JavaScript library for quantity calculation and unit conversion


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JS-quantities is originally a JavaScript port of Kevin Olbrich’s library Ruby Units (http://github.com/olbrich/ruby-units).

The library aims to simplify the handling of units for scientific calculations involving quantities.

JS-quantities is built as UMD and ES modules and can be used with Node.js and browsers. It has no dependencies.


Install with npm install js-quantities or download latest release v1.8.0 as:



// As CommonJS module
const Qty = require('js-quantities');

// As ES module
import Qty from 'js-quantities/esm';


<script src='quantities.js'></script>

In this case, it will define a global variable Qty.

define(['quantities'], function(Qty) {
<script type="module">
  import Qty from "./path/to/quantities.mjs";
  // ...



Instances of quantities are made by means of Qty() method. Qty can both be used as a constructor (with new) or as a factory (without new):

qty = new Qty('23 ft'); // constructor
qty = Qty('23 ft'); // factory

Qty constructor accepts strings, numbers and Qty instances as initializing values.

If scalars and their respective units are available programmatically, the two argument signature may be useful:

qty = new Qty(124, 'cm'); // => 1.24 meter
qty = Qty(124, 'cm'); // => 1.24 meter

For the sake of simplicity, one will use the factory way below but using new Qty() is equivalent.

qty = Qty('1m'); // => 1 meter
qty = Qty('m'); // =>  1 meter (scalar defaults to 1)

qty = Qty('1 N*m');
qty = Qty('1 N m'); // * is optional

qty = Qty('1 m/s');

qty = Qty('1 m^2/s^2');
qty = Qty('1 m^2 s^-2'); // negative powers
qty = Qty('1 m2 s-2'); // ^ is optional

qty = Qty('1 m^2 kg^2 J^2/s^2 A');

qty = Qty('1.5'); // unitless quantity
qty = Qty(1.5); // number as initializing value

qty = Qty('1 attoparsec/microfortnight');

qtyCopy = Qty(qty); // quantity could be copied when used as
                    // initializing value

Qty.parse utility method is also provided to parse and create quantities from strings. Unlike the constructor, it will return null instead of throwing an error when parsing an invalid quantity.

Qty.parse('1 m'); // => 1 meter
Qty.parse('foo') // => null

Available well-known kinds

Qty.getKinds(); // => Array of names of every well-known kind of units

Available units of a particular kind

Qty.getUnits('currency'); // => [ 'dollar', 'cents' ]
// Or all alphabetically sorted
Qty.getUnits(); // => [ 'acre','Ah','ampere','AMU','angstrom']

Alternative names of a unit

Qty.getAliases('m'); // => [ 'm', 'meter', 'meters', 'metre', 'metres' ]

Quantity compatibility, kind and various queries

qty1.isCompatible(qty2); // => true or false

qty.kind(); // => 'length', 'area', etc...

qty.isUnitless(); // => true or false
qty.isBase(); // => true if quantity is represented with base units


qty.toBase(); // converts to SI units (10 cm => 0.1 m) (new instance)

qty.toFloat(); // returns scalar of unitless quantity
               // (otherwise throws error)

qty.to('m'); // converts quantity to meter if compatible
             // or throws an error (new instance)
qty1.to(qty2); // converts quantity to same unit of qty2 if compatible
               // or throws an error (new instance)

qty.inverse(); // converts quantity to its inverse
               // ('100 m/s' => '.01 s/m')
// Inverses can be used, but there is no special checking to
// rename the units
Qty('10ohm').inverse() // '.1/ohm'
                       // (not '.1S', although they are equivalent)
// however, the 'to' command will convert between inverses also
Qty('10ohm').to('S') // '.1S'

Qty.swiftConverter() is a fast way to efficiently convert large array of Number values. It configures a function accepting a value or an array of Number values to convert.

var convert = Qty.swiftConverter('m/h', 'ft/s'); // Configures converter

// Converting single value
var converted = convert(2500); // => 2.278..

// Converting large array of values
var convertedSerie = convert([2500, 5000, ...]); // => [2.278.., 4.556.., ...]

The main drawback of this conversion method is that it does not take care of rounding issues.


qty1.eq(qty2); // => true if both quantities are equal (1m == 100cm => true)
qty1.same(qty2); // => true if both quantities are same (1m == 100cm => false)
qty1.lt(qty2); // => true if qty1 is stricty less than qty2
qty1.lte(qty2); // => true if qty1 is less than or equal to qty2
qty1.gt(qty2); // => true if qty1 is stricty greater than qty2
qty1.gte(qty2); // => true if qty1 is greater than or equal to qty2

qty1.compareTo(qty2); // => -1 if qty1 < qty2,
                      // => 0 if qty1 == qty2,
                      // => 1 if qty1 > qty2



Qty#toPrec(precision) : returns the nearest multiple of quantity passed as precision.

var qty = Qty('5.17 ft');
qty.toPrec('ft'); // => 5 ft
qty.toPrec('0.5 ft'); // => 5 ft
qty.toPrec('0.25 ft'); // => 5.25 ft
qty.toPrec('0.1 ft'); // => 5.2 ft
qty.toPrec('0.05 ft'); // => 5.15 ft
qty.toPrec('0.01 ft'); // => 5.17 ft
qty.toPrec('0.00001 ft'); // => 5.17 ft
qty.toPrec('2 ft'); // => 6 ft
qty.toPrec('2'); // => 6 ft

var qty = Qty('6.3782 m');
qty.toPrec('dm'); // => 6.4 m
qty.toPrec('cm'); // => 6.38 m
qty.toPrec('mm'); // => 6.378 m
qty.toPrec('5 cm'); // => 6.4 m
qty.toPrec('10 m'); // => 10 m
qty.toPrec(0.1); // => 6.3 m

var qty = Qty('1.146 MPa');
qty.toPrec('0.1 bar'); // => 1.15 MPa

Formatting quantities

Qty#toString returns a string using the canonical form of the quantity (that is it could be seamlessly reparsed by Qty).

var qty = Qty('1.146 MPa');
qty.toString(); // => '1.146 MPa'

As a shorthand, units could be passed to Qty#toString and is equivalent to successively call Qty#to then Qty#toString.

var qty = Qty('1.146 MPa');
qty.toString('bar'); // => '11.46 bar'
qty.to('bar').toString(); // => '11.46 bar'

Qty#toString could also be used with any method from Qty to make some sort of formatting. For instance, one could use Qty#toPrec to fix the maximum number of decimals:

var qty = Qty('1.146 MPa');
qty.toPrec(0.1).toString(); // => '1.1 MPa'
qty.to('bar').toPrec(0.1).toString(); // => '11.5 bar'

For advanced formatting needs as localization, specific rounding or any other custom customization, quantities can be transformed into strings through Qty#format according to optional target units and formatter. If target units are specified, the quantity is converted into them before formatting.

Such a string is not intended to be reparsed to construct a new instance of Qty (unlike output of Qty#toString).

If no formatter is specified, quantities are formatted according to default js-quantities’ formatter and is equivalent to Qty#toString.

var qty = Qty('1.1234 m');
qty.format(); // same units, default formatter => '1.234 m'
qty.format('cm'); // converted to 'cm', default formatter => '123.45 cm'

Qty#format could delegates formatting to a custom formatter if required. A formatter is a callback function accepting scalar and units as parameters and returning a formatted string representing the quantity.

var configurableRoundingFormatter = function(maxDecimals) {
  return function(scalar, units) {
    var pow = Math.pow(10, maxDecimals);
    var rounded = Math.round(scalar * pow) / pow;

    return rounded + ' ' + units;

var qty = Qty('1.1234 m');

// same units, custom formatter => '1.12 m'

// convert to 'cm', custom formatter => '123.4 cm'
qty.format('cm', configurableRoundingFormatter(1));

Custom formatter can be configured globally by setting Qty.formatter.

Qty.formatter = configurableRoundingFormatter(2);
var qty = Qty('1.1234 m');
qty.format(); // same units, current default formatter => '1.12 m'


Like ruby-units, JS-quantities makes a distinction between a temperature (which technically is a property) and degrees of temperature (which temperatures are measured in).

Temperature units (i.e., ‘tempK’) can be converted back and forth, and will take into account the differences in the zero points of the various scales. Differential temperature (e.g., ‘100 degC’) units behave like most other units.

Qty('37 tempC').to('tempF') // => 98.6 tempF

JS-quantities will throw an error if you attempt to create a temperature unit that would fall below absolute zero.

Unit math on temperatures is fairly limited.

Qty('100 tempC').add('10 degC')  // 110 tempC
Qty('100 tempC').sub('10 degC')  // 90 tempC
Qty('100 tempC').add('50 tempC') // throws error
Qty('100 tempC').sub('50 tempC') // 50 degC
Qty('50 tempC').sub('100 tempC') // -50 degC
Qty('100 tempC').mul(scalar)     // 100*scalar tempC
Qty('100 tempC').div(scalar)     // 100/scalar tempC
Qty('100 tempC').mul(qty)        // throws error
Qty('100 tempC').div(qty)        // throws error
Qty('100 tempC*unit')            // throws error
Qty('100 tempC/unit')            // throws error
Qty('100 unit/tempC')            // throws error
Qty('100 tempC').inverse()       // throws error
Qty('100 tempC').to('degC') // => 100 degC

This conversion references the 0 point on the scale of the temperature unit

Qty('100 degC').to('tempC') // => -173.15 tempC

These conversions are always interpreted as being relative to absolute zero. Conversions are probably better done like this…

Qty('0 tempC').add('100 degC') // => 100 tempC


Every error thrown by JS-quantities is an instance of Qty.Error.

try {
  // code triggering an error inside JS-quantities
catch(e) {
  if(e instanceof Qty.Error) {
    // ...
  else {
    // ...


Tests are implemented with Jasmine (https://github.com/pivotal/jasmine). You could use both HTML and jasmine-node runners.

To execute specs through HTML runner, just open SpecRunner.html file in a browser to execute them.

To execute specs through jasmine-node, launch:

make test

Performance regression test

There is a small benchmarking HTML page to spot performance regression between currently checked-out quantities.js and any committed version. Just execute:

make bench

then open

Checked-out version is benchmarked against HEAD by default but it could be changed by passing any commit SHA on the command line. Port (default 3000) is also configurable.

make bench COMMIT=e0c7fc468 PORT=5000

TypeScript type declarations

A TypeScript declaration file is published on DefinitelyTyped.

It could be installed with npm install @types/js-quantities.


Feedback and contributions are welcomed.

Pull requests must pass tests and linting. Please make sure that make test and make lint return no errors before submitting.