cm^3)
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// Imperial/US volume
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unit('1 in^3') // cubic inch
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unit('1 ft^3') // cubic foot
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unit('1 gallon') // US gallon
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unit('1 quart') // US quart
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unit('1 pint') // US pint  
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unit('1 cup') // US cup
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unit('1 floz') // fluid ounce
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unit('1 tablespoon') // tablespoon
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unit('1 teaspoon') // teaspoon
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// UK Imperial
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unit('1 gallon_imp') // Imperial gallon
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unit('1 pint_imp') // Imperial pint
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// Conversions
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to(unit('1 gallon'), 'liter') // ~3.785 liter
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to(unit('1 liter'), 'gallon') // ~0.264 gallon
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to(unit('1 m^3'), 'liter') // 1000 liter
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```
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### Temperature Units
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```typescript
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// Temperature scales
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unit('0 celsius') // Celsius  
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unit('32 fahrenheit') // Fahrenheit
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unit('273.15 kelvin') // Kelvin
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unit('491.67 rankine') // Rankine
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// Temperature conversions (handle offset scales correctly)
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to(unit('0 celsius'), 'fahrenheit') // 32 fahrenheit
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to(unit('100 celsius'), 'kelvin') // 373.15 kelvin
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to(unit('68 fahrenheit'), 'celsius') // 20 celsius
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// Temperature differences (no offset)  
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const tempDiff = unit('10 degC') // 10 degree difference
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to(tempDiff, 'degF') // 18 degF (difference)
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```
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### Speed and Velocity Units
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```typescript
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// Speed units
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unit('1 m/s') // meter per second
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unit('1 km/h') // kilometer per hour  
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unit('1 mph') // mile per hour
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unit('1 ft/s') // foot per second
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unit('1 knot') // nautical mile per hour
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// Special speeds
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unit('1 c') // speed of light
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unit('1 mach') // speed of sound (approximate)
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// Conversions
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to(unit('60 mph'), 'km/h') // ~96.56 km/h
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to(unit('100 km/h'), 'm/s') // ~27.78 m/s
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to(unit('1 knot'), 'm/s') // ~0.514 m/s
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```
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### Energy and Power Units
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```typescript
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// Energy units
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unit('1 J') // joule
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unit('1 kJ') // kilojoule  
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unit('1 cal') // calorie
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unit('1 kcal') // kilocalorie (food calorie)
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unit('1 Btu') // British thermal unit
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unit('1 kWh') // kilowatt hour
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unit('1 eV') // electron volt
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unit('1 erg') // erg (CGS)
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// Power units  
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unit('1 W') // watt
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unit('1 kW') // kilowatt
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unit('1 MW') // megawatt
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unit('1 hp') // horsepower
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unit('1 PS') // metric horsepower
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// Conversions
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to(unit('1 kWh'), 'J') // 3600000 J
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to(unit('1 hp'), 'W') // ~745.7 W
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to(unit('1 cal'), 'J') // ~4.184 J
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```
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### Pressure Units
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```typescript
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// Pressure units
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unit('1 Pa') // pascal
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unit('1 kPa') // kilopascal  
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unit('1 MPa') // megapascal
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unit('1 bar') // bar
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unit('1 atm') // atmosphere
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unit('1 mmHg') // millimeter mercury
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unit('1 psi') // pound per square inch
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unit('1 torr') // torr
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// Conversions
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to(unit('1 atm'), 'Pa') // 101325 Pa
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to(unit('1 bar'), 'psi') // ~14.504 psi
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to(unit('760 mmHg'), 'atm') // 1 atm
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```
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### Electrical Units
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```typescript
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// Basic electrical units
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unit('1 A') // ampere (current)
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unit('1 V') // volt (voltage)  
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unit('1 ohm') // ohm (resistance)
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unit('1 W') // watt (power)
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unit('1 F') // farad (capacitance)
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unit('1 H') // henry (inductance)
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unit('1 C') // coulomb (charge)
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unit('1 S') // siemens (conductance)
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// Derived units
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unit('1 kW') // kilowatt
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unit('1 mA') // milliampere
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unit('1 kV') // kilovolt  
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unit('1 kOhm') // kiloohm
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unit('1 uF') // microfarad
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unit('1 mH') // millihenry
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// Electrical calculations
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const i = unit('2 A')
479
const v = unit('12 V')  
480
const p = multiply(v, i) // 24 W
481
const r = divide(v, i) // 6 ohm
482
```
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### Frequency and Angular Units
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```typescript
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// Frequency
488
unit('1 Hz') // hertz
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unit('1 kHz') // kilohertz
490
unit('1 MHz') // megahertz  
491
unit('1 GHz') // gigahertz
492
unit('1 rpm') // revolutions per minute
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// Angular units
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unit('1 rad') // radian
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unit('1 deg') // degree  
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unit('1 grad') // gradian
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unit('1 arcmin') // arc minute
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unit('1 arcsec') // arc second
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unit('1 revolution') // full rotation
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// Conversions
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to(unit('180 deg'), 'rad') // π rad
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to(unit('1 revolution'), 'deg') // 360 deg
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to(unit('60 rpm'), 'Hz') // 1 Hz
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```
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## Unit Splitting and Formatting
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### Split Units into Components
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```typescript
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splitUnit(unit: Unit, parts: string[]): Unit[]
514
```
515
{ .api }
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```typescript
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// Split time into hours, minutes, seconds
519
const duration = unit('7463 second')
520
splitUnit(duration, ['hour', 'minute', 'second'])
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// [2 hour, 4 minute, 23 second] (2h 4m 23s)
522

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// Split length into feet and inches  
524
const height = unit('73 inch')  
525
splitUnit(height, ['ft', 'inch'])
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// [6 ft, 1 inch] (6'1")
527

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// Split weight into pounds and ounces
529
const weight = unit('2.75 lb')
530
splitUnit(weight, ['lb', 'ounce'])  
531
// [2 lb, 12 ounce]
532

533
// Custom formatting with split units
534
function formatTime(seconds) {
535
  const parts = splitUnit(unit(seconds, 'second'), ['hour', 'minute', 'second'])
536
  return parts.map(part => part.toString()).join(' ')
537
}
538
formatTime(3661) // "1 hour 1 minute 1 second"
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```
540

541
## Physical Constants with Units
542

543
Math.js includes many physical constants with proper units:
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545
```typescript
546
// Fundamental constants
547
speedOfLight // ~299792458 m/s
548
planckConstant // ~6.626e-34 J*s  
549
electronCharge // ~1.602e-19 C
550
electronMass // ~9.109e-31 kg
551
protonMass // ~1.673e-27 kg
552
avogadro // ~6.022e23 mol^-1
553
boltzmann // ~1.381e-23 J/K
554
gravity // 9.80665 m/s^2 (standard)
555

556
// Usage in calculations
557
const energy = multiply(electronMass, pow(speedOfLight, 2))
558
// E = mc² for electron rest energy
559

560
const force = multiply(unit('70 kg'), gravity) // Weight of 70kg person  
561
to(force, 'lbf') // Convert to pounds-force
562
```
563

564
## Advanced Unit Operations
565

566
### Unit Validation and Testing
567

568
```typescript
569
// Check if value has a unit  
570
isUnit(unit('5 meter')) // true
571
isUnit(5) // false
572

573
// Check if unit has numeric value
574
hasNumericValue(unit('5 meter')) // true
575
hasNumericValue(unit('meter')) // false (no numeric part)
576

577
// Get unit components
578
const speed = unit('60 km/h')
579
speed.value // 60
580
speed.units // [{ unit: 'm', power: 1 }, { unit: 's', power: -1 }] (SI base)
581
speed.toString() // "60 km / h"
582
```
583

584
### Unit Compatibility
585

586
```typescript
587
// Units are compatible if they have the same dimension
588
function areCompatible(unit1, unit2) {
589
  try {
590
    to(unit1, unit2.units)
591
    return true
592
  } catch (error) {
593
    return false  
594
  }
595
}
596

597
areCompatible(unit('5 meter'), unit('10 foot')) // true (both length)
598
areCompatible(unit('5 kg'), unit('10 second')) // false (different dimensions)
599

600
// Get dimension of unit
601
function getDimension(unit_val) {
602
  // Convert to SI base units to see fundamental dimensions  
603
  return unit_val.to('SI')
604
}
605
```
606

607
### Working with Dimensionless Units
608

609
```typescript
610
// Dimensionless quantities
611
unit('5') // Just number 5
612
unit('0.5') // 0.5 (could be percentage, ratio, etc.)
613

614
// Converting to dimensionless
615
const ratio = divide(unit('10 meter'), unit('5 meter')) // 2 (dimensionless)
616

617
// Angles are dimensionless but have units for clarity
618
const angle = unit('45 deg')
619
to(angle, 'rad') // π/4 rad
620
sin(angle) // sin(45°) = √2/2
621
```
622

623
## Unit System Conversions
624

625
### Metric (SI) System
626

627
```typescript
628
// Base SI units: meter, kilogram, second, ampere, kelvin, mole, candela
629
const siUnits = {
630
  length: unit('1 m'),
631
  mass: unit('1 kg'),  
632
  time: unit('1 s'),
633
  current: unit('1 A'),
634
  temperature: unit('1 K'), 
635
  amount: unit('1 mol'),
636
  luminosity: unit('1 cd')
637
}
638

639
// Convert measurements to SI
640
function toSI(measurement) {
641
  return measurement.to('SI')
642
}
643
```
644

645
### Imperial/US System
646

647
```typescript
648
// Common US/Imperial units
649
const usUnits = {
650
  length: unit('1 ft'),
651
  mass: unit('1 lb'),
652
  volume: unit('1 gallon'),  
653
  temperature: unit('1 fahrenheit'),
654
  area: unit('1 acre'),
655
  speed: unit('1 mph')
656
}
657

658
// Convert between systems
659
function toMetric(usValue, targetUnit) {
660
  return to(usValue, targetUnit)
661
}
662

663
toMetric(unit('70 fahrenheit'), 'celsius') // 21.11 celsius
664
toMetric(unit('60 mph'), 'km/h') // 96.56 km/h
665
```
666

667
## Performance and Memory Considerations
668

669
### Efficient Unit Operations
670

671
```typescript
672
// Pre-compile unit conversions for repeated use
673
const meterToFeet = (meters) => to(unit(meters, 'meter'), 'ft')
674
const celsiusToFahrenheit = (celsius) => to(unit(celsius, 'celsius'), 'fahrenheit')
675

676
// Batch conversions
677
const distances_m = [100, 200, 300, 500, 1000]
678
const distances_ft = distances_m.map(meterToFeet)
679

680
// Use unit arithmetic instead of converting to numbers
681
const area_m2 = multiply(unit('10 m'), unit('5 m')) // 50 m^2
682
const area_ft2 = to(area_m2, 'ft^2') // More accurate than converting first
683
```
684

685
### Working with Large Unit Systems
686

687
```typescript
688
// For applications with many custom units, create them systematically
689
function createCurrencySystem(baseCurrency, rates) {
690
  Object.entries(rates).forEach(([currency, rate]) => {
691
    createUnit(currency, `${rate} ${baseCurrency}`)
692
  })
693
}
694

695
createCurrencySystem('USD', {
696
  EUR: 0.85,
697
  GBP: 0.73,
698
  JPY: 110,
699
  CAD: 1.25
700
})
701

702
// Now can convert between currencies
703
const price = unit('100 USD')
704
to(price, 'EUR') // 85 EUR
705
```
706

707
## Common Unit Patterns
708

709
### Engineering Calculations
710

711
```typescript
712
// Electrical engineering
713
function ohmsLaw(voltage, current, resistance) {
714
  if (voltage && current) return { resistance: divide(voltage, current) }
715
  if (voltage && resistance) return { current: divide(voltage, resistance) }  
716
  if (current && resistance) return { voltage: multiply(current, resistance) }
717
}
718

719
const result = ohmsLaw(unit('12 V'), unit('2 A'))
720
result.resistance // 6 ohm
721

722
// Mechanical engineering  
723
function kineticEnergy(mass, velocity) {
724
  return multiply(0.5, multiply(mass, pow(velocity, 2)))
725
}
726

727
const ke = kineticEnergy(unit('1000 kg'), unit('25 m/s'))
728
to(ke, 'kJ') // 312.5 kJ
729

730
// Fluid dynamics
731
function reynoldsNumber(density, velocity, length, viscosity) {
732
  return divide(multiply(density, multiply(velocity, length)), viscosity)
733
}
734
```
735

736
### Scientific Applications
737

738
```typescript
739
// Chemistry: ideal gas law PV = nRT
740
function idealGas(pressure, volume, moles, temperature) {
741
  const R = unit('8.314 J/(mol*K)') // Gas constant
742
  
743
  if (!pressure) return divide(multiply(moles, multiply(R, temperature)), volume)
744
  if (!volume) return divide(multiply(moles, multiply(R, temperature)), pressure)
745
  if (!moles) return divide(multiply(pressure, volume), multiply(R, temperature))
746
  if (!temperature) return divide(multiply(pressure, volume), multiply(moles, R))
747
}
748

749
// Physics: wavelength-frequency relationship  
750
function wavelengthFrequency(wavelength, frequency) {
751
  if (wavelength) return divide(speedOfLight, wavelength)
752
  if (frequency) return divide(speedOfLight, frequency)
753
}
754

755
const freq = wavelengthFrequency(unit('500 nm')) // Green light frequency
756
to(freq, 'THz') // ~600 THz
757
```
758

759
### Everyday Calculations
760

761
```typescript
762
// Cooking conversions
763
function convertRecipe(ingredient, targetUnit) {
764
  return to(ingredient, targetUnit)  
765
}
766

767
convertRecipe(unit('2 cup'), 'ml') // ~473 ml
768
convertRecipe(unit('350 fahrenheit'), 'celsius') // ~177 celsius
769

770
// Travel calculations  
771
function travelTime(distance, speed) {
772
  return divide(distance, speed)
773
}
774

775
const time = travelTime(unit('300 km'), unit('80 km/h'))
776
to(time, 'hour') // 3.75 hour = 3h 45m
777

778
// Fuel efficiency
779
function fuelConsumption(distance, fuel) {
780
  return divide(distance, fuel)
781
}
782

783
const efficiency = fuelConsumption(unit('400 mile'), unit('12 gallon'))
784
to(efficiency, 'km/l') // ~14.1 km/l
785
```
786

787
## Error Handling with Units
788

789
```typescript
790
import { DimensionError } from 'mathjs'
791

792
try {
793
  // Incompatible unit operations
794
  add(unit('5 meter'), unit('3 second')) // DimensionError
795
} catch (error) {
796
  if (error instanceof DimensionError) {
797
    console.log('Cannot add different unit dimensions')  
798
  }
799
}
800

801
try {
802
  // Invalid unit conversion
803
  to(unit('5 kg'), 'meter') // DimensionError
804
} catch (error) {
805
  console.log('Cannot convert mass to length')
806
}
807

808
// Safe unit operations
809
function safeAdd(unit1, unit2) {
810
  try {
811
    return add(unit1, unit2)
812
  } catch (error) {
813
    return null // or throw custom error
814
  }
815
}
816

817
function safeConvert(value, targetUnit) {
818
  try {
819
    return to(value, targetUnit)  
820
  } catch (error) {
821
    return value // Return original if conversion fails
822
  }
823
}
824
```
825

826
## Chain Operations with Units
827

828
```typescript
829
// Units work seamlessly with chain operations
830
const result = chain(unit('100 mile'))
831
  .to('km')        // Convert to kilometers  
832
  .multiply(2)     // Double the distance
833
  .divide(unit('80 km/h')) // Divide by speed to get time
834
  .to('hour')      // Convert to hours
835
  .done()          // ~5 hours
836

837
// Complex unit calculations in chain
838
const power = chain(unit('12 V'))
839
  .multiply(unit('5 A'))  // 60 W
840
  .multiply(unit('2 hour')) // 120 Wh  
841
  .to('kWh')              // 0.12 kWh
842
  .done()
843
```