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UnitMath is a JavaScript library for unit conversion and arithmetic.

Build Status codecov


npm install unitmath

UnitMath is still in the early stages of development. The following API may be incomplete, or refer to features that are not yet implemented.


const unit = require('unitmath')

unit('40 km').to('mile')  // 62.1371192237334 mile
unit('5 m').div('2 s')   // 2.5 m / s

Creating Units

To create a unit, use the unit factory function, passing either a single string, or a number and a string:

// String
let a = unit('40 mile')
let b = unit('hour')

// Number and string
let g = unit(9.8, 'm/s^2')
let h = unit(19.6, 'm')

// Two strings
let k = unit('45', 'W / m K')

Units can be simple (4 kg) or compound (8.314 J/mol K). They may also be valueless (hour). Below are more examples of parsing units:

unit('8.314 kg m^2 / s^2 mol K')
unit('kW / kg K')

Parentheses are not used. Any units appearing before a / are in the numerator of the resulting unit, and any units appearing after the / are in the denominator. Any *'s will be ignored.

Converting Units

The to method converts one unit to another. The two units must be consistent, that is, have the same dimension.

unit('40 mile').to('km') // 64.37376 km
unit('kg').to('lbm') // 2.20462262184878 lbm

The simplify method will attempt to convert the unit to a simpler form.

unit('10 / s').simplify() // 10 Hz
unit('J / m').simplify() // N

The split method will convert one unit into an array of units like so:

unit('10 km').split([ 'mi', 'ft', 'in' ]) // [ 6 mi, 1128 ft, 4.78740157486361 in ]
unit('51.4934 deg').split([ 'deg', 'arcmin', 'arcsec' ]) // [ 51 deg, 29 arcmin, 36.24 arcsec ]


Use the methods add, sub, mul, div, pow, sqrt, and others to perform arithmetic on units. Multiple operations can be chained together:

let g = unit(9.8, 'm/s^2')
let h = unit(19.6, 'm')
h.mul(2).div(g).sqrt()   // 2 s

Strings and numbers are implicitly converted to units within a chained expression. When chaining operators, they will execute in order from left to right, so your "expression" may not follow the usual, mathematical order of operations.

unit('3 ft').add('6 in').mul(2)   // 7 ft

All of the operators are also available on the unit namespace:

unit.mul(unit.add('3 ft', '6 in'), 2)

Units are immutable, so every operation on a unit creates a new unit.


Use either the format method to format a unit as a string. toString is an alias for format.

unit('1 lb').to('kg').format() // '0.45359237 kg'

The format and toString methods accept several options; see Configuring.

You can also define custom formatters; see Custom Formatter.


UnitMath can be configured using unit.config(options). The function returns a new instance of UnitMath with the specified configuration options:

const unit = require('unitmath').config({ system: 'us' })

To query the current configuration, call unit.config() with no arguments.

Some options can be passed directly to format without first invoking the config() method:

unit('1 lb').to('kg').format({ prefix: 'always', prefixMin: 1, precision: 4 }) // '453.6 g'

These are the available options and their defaults:

  • parentheses: false. When formatting a unit, group the numerator and/or denominator in parentheses if multiple units are present.

    unit('45 W / m K').format({ parentheses: true }) // 45 W / (m K)
  • precision: 15. The number of significant figures to output when converting a unit to a string. Reducing this can help reduce the appearance of round-off errors. A value of 0 will disable rounding entirely.

    unit('180 deg').to('rad').format({ precision: 6 }) // 3.14159 rad
  • prefix: 'auto'. When formatting a unit, this option will specify whether the toString and format methods are allowed to choose an appropriately sized prefix in case of very small or very large quantities. Possible values are 'auto', 'always', or 'never'. If 'auto' is chosen, then a prefix is always chosen unless the unit was constructed using the to() method.

  • prefixMin: 0.1. When choosing a prefix, the smallest formatted value of a unit that is allowed.

  • prefixMax: 1000. When choosing a prefix, the largest formatted value of a unit that is allowed.

  • prefixesToChooseFrom: 'common'. When choosing a prefix, whether to consider all allowed prefixes or just the common ones for that unit. Possible values are 'common' and 'all'.

  • simplify: 'auto'. Specifies if UnitMath should attempt to simplify the units before formatting as a string. Possible values are 'auto', 'always', or 'never'. If 'auto' or 'always', then u.toString() essentially becomes equivalent to u.simplify().toString(). The original u is never modified. When 'auto' is used, simplification is skipped if the unit is valueless or was constructed using the to() method.

  • simplifyThreshold: 2. A factor that affects whether the format method will output the original unit or a simplified version. The original unit will always be output unless the 'complexity' of the unit is reduced by an amount equal to or greater than the simplifyThreshold. A lower value results in more units being simplified, while a higher number results in fewer units being simplified. The complexity of a unit is roughly equal to the number of 'symbols' that are required to write the unit.

    unit('8 kg m / s^2').format() // 8 N
    unit('8 kg m / s^2').format({ simplifyThreshold: 6 })) // 8 kg m / s^2
  • system: 'auto'. The unit system to use when simplifying a unit. Available systems are si, cgs, us, and auto. When system === 'auto', UnitMath will try to infer the unit system from the individual units that make up that unit.

    unit = unit.config({ system: 'auto' })
    unit('150 lbf').div('10 in^2').toString()  // "15 psi"
    unit('400 N').div('10 cm^2').toString()  // "400 kPa"
  • subsystem: 'auto' Not yet implemented. The subsystem, or technical field, etc., to use when simplifying a unit. It can provide additional hints about which units to use when there are multiple options within the same system. Available subsystems are 'mechanics', 'chemistry', 'electricity_and_magnetism', etc. When subsystem === 'auto', UnitMath will try to infer the subsystem from the individual units that make up that unit:

    // Proposed, but not yet implemented
    unit = unit.config({ subsystem: 'auto' })
    unit('240 V').mul('5 A').mul('1 hr').toString()  // "1.2 kWh"
    unit('4000 kg').mul('9.8 m/s^2').mul('100 m').toString()  // "3.92 MJ"
  • definitions. An object that allows you to add to, modify, or remove the built-in units. See User-Defined Units for complete details.

    unit = unit.config({
      definitions: {
        units: {
          furlong: '220 yards',
          fortnight: '2 weeks'
    unit('6 furlongs/fortnight').to('m/s') // 0.000997857142857143 m / s
  • type. An object that allows UnitMath to work with custom numeric types. See Custom Types for complete details and examples.

Because unit.config(options) returns a new instance of UnitMath, is is technically possible to perform operations between units created from different instances. The resulting behavior is undefined, however, so it is probably best to avoid doing this.

Important: unit.config(options) returns a new instance of the factory function, so you must assign the return value of unit.config(options) to some variable, otherwise the new options won't take effect:

let unit = require('unitmath')

unit.config(options) // This has no effect

Extending UnitMath

User-Defined Units

To create a user-defined unit, pass a definitions object to unit.config():

unit = unit.config({
  definitions: {
    units: {
      lightyear: '9460730472580800 m'

unit('1 lightyear').to('mile') // 5878625373183.608 mile

The definitions contains five keys which allow additional customization of the unit system. These are: units, prefixes, unitSystems, baseQuantities, and quantities.


This object contains the units that are made available by UnitMath. Each key in definitions.units becomes a new unit. The easiest way to define a unit is to provide a string representation in terms of other units:

units: {
  minute: '60 seconds',
  newton: '1 kg m/s^2'

You can also supply an object for additional customization. These are all the options you can specify:

  • value: (Required) The value of the unit. It can be a string or an array containing two items:

    units: {
      minute: { value: '60 seconds' },
      newton: { value: '1 kg m/s^2' }


    units: {
      minute: { value: [ 60, 'seconds' ] },
      newton: { value: [ 1, 'kg m/s^2' ] }
  • quantity: This is required for base units that are not defined in terms of other units, such as meter and second. For derived units, such as joule, the quantity is determined automatically from the unit's value.

    units: {
      seconds: { quantity: 'TIME', value: 1 }
  • prefixes: Specifies which group of prefixes will be allowed when parsing the unit. The default is 'NONE'.

    units: {
      // Will parse 'nanometer', 'micrometer', 'millimeter', 'kilometer', 'megameter', etc.
      meter: { prefixes: 'LONG', ... },
      // Will parse 'nm', 'um', 'mm', 'km', 'Mm', etc.
      m: { prefixes: 'SHORT', ... }
  • commonPrefixes: A string array that specifies which of the allowed prefixes will be used when formatting a unit. If this option is omitted, the unit will be formatted using the original prefix, or none at all.

    units: {
      L: {
        prefixes: 'SHORT',
        // Will format only as 'nL', 'uL', 'mL', and 'L'.
        commonPrefixes: ['n', 'u', 'm', ''],
        value: '1e-3 m^3',
      lumen: {
        prefixes: 'LONG',
        value: '1 cd sr'
        // commonPrefixes not given, so lumen will only be formatted as "lumen", but could be parsed as "millilumen", etc.
  • aliases: Shortcut to create additional units with identical definitions.

    units: {
      meter: { ... , aliases: [ 'meters' ] }
  • offset: Used when the zero-value of this unit is different from the zero-value of the base unit.

    units: {
      celsius: {
        value: '1 K',
        offset: 273.15
  • autoAddToSystem: An optional string value, such as 'si', 'us', or 'auto'. Causes this unit to automatically be added to the specified unit system. A value of 'auto' will cause UnitMath to infer the system from the unit's value. This is a shortcut for setting definitions.quantities and definitions.unitSystem directly:

    definitions: {
      units: {
        snap: {
          value: '1 m/s^4',
          autoAddToUnitSystem: 'si'

    Which is equivalent to the following:

    definitions: {
      units: {
        snap: {
          value: '1 m/s^4'
      unitSystems: {
        si: {
          snap_QUANTITY: 'snap'
      quantities: {
        snap_QUANTITY: 'LENGTH TIME^-4'

    This causes snap to become part of the si system, which means it may be used to simplify a Unit:

    unit('1 m/s^2').div('2 s^2').simplify() // 0.5 snap

    You may also set the global option definitions.autoAddToSystem in order to set the same value for all user-defined units at the same time.

    Behind the scenes, autoAddToSystem automatically adds the necessary entries to definitions.quantities and definitions.unitSystem so that the unit becomes a member of the specified unit system, allowing it to be used to simplify a Unit. If, however, existing values for definitions.quantities or definitions.unitSystem already exist in the user-defined or built-in units, autoAddToSystem will not override them. If you need more control over how your unit systems are set up, you should set definitions.quantities and definitions.unitSystem directly, rather than using autoAddToSystem.


The definitions.prefixes object is used to define strings and associated multipliers that are prefixed to units to change their value. For example, the 'k' prefix in km multiplies the value of the m unit by 1000.

prefixes: {
  NONE: { '': 1 },
  SHORT: {
    m: 0.001,
    '': 1,
    k: 1000
  LONG: {
    milli: 0.001,
    '': 1,
    kilo: 1000


The definitions.baseQuantities array defines the dimensionally-independent quantities (LENGTH, MASS, LUMINOUS_INTENSITY, etc.) that form the basis of all units. To add a new base quantity, supply the new base quantity or quantities in an array:

baseQuantities: [ 'MY_NEW_BASE_QUANTITY' ]


The definitions.quantities object defines other quantities that are derived from the base quantities, such as FORCE. These are used in the unitSystems object to specify the preferred unit for that quantity. The syntax for defining a quantity is more strict than that used to parse units generally. The format of a quantity is: zero or more terms, separated by spaces, where each term comprises a baseQuantity, optionally followed by a caret ^ and a floating point number:

quantities: {


The definitions.unitSystems object defines the preferred units to use with a particular unit system. Any or all of the quantities in a unit system may be assigned a single unit, optionally with a prefix, that will be used when formatting a matching unit in that system.

unitSystems: {
  si: {
    CURRENT: 'A',
    MASS: 'kg',


A boolean value indicating whether to skip creation of the built-in units. If true, only the user-defined units and quantities defined in definitions will be created.


Global setting that applies autoAddToSystem to all user-defined units simultaneously. This means that the following two definitions are equivalent:

definitions: {
  units: {
    snap: '1 m/s^4'
  autoAddToUnitSystem: 'auto'
definitions: {
  units: {
    snap: {
      value: '1 m/s^4',
      autoAddToUnitSystem: 'auto'

For more information, see definitions.units.autoAddToUnit.

Querying current unit definitions

You can view all the current definitions by calling unit.definitions(). This object contains all the built-in units, prefixes, unit systems, base quantities, and quantities. If you have configured UnitMath with additional definitions, these will also be included in the return value from unit.definitions().


Below is an abbreviated sample output from unit.definitions(). It can serve as a starting point to create your own definitions.

{ units:
   { '': { quantity: 'UNITLESS', value: 1 },
      { quantity: 'LENGTH',
        prefixes: 'LONG',
        commonPrefixes: [ 'nano', 'micro', 'milli', 'centi', '', 'kilo' ],
        value: 1,
        aliases: [ 'meters' ] },
      { prefixes: 'SHORT',
        commonPrefixes: [ 'n', 'u', 'm', 'c', '', 'k' ],
        value: '1 meter' },
     inch: { value: '0.0254 meter', aliases: [ 'inches', 'in' ] },
     foot: { value: '12 inch', aliases: [ 'ft', 'feet' ] },
     yard: { value: '3 foot', aliases: [ 'yd', 'yards' ] },
     mile: { value: '5280 ft', aliases: [ 'mi', 'miles' ] },
     ... },
   { NONE: { '': 1 },
      { '': 1,
        da: 10,
        h: 100,
        k: 1000,
        d: 0.1,
        c: 0.01,
        m: 0.001,
        ... },
     ... },
   { si:
      { AMOUNT_OF_SUBSTANCE: 'mol',
        CAPACITANCE: 'F',
        CURRENT: 'A',
        MASS: 'kg'
        ... },
     ... },
   [ 'MASS',
     'SOLID_ANGLE' ],
   { UNITLESS: '',
     AREA: 'LENGTH^2',
     ... } }

Custom Types

You can easily extend UnitMath to work with custom types. The type option is an object containing several key/value pairs, where each value is a function that replaces the normal +, -, *, /, and other arithmetic operators used internally by UnitMath.

Example using Decimal.js as the custom type:

const Decimal = require('decimal.js')
const unit = require('unitmath').config({
  type: {
    clone: Decimal,
    conv: Decimal,
    add: (a, b) => a.add(b),
    sub: (a, b) => a.sub(b),
    mul: (a, b) => a.mul(b),
    div: (a, b) => a.div(b),
    pow: (a, b) => a.pow(b),
    eq: (a, b) => a.eq(b),
    lt: (a, b) =>,
    le: (a, b) => a.lte(b),
    gt: (a, b) =>,
    ge: (a, b) => a.gte(b),
    abs: (a) => a.abs(),
    round: (a) => a.round(),
    trunc: (a) => Decimal.trunc(a),
    format: a => a.toString()

let u = unit('2.74518864784926316174649567946 m')

Below is a list of functions, their signatures, and when they are required.

Required always:

  • clone: (a: T) => T
  • conv: (a: number | string | T) => T
  • add: (a: T, b: T) => T
  • sub: (a: T, b: T) => T
  • mul: (a: T, b: T) => T
  • div: (a: T, b: T) => T
  • pow: (a: T, b: number) => T

Required for prefix = 'auto' or 'always':

  • abs: (a: T) => T
  • lt: (a: T, b: T) => boolean
  • gt: (a: T, b: T) => boolean
  • le: (a: T, b: T) => boolean
  • ge: (a: T, b: T) => boolean

Required for specific functions:

  • eq: (a: T, b: T) => boolean (Required for equals function)
  • round: (a: T) => T (Required for split function)
  • trunc: (a: T) => T (Required for split function)


  • format: (a: T, options: any) => string

The add, sub, mul, div, and pow functions replace +, -, *, /, and Math.pow, respectively. The clone function should return a clone of your custom type (same value, different object).

The conv function must, at a minimum, be capable of converting both strings and numbers into your custom type. If given a custom type, it should return it unchanged, or return a clone. Among other things, the conv function is used by UnitMath to convert the values of the built-in units to your custom type.

UnitMath will also use the conv function when constructing units from numbers and strings. If your custom type is representable using decimal or scientific notation (such as 6.022e+23), you can include both the value and the units in a single string:

// Supply a single string, and the numeric portion will be parsed using type.conv
unit('3.1415926535897932384626433832795 rad')

If your custom type cannot be represented in decimal or scientific notation, such as is the case with complex numbers and fractions, you will have to use the more generic two-argument constructor, supplying either two strings, a number and a string, or your custom type and a string:

unit('1 / 2', 'kg') // Supply two strings

unit(0.5, 'kg') // Supply a number and a string

unit(Fraction(1, 2), 'kg') // Supply the value directly

The functions clone, conv, add, sub, mul, div, and pow are always required. Omitting any of these will cause the config method to throw an error. The other functions are conditionally required, and you will receive an error if you attempt something that depends on a function you haven't provided.

Custom Formatter

UnitMath will use your type's toString method when formatting a unit. You can use a different formatter by setting the type.format function. This works even if you are not using custom types. Any arguments you pass to the unit's format or toString method will also be passed to your custom format function:

let unitFunny = require('../index.js').config({
  type: {
    format: (a, b, c) => b + a.toString().split('').reverse().join(c)

unitFunny('3.14159 rad').toString('$', '_') // '$9_5_1_4_1_._3 rad'

API Reference

Factory Function

  • unit(value: number, unitString: string)
    unit(value: number)
    unit(valueAndUnitString: string)

    Creates a unit with the specified value and unit string. If valueAndUnitString is supplied, it must specify both the numeric portion and the units portion of the unit.

    const unit = require('unitmath')
    unit(60, 'mile/hour') // 60 mile / hour
    unit(60) // 60
    unit('60 mile/hour') // 60 mile / hour
    unit('mile/hour') // mile / hour
    unit() // Empty unit

    The string used to specify the unit (valueAndUnitString or unitString) must be in the following format:

    numerator, denominator:
    unitPiece [unitPiece ...]
    value, power:
    Any floating-point number

Member Functions

  • #clone()

    Returns a clone of this unit.

    let a = unit('40 m/s') // 40 m / s
    let b = a.clone() // 40 m / s
  • #add(other: unit | string | number)

    Adds this unit to another unit and returns a new unit. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.

    let a = unit('20 kW')
    let b = unit('300 W')
    a.add(b) // 20.3 kW
  • #sub(other: unit | string | number)

    Adds another unit from this unit and returns a new unit. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.

    let a = unit('20 kW')
    let b = unit('300 W')
    a.sub(b) // 19.7 kW
  • #mul(other: unit | string | number)

    Multipies this unit and another unit and returns a new unit. If a string or number is supplied as an argument, it is converted to a unit.

    let a = unit('8 m')
    let b = unit('200 N')
    a.mul(b) // 16 kJ
  • #div(other: unit | string | number)

    Divides this unit by another unit and returns a new unit. If a string or number is supplied as an argument, it is converted to a unit.

    let a = unit('64 kJ')
    let b = unit('16 s')
    a.mul(b) // 4 kW
  • #pow(p: number)

    Raises this unit to the power p and returns a new unit.

    unit('10 m').pow(3) // 1000 m^3
  • #sqrt()

    Returns the square root of this unit.

    unit('1 heactare').sqrt() // 100 m
  • #abs()

    Returns the absolute value of this unit. If the unit has an offset, such as degC, this is applied before taking the absolute value.

    unit('-5 m / s').abs() // 5 m / s
    unit('300 degC').abs() // -246.3 degC
  • #to(target: unit | string)

    Converts this unit to the specified target unit or string. The returned unit will be "fixed", so it will not be auto-simplified or auto-prefixed in format(). If to() is called on the unit with no arguments, then a "fixed" clone of the unit is returned.

    let r = unit('10 kg / m^2 s^3 A^2')
    r.format() // 10 ohm'kohm').format() // 0.01 kohm // 10 kg m^2 / s^3 A^2
  • #toSI()

    Returns a new unit that is the SI representation of this unit.

    unit('10 ft/s').toSI() // 3.048 m / s
  • getValue()

    Returns the value of this unit, or null if the unit is valueless.

  • #setValue(x: number | string | custom)

    Returns a copy of this unit but with its value replaced with the given value. Useful if you would like to perform your own operations on a unit's value. If supplied with no arguments, or null, will remove the value from the unit.

    unit('10 m').setValue(20) // 20 m
    unit('m').setValue(20) // 20 m
    unit('10 ft').setValue(20) // 20 ft
    unit('10 ft').setValue() // ft
  • getNormalizedValue()

    Returns the value of this unit if it were to be converted to SI base units (or whatever base units that are defined). Returns null if the unit is valueless.

  • setNormalizedValue()

    Returns a copy of this unit but with its value replaced with the given normalized value.

  • #simplify()

    Attempts to simplify the unit, and returns the simplified unit (or a clone of the original if unsuccessful). simplify() is called when a unit is being formatted as a string whenever the config option simplify is 'auto' or 'always'.

    unit('10 N m').simplify() // 10 J
  • #split(Array(string | unit))

    Converts this unit into an array of units, where the sum of the resulting units is equal to this unit, and where each of the resulting units is the result of truncating this unit to an integer, and then passing the remainder to the next unit, until the final unit, which takes up all the remainder.

    unit('51.4934 deg').split([ 'deg', 'arcmin', 'arcsec' ]) // [ 51 deg, 29 arcmin, 36.24 arcsec ]
  • #getUnits()

    Returns a clone of this unit with the value removed.

    unit('8.314 J / mol K').getUnits() // J / mol K
  • #isCompound()

    Returns true if this unit's unit list contains two or more units, or one unit with a power not equal to 1.

    unit('34 kg').isCompound() // false
    unit('34 kg/s').isCompound() // true
    unit('34 kg^2').isCompound() // true
    unit('34 N').isCompound() // false
    unit('34 kg m / s^2').isCompound() // true
  • #getQuantities()

    Returns a string array of all of this unit's matching quantities.

    unit('5 m/s^2').getQuantities(), // ['ACCELERATION']
    unit('5 m^2').getQuantities(), // ['AREA']
    unit('5 kg m^2 / s^2').getQuantities(), // ['ENERGY', 'TORQUE']
    unit('5 A/m').getQuantities(), // ['MAGNETIC_FIELD_STRENGTH']
    unit('5 kg m s K A rad bits').getQuantities(), // []
  • #hasQuantity(quantity: string)

    Returns true if this unit matches the given quantity.

    unit('5 m/s^2').hasQuantity('ACCELERATION') // true
  • #equalQuantity(other: unit | string)

    Returns true if this unit and another unit have equal quantities or dimensions.

    unit('5 m/s^2').equalQuantity('4 ft/s^2')) // true
  • #equals(other: unit | string)

    Returns true if the two units represent the same values.

    unit('3 ft').equals('1 yard') // true
  • #compare(other: unit | string)

    Returns a value indicating whether this unit is less than (-1), greater than (1), or equal to (0), another unit.

    unit('30 min').compare('1 hour') // -1
    unit('60 min').compare('1 hour') // 0
    unit('90 min').compare('1 hour') // 1
  • #lessThan(other: unit | string)

    Compares this and another unit and returns true if this unit is less than the other.

  • #lessThanOrEqual(other: unit | string)

    Compares this and another unit and returns true if this unit is less than or equal to the other.

  • #greaterThan(other: unit | string)

    Compares this and another unit and returns true if this unit is greater than the other.

  • #greaterThanOrEqual(other: unit | string)

    Compares this and another unit and returns true if this unit is greater than or equal to the other.

  • #format(options)

    Formats this unit as a string. Formatting options can be supplied which will override the configured options. See Configuring for a list of all options and their effects.

    If the prefix or simplify options are set to 'auto' or 'always', the toString and format methods will try to simplify the unit before outputting. This can be prevented by calling .to() on a unit with no parameters, which will return a new unit that will not be simplified automatically.

  • #toString(options)

    Alias for format(options)

Namespace Functions

  • config()

    Returns the current configuration.

    const unit = require('unitmath')
  • config(options:object)

    Configure a new unit namespace with the given options (see Configuring)

    const unit = require('unitmath').config({ option1, option2, ... })
  • exists(singleUnitString:string)

    Tests if the given unit, optionally with a prefix, exists.

    const unit = require('unitmath')
    unit.exists('km') // true
  • unit.definitions()

    Return the current unit definitions in effect. (User's own definitions can be queried through unit.config().definitions.)

  • unit.add(a: unit | string | number, b: unit | string | number)

    Alias for a.add(b).

    unit.add('4 ft', '1 yd') // 7 ft
  • unit.sub(a: unit | string | number, b: unit | string | number)

    Alias for a.sub(b).

    unit.sub('4 ft', '1 yd') // 1 ft
  • unit.mul(a: unit | string | number, b: unit | string | number)

    Alias for a.mul(b).

    unit.mul('4 ft', '1 yd') // 12 ft^2
  • unit.div(a: unit | string | number, b: unit | string | number)

    Alias for a.div(b).

    unit.div('4 ft', '1 yd') // 1.33333333333333
  • unit.pow(a: unit | string | number, b: number)

    Alias for a.pow(b).

    unit.pow('4 ft', 2) // 16 ft^2
  • unit.sqrt(a: unit | string | number)

    Alias for a.sqrt().

    unit.sqrt('16 ft^2') // 4 ft
  • unit.abs(a: unit | string | number)

    Alias for a.abs().

    unit.abs('-5 m / s') // 5 m / s
    unit.abs('300 degC') // -246.3 degC
  • unit | string | number, b: unit | string)

    Alias for

  • unit.toSI(a: unit | string | number)

    Alias for a.toSI().


This is a community-supported project; all contributions are welcome. Please open an issue or submit a pull request.


Many thanks to Jos de Jong (@josdejong), the original author of Unit.js, who suggested the idea of splitting the file off from Math.js and into its own library.



UnitMath is released under the Apache-2.0 license.


JavaScript library for unit conversion and arithmetic




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