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An isomorphic, load-anywhere function to convert a bytes value (e.g. 3456) to a human-readable string ('3.5 kB'). Choose between metric or IEC units (summarised below) or specify your own custom units.

Value Metric Metric (octet)
1000 kB kilobyte ko kilooctet
1000^2 MB megabyte Mo megaoctet
1000^3 GB gigabyte Go gigaoctet
1000^4 TB terabyte To teraoctet
1000^5 PB petabyte Po petaoctet
1000^6 EB exabyte Eo exaoctet
1000^7 ZB zettabyte Zo zettaoctet
1000^8 YB yottabyte Yo yottaoctet
Value IEC IEC (octet)
1024 KiB kibibyte Kio kibioctet
1024^2 MiB mebibyte Mio mebioctet
1024^3 GiB gibibyte Gio gibioctet
1024^4 TiB tebibyte Tio tebioctet
1024^5 PiB pebibyte Pio pebioctet
1024^6 EiB exbibyte Eio exbioctet
1024^7 ZiB zebibyte Zio zebioctet
1024^8 YiB yobibyte Yio yobioctet


By default, byteSize converts the input number to a human readable string with metric units and a precision of 1.

> const byteSize = require('byte-size')

> byteSize(1580)
{ value: '1.6', unit: 'kB', long: 'kilobytes' }

The object returned by byteSize defines a toString method therefore can be used directly in string context.

> `Filesize: ${byteSize(12400)}`
'Filesize: 12.4 kB'

Override the default toString behaviour by setting options.toStringFn.

> function toStringFn () {
  return `**${this.value}${this.unit}**`

> `Filesize: ${byteSize(12400, { toStringFn })}`
'Filesize: **12.4kB**'

Beside the default of metric, there are three other built-in units available: metric_octet, iec and iec_octet.

> byteSize(1580, { units: 'iec' })
{ value: '1.5', unit: 'KiB', long: 'kibibytes' }

> byteSize(1580, { units: 'iec_octet' })
{ value: '1.5', unit: 'Kio', long: 'kibioctets' }

> byteSize(1580, { units: 'metric_octet' })
{ value: '1.6', unit: 'ko', long: 'kilooctets' }

You can adjust the precision.

> byteSize(1580, { units: 'iec', precision: 3 })
{ value: '1.543', unit: 'KiB', long: 'kibibytes' }

> byteSize(1580, { units: 'iec', precision: 0 })
{ value: '2', unit: 'KiB', long: 'kibibytes' }

Define custom units by passing an object containing one or more additional conversion tables to options.customUnits. In options.units, specify the name of a property from the customUnits object.

> const customUnits = {
  simple: [
    { from: 0   , to: 1e3 , unit: ''  },
    { from: 1e3 , to: 1e6 , unit: 'K', long: 'thousand' },
    { from: 1e6 , to: 1e9 , unit: 'Mn', long: 'million' },
    { from: 1e9 , to: 1e12, unit: 'Bn', long: 'billion' }

> const { value, unit } = byteSize(10000, { customUnits, units: 'simple' })

> `${value}${unit}`

Override the built-in defaults for the duration of the process by passing an options object to byteSize.defaultOptions(). This results in cleaner code in cases where byteSize is used often with the same options.

> byteSize.defaultOptions({
  units: 'simple',
  precision: 2,
  customUnits: {
    simple: [
      { from: 0, to: 1e3, unit: '' },
      { from: 1e3, to: 1e6, unit: 'k' },
      { from: 1e6, to: 1e9, unit: 'm' },
      { from: 1e9, to: 1e12, unit: 'bn' },
  toStringFn: function () {
    return this.value + this.unit

> [2400, 16400, 3991200].map(byteSize).join(', ')
'2.40k, 16.40k, 3.99m'


byteSize(bytes, [options]) ⇒ object

Returns an object with the spec { value: string, unit: string, long: string }. The returned object defines a toString method meaning it can be used in any string context.

Kind: Exported function

Param Type Description
bytes number The bytes value to convert.
[options] object Optional config.
[options.precision] number Number of decimal places. Defaults to 1.
[options.units] string Specify 'metric', 'iec', 'metric_octet', 'iec_octet' or the name of a property from the custom units table in options.customUnits. Defaults to metric.
[options.customUnits] object An object containing one or more custom unit lookup tables.
[options.toStringFn] function A toString function to override the default.
[options.locale] string | Array.<string> Node >=13 or modern browser only - on earlier platforms this option is ignored. The locale to use for number formatting (e.g. 'de-DE'). Defaults to your system locale. Passed directed into Intl.NumberFormat().


Set the default byteSize options for the duration of the process.

Kind: static method of byteSize

Param Type Description
options object A byteSize options object.

Load anywhere

This library is compatible with Node.js, the Web and any style of module loader. It can be loaded anywhere, natively without transpilation.

Node.js (CommonJS):

const byteSize = require('byte-size')

Node.js (ECMAScript Module):

import byteSize from 'byte-size'

Within a modern browser ECMAScript Module:

import byteSize from './node_modules/byte-size/index.js'

Browser global (adds window.byteSize):

<script src="./node_modules/byte-size/dist/index.js"></script>

© 2014-23 Lloyd Brookes <>.

Tested by test-runner. Documented by jsdoc-to-markdown.