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Oct 18, 2018


A small, fast JavaScript library for arbitrary-precision decimal arithmetic.

The little sister to bignumber.js and decimal.js. See here for some notes on the difference between them.


  • Faster, smaller and easier-to-use than JavaScript versions of Java's BigDecimal
  • Only 5.9 KB minified and 2.7 KB gzipped
  • Simple API
  • Replicates the toExponential, toFixed and toPrecision methods of JavaScript's Number type
  • Includes a sqrt method
  • Stores values in an accessible decimal floating point format
  • No dependencies
  • Comprehensive documentation and test set

Set up

The library is the single JavaScript file big.js (or big.min.js, which is big.js minified).


<script src='path/to/big.js'></script>


$ npm install big.js
const Big = require('big.js');

ES6 module:

import Big from './big.mjs';


In all examples below, var, semicolons and toString calls are not shown. If a commented-out value is in quotes it means toString has been called on the preceding expression.

The library exports a single function, Big, the constructor of Big number instances. It accepts a value of type number, string or Big number object.

x = new Big(123.4567)
y = Big('123456.7e-3')             // 'new' is optional
z = new Big(x)
x.eq(y) && x.eq(z) && y.eq(z)      // true

A Big number is immutable in the sense that it is not changed by its methods.

0.3 - 0.1                          // 0.19999999999999998
x = new Big(0.3)
x.minus(0.1)                       // "0.2"
x                                  // "0.3"

The methods that return a Big number can be chained.

x.sqrt().div(y).pow(3).gt(y.mod(z))    // true

Like JavaScript's Number type, there are toExponential, toFixed and toPrecision methods.

x = new Big(255.5)
x.toExponential(5)                 // "2.55500e+2"
x.toFixed(5)                       // "255.50000"
x.toPrecision(5)                   // "255.50"

The arithmetic methods always return the exact result except div, sqrt and pow (with negative exponent), as these methods involve division.

The maximum number of decimal places and the rounding mode used to round the results of these methods is determined by the value of the DP and RM properties of the Big number constructor.

Big.DP = 10
Big.RM = 1

x = new Big(2);
y = new Big(3);
z = x.div(y)                       // "0.6666666667"
z.sqrt()                           // "0.8164965809"
z.pow(-3)                          // "3.3749999995"
z.times(z)                         // "0.44444444448888888889"
z.times(z).round(10)               // "0.4444444445"

Multiple Big number constructors can be created, each with an independent configuration.

The value of a Big number is stored in a decimal floating point format in terms of a coefficient, exponent and sign.

x = new Big(-123.456);
x.c                                // [1,2,3,4,5,6]    coefficient (i.e. significand)
x.e                                // 2                exponent
x.s                                // -1               sign

For further information see the API reference from the doc folder.


The test directory contains the test scripts for each Big number method.

The tests can be run with Node.js or a browser.

To run all the tests

$ npm test

To test a single method

$ node test/toFixed

For the browser, see single-test.html and every-test.html in the test/browser directory.

big-vs-number.html is a simple application that enables some of the methods of big.js to be compared with those of JavaScript's Number type.


The perf directory contains two legacy applications and a lib directory containing the BigDecimal libraries used by both.

big-vs-bigdecimal.html tests the performance of big.js against the JavaScript translations of two versions of BigDecimal, its use should be more or less self-explanatory.

The BigDecimal in the npm registry is the GWT version. It has some bugs, see the Node.js script perf/lib/bigdecimal_GWT/bugs.js for examples of flaws in its remainder, divide and compareTo methods.

bigtime.js is a Node.js command-line application which tests the performance of big.js against the GWT version of BigDecimal from the npm registry.

For example, to compare the time taken by the big.js plus method and the BigDecimal add method

$ node bigtime plus 10000 40

This will time 10000 calls to each, using operands of up to 40 random digits and will check that the results match.

For help

$ node bigtime -h


If uglify-js is installed globally

$ npm install uglify-js -g


$ npm run build

will create big.min.js.


The DefinitelyTyped project has a Typescript type definitions file for big.js.

$ npm install --save-dev @types/big.js

Any questions about the TypeScript type definitions file should be addressed to the DefinitelyTyped project.



Open an issue, or email Michael




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