pretty-bytes

Convert bytes to a human readable string: 1337 → 1.34 kB

Useful for displaying file sizes for humans.

Note that it uses base-10 (e.g. kilobyte). Read about the difference between kilobyte and kibibyte.

Install

npm install pretty-bytes

Usage

import prettyBytes from 'pretty-bytes';

prettyBytes(1337);
//=> '1.34 kB'

prettyBytes(100);
//=> '100 B'

// Display with units of bits
prettyBytes(1337, {bits: true});
//=> '1.34 kbit'

// Display file size differences
prettyBytes(42, {signed: true});
//=> '+42 B'

// Localized output using German locale
prettyBytes(1337, {locale: 'de'});
//=> '1,34 kB'

// Fixed width for alignment (useful for progress bars and tables)
prettyBytes(1337, {fixedWidth: 8});
//=> ' 1.34 kB'

API

prettyBytes(number, options?)

number

Type: number | bigint

The number to format.

A bigint of any size is accepted. Past roughly 1e332 bytes the scaled value no longer fits a number, and it is written out in full from the exact quotient, so it carries no grouping and no localized decimal separator, the same as a value below 1e-100.

Byte counts are otherwise formatted with the precision of a JavaScript number, which holds about 16 significant digits, so above roughly 1e17 bytes the trailing digits of the output may not be exact. A value that rounds up onto a unit boundary can therefore be printed in the larger unit, so 999499999999999999 bytes is 1 EB rather than 999 PB.

[!NOTE] Rounding uses Number#toPrecision, so at an exact tie the direction comes from how that value is stored as a floating point number. 1005 bytes is 1.005 kB and rounds down to 1 kB, while 1125 bytes is 1.125 kB and rounds up to 1.13 kB.

options

Type: object

signed

Type: boolean
Default: false

Include plus sign for positive numbers. If the difference is exactly zero a space character will be prepended instead for better alignment.

bits

Type: boolean
Default: false

Format the number as bits instead of bytes. This can be useful when, for example, referring to bit rate.

import prettyBytes from 'pretty-bytes';

prettyBytes(1337, {bits: true});
//=> '1.34 kbit'

binary

Type: boolean
Default: false

Format the number using the Binary Prefix instead of the SI Prefix. This can be useful for presenting memory amounts. However, this should not be used for presenting file sizes.

import prettyBytes from 'pretty-bytes';

prettyBytes(1000, {binary: true});
//=> '1000 B'

prettyBytes(1024, {binary: true});
//=> '1 KiB'

locale

Type: boolean | string | string[]
Default: false

• If false: Output won't be localized.
• If true: Localize the output using the system/browser locale.
• If string: Expects a BCP 47 language tag (For example: en, de, …)
• If string[]: Expects a list of BCP 47 language tags (For example: en, de, …)

[!IMPORTANT] Only the number and decimal separator are localized. The unit title is not and will not be localized.

[!NOTE] A value below 1e-100 bytes needs more than the 100 fraction digits Intl.NumberFormat accepts, so it is written out with plain digits instead of a localized decimal separator.

import prettyBytes from 'pretty-bytes';

prettyBytes(1337, {locale: 'de'});
//=> '1,34 kB'

minimumFractionDigits

Type: number
Default: undefined

The minimum number of fraction digits to display.

If neither minimumFractionDigits nor maximumFractionDigits is set, the default behavior is to round to 3 significant digits.

Must be an integer between 0 and 100. Throws a TypeError for invalid values.

Must not be greater than maximumFractionDigits, which throws a RangeError when both are set.

[!NOTE] When minimumFractionDigits or maximumFractionDigits is specified, values are truncated instead of rounded to provide more intuitive results for file sizes.

import prettyBytes from 'pretty-bytes';

// Show the number with at least 3 fractional digits
prettyBytes(1900, {minimumFractionDigits: 3});
//=> '1.900 kB'

prettyBytes(1900);
//=> '1.9 kB'

maximumFractionDigits

Type: number
Default: undefined

The maximum number of fraction digits to display.

If neither minimumFractionDigits nor maximumFractionDigits is set, the default behavior is to round to 3 significant digits.

Must be an integer between 0 and 100. Throws a TypeError for invalid values.

Must not be less than minimumFractionDigits, which throws a RangeError when both are set.

[!NOTE] When minimumFractionDigits or maximumFractionDigits is specified, values are truncated instead of rounded to provide more intuitive results for file sizes.

import prettyBytes from 'pretty-bytes';

// Show the number with at most 1 fractional digit
prettyBytes(1920, {maximumFractionDigits: 1});
//=> '1.9 kB'

prettyBytes(1920);
//=> '1.92 kB'

space

Type: boolean
Default: true

Put a space between the number and unit.

import prettyBytes from 'pretty-bytes';

prettyBytes(1920, {space: false});
//=> '1.92kB'

prettyBytes(1920);
//=> '1.92 kB'

nonBreakingSpace

Type: boolean
Default: false

Use a non-breaking space instead of a regular space to prevent the unit from wrapping to a new line.

Has no effect when space is false.

import prettyBytes from 'pretty-bytes';

prettyBytes(1337, {nonBreakingSpace: true});
//=> '1.34\u00A0kB'

prettyBytes(1337, {space: false, nonBreakingSpace: true});
//=> '1.34kB'

fixedWidth

Type: number
Default: undefined

Pad the output to a fixed width by right-aligning it.

Useful for creating aligned columns in tables or progress bars.

If the output is longer than the specified width, no padding is applied.

Must be a non-negative integer. Throws a TypeError for invalid values.

import prettyBytes from 'pretty-bytes';

prettyBytes(1337, {fixedWidth: 10});
//=> '   1.34 kB'

prettyBytes(100_000, {fixedWidth: 10});
//=> '    100 kB'

// Useful for progress bars and tables
[1000, 10_000, 100_000].map(bytes => prettyBytes(bytes, {fixedWidth: 8}));
//=> ['    1 kB', '   10 kB', '  100 kB']

FAQ

Why kB and not KB?

k is the standardized SI prefix for kilo.

Related

• pretty-bytes-cli - CLI for this module
• pretty-ms - Convert milliseconds to a human readable string