JavaScript To Convert Bytes To MB, KB, Etc

bytesToSize.js

// from http://scratch99.com/web-development/javascript/convert-bytes-to-mb-kb/
function bytesToSize(bytes) {
    var sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB'];
    if (bytes == 0) return 'n/a';
    var i = parseInt(Math.floor(Math.log(bytes) / Math.log(1024)));
    if (i == 0) return bytes + ' ' + sizes[i];
    return (bytes / Math.pow(1024, i)).toFixed(1) + ' ' + sizes[i];
};

A little bit of code for people who want the reverse.
Basically adapted this from https://stackoverflow.com/a/31625253/104461

function printpower (n, base, power) {
  if (base === 2) { // 1 << power is approx 10x faster than Math.pow(2, power) 
    console.log(n * (1 << power))
  } else {
    console.log(n * Math.pow(base, power))
  }
}

const humanReadable = '86 MB'

const [n, abbreviation] = humanReadable.split(/\s+/)
if (abbreviation) {
  if (/K(iB)?$/.test(abbreviation)) {
    printpower(n, 2, 10)
  } else if (/M(iB)?$/.test(abbreviation)) {
    printpower(n, 2, 20)
  } else if (/G(iB)?$/.test(abbreviation)) {
    printpower(n, 2, 30)
  } else if (/T(iB)?$/.test(abbreviation)) {
    printpower(n, 2, 40)
  } else if (/KB$/.test(abbreviation)) {
    printpower(n, 10, 3)
  } else if (/MB$/.test(abbreviation)) {
    printpower(n, 10, 6)
  } else if (/GB$/.test(abbreviation)) {
    printpower(n, 10, 9)
  } else if (/TB$/.test(abbreviation)) {
    printpower(n, 10, 12)
  }
} else {
  console.log(n)
}

Edit: the if statement is probably pretty expensive, and so are the Regular Expressions (at a minimum, should be compiled once as const variables), so there is room for incremental optimization!

Thanks all of you

TypeScript version:

export function bytesToSize(bytes: number): string {
  const sizes: string[] = ['Bytes', 'KB', 'MB', 'GB', 'TB']
  if (bytes === 0) return 'n/a'
  const i: number = parseInt(Math.floor(Math.log(bytes) / Math.log(1024)).toString())
  if (i === 0) return `${bytes} ${sizes[i]}`
  return `${(bytes / Math.pow(1024, i)).toFixed(1)} ${sizes[i]}`
}

Strength and power of open source community. I'm shocked and impressed. Thanks to all, guys.

function bytesToSize(bytes) {
  const units = ["byte", "kilobyte", "megabyte", "terabyte", "petabyte"];
  const unit = Math.floor(Math.log(bytes) / Math.log(1024));
  return new Intl.NumberFormat("en", {style: "unit", unit: units[unit]}).format(bytes / 1024 ** unit);
}

To expand on the typescript version, I don't think the parseInt(...) is required. Also, in the unlikely case that you exceed 999.9 TB, you end up in xxx.x undefined territory.

Here is a fixed Typescript version:

export function bytesToSize(bytes: number): string {
  const sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB'];
  if (bytes === 0) return 'n/a';
  const i = Math.min(Math.floor(Math.log(bytes) / Math.log(1024)), sizes.length - 1);
  if (i === 0) return `${bytes} ${sizes[i]}`;
  return `${(bytes / (1024 ** i)).toFixed(1)} ${sizes[i]}`;
}

With Jest spec:

describe('bytesToSize', () => {
  test.each([
    [0, 'n/a'],
    [1, '1 Bytes'],
    [2000, '2.0 KB'],
    [30000, '29.3 KB'],
    [400000, '390.6 KB'],
    [5000000, '4.8 MB'],
    [60000000, '57.2 MB'],
    [700000000, '667.6 MB'],
    [8000000000, '7.5 GB'],
    [90000000000, '83.8 GB'],
    [100000000000, '93.1 GB'],
    [1100000000000, '1.0 TB'],
    [12000000000000, '10.9 TB'],
    [130000000000000, '118.2 TB'],
    [1400000000000000, '1273.3 TB'],
    [15000000000000000, '13642.4 TB'],
  ])('.bytesToSize(%p)', (
    val: number,
    expected: string,
  ) => {
    expect(
      bytesToSize(val),
    ).toBe(expected);
  });
});

Using the Intl.NumberFormat for local friendly conversions

export function bytesToSize(bytes: number): string {
  const units = ['byte', 'kilobyte', 'megabyte', 'gigabyte', 'terabyte'];

  const navigatorLocal = navigator.languages && navigator.languages.length >= 0 ? navigator.languages[0] : 'en-US'
  const unitIndex      = Math.max(0, Math.min(Math.floor(Math.log(bytes) / Math.log(1024)), units.length - 1));

  return Intl.NumberFormat(navigatorLocal, {
    style: 'unit',
    unit : units[unitIndex]
  }).format(bytes / (1024 ** unitIndex))
}

The sizes should be named Mebibyte (MiB), Gibibyte (GiB) etc if you use 1024 for the calculation.

Or if you want to calculate Megabyte, Gigabyte etc. replace the '1024' in the code by '1000'.

@HaukeHa Thank you for noticing 👍 

I use the following code:

export function convertBytes(bytes: number, options: { useBinaryUnits?: boolean; decimals?: number } = {}): string {
  const { useBinaryUnits = false, decimals = 2 } = options;

  if (decimals < 0) {
    throw new Error(`Invalid decimals ${decimals}`);
  }

  const base = useBinaryUnits ? 1024 : 1000;
  const units = useBinaryUnits
    ? ["Bytes", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"]
    : ["Bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];

  const i = Math.floor(Math.log(bytes) / Math.log(base));

  return `${(bytes / Math.pow(base, i)).toFixed(decimals)} ${units[i]}`;
}

and basic tests:

import { describe, expect } from "vitest";
import { convertBytes } from "./bytes";

describe("convertBytes", () => {
  const fileSizeInBytes = 2048;

  describe("default conversion", () => {
    it("should convert to KB", () => {
      const resultKB = convertBytes(fileSizeInBytes);
      expect(resultKB).toBe("2.05 KB");
    });

    it("should convert to MB", () => {
      const resultMB = convertBytes(fileSizeInBytes * 1000);
      expect(resultMB).toBe("2.05 MB");
    });

    it("should throw an error for invalid decimals", () => {
      expect(() => convertBytes(fileSizeInBytes, { decimals: -1 })).toThrowError("Invalid decimals -1");
    });
  });

  describe("conversion to binary units", () => {
    it("should convert to KiB", () => {
      const resultKiB = convertBytes(fileSizeInBytes, { useBinaryUnits: true });
      expect(resultKiB).toBe("2.00 KiB");
    });

    it("should convert to MiB", () => {
      const resultMiB = convertBytes(fileSizeInBytes * 1024, { useBinaryUnits: true });
      expect(resultMiB).toBe("2.00 MiB");
    });
  });

  it("should handle very large bytes (Number.MAX_SAFE_INTEGER)", () => {
    const result = convertBytes(Number.MAX_SAFE_INTEGER);
    expect(result).toBe("9.01 PB");
  });
});

  const units = ["byte", "kilobyte", "megabyte", "terabyte", "petabyte"];

you lost "gigabyte"

@gynekolog I implemented a simple word declension for the word "byte".

export const convertBytes = (bytes: number, options: { useBinaryUnits?: boolean; decimals?: number } = {}): string => {
    const {useBinaryUnits = false, decimals = 2} = options;

    if (decimals < 0)
        throw new Error(`Invalid decimals ${decimals}`);

    const declension = <T extends unknown>(num: number, dictionary: T[]): T =>
        (num = Math.abs(num)) === 1
            ? dictionary[1]
            : (num > 1 && num < 5)
                ? dictionary[2]
                : dictionary[0];

    const base = useBinaryUnits ? 1024 : 1000;
    const i = Math.floor(Math.log(Math.abs(bytes)) / Math.log(base));
    const value = bytes / Math.pow(base, i);
    const units = useBinaryUnits
        ? [["Bytes", "Byte", "Bytes"], "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"]
        : [["Bytes", "Byte", "Bytes"], "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];

    let unit = units[i];

    return `${value.toFixed(decimals)} ${Array.isArray(unit) ? declension(value, unit) : unit}`;
}

Example:

convertBytes(0); // 0 Bytes
convertBytes(1); // 1 Byte
convertBytes(2); // 2 Bytes
convertBytes(3); // 3 Bytes
convertBytes(6); // 6 Bytes
convertBytes(987); // 987 Bytes
convertBytes(34523465); // 34.52 MB

I'm from Slovakia, so in my case it would be:

const units = useBinaryUnits
    ? [["Bajtov", "Bajt", "Bajty"], "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"]
    : [["Bajtov", "Bajt", "Bajty"], "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];

convertBytes(0); // 0 Bajtov
convertBytes(1); // 1 Bajt
convertBytes(2); // 2 Bajty
convertBytes(3); // 3 Bajty
convertBytes(4); // 4 Bajty
convertBytes(5); // 5 Bajtov
convertBytes(6); // 6 Bajtov
convertBytes(987); // 987 Bajtov
convertBytes(34523465); // 34.52 MB

@weroro-sk Good point, thanks for sharing! I gave it a try my way just for fun. The code uses Intl.PluralRules, but the whole function is more robust now. Feel free to check it out.

Shouldn't 999999999 return 1 GB and not 1000 MB, since 1000000000 returns 1 GB.

If you find yourself tweaking this snippet frequently, smart-unit might be worth a look — it handles this out of the box with configurable decimal places, TypeScript support, and bidirectional format/parse:

const size = new SmartUnit(['B', 'KB', 'MB', 'GB', 'TB'], { baseDigit: 1024 });
size.format(1234);    // "1.21KB"
size.parse('1.5GB'); // 1610612736

And bytes formatting is just one use case — the same API works for any unit system with variable ratios, like time, length, frequency, or anything custom:

const time = new SmartUnit(['ms', 1000, 's', 60, 'm', 60, 'h']);
time.format(5400000); // "1.5h"