Correct way to convert size in bytes to KB, MB, GB in JavaScript

This solution includes a decimal point and a tenths-place digit if presenting less than ten of KB or greater units

const units = ['bytes', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB'];
   
function niceBytes(x){

  let l = 0, n = parseInt(x, 10) || 0;

  while(n >= 1024 && ++l){
      n = n/1024;
  }
  
  return(n.toFixed(n < 10 && l > 0 ? 1 : 0) + ' ' + units[l]);
}

Results:

niceBytes(7)                   // 7 bytes
niceBytes(435)                 // 435 bytes
niceBytes(3398)                // 3.3 KiB
niceBytes(490398)              // 479 KiB
niceBytes(6544528)             // 6.2 MiB
niceBytes(23483023)            // 22 MiB
niceBytes(3984578493)          // 3.7 GiB
niceBytes(30498505889)         // 28 GiB
niceBytes(9485039485039445)    // 8.4 PiB

Note: Use the "Kib", etc. abbreviations when using base 1024, because "KB", etc. actually refers to base 1000. If you wish to calculate the value to base 1000, then just replace 1024 with 1000 in this function, and use the appropriate units (kB, MB, etc.)

function formatBytes(bytes) {
    var marker = 1024; // Change to 1000 if required
    var decimal = 3; // Change as required
    var kiloBytes = marker; // One Kilobyte is 1024 bytes
    var megaBytes = marker * marker; // One MB is 1024 KB
    var gigaBytes = marker * marker * marker; // One GB is 1024 MB
  
    // return bytes if less than a KB
    if(bytes < kiloBytes) return bytes + " Bytes";
    // return KB if less than a MB
    else if(bytes < megaBytes) return(bytes / kiloBytes).toFixed(decimal) + " KB";
    // return MB if less than a GB
    else if(bytes < gigaBytes) return(bytes / megaBytes).toFixed(decimal) + " MB";
    // return GB if less than a TB
    else return(bytes / gigaBytes).toFixed(decimal) + " GB";
}

Here's a one liner:

val => ['Bytes','Kb','Mb','Gb','Tb'][Math.floor(Math.log2(val)/10)]

Or even:

v => 'BKMGT'[~~(Math.log2(v)/10)]

With count:

function shortenBytes(n) {
    const k = n > 0 ? Math.floor((Math.log2(n)/10)) : 0;
    const rank = (k > 0 ? 'KMGT'[k - 1] : '') + 'b';
    const count = Math.floor(n / Math.pow(1024, k));
    return count + rank;
}

There are 2 real ways to represent sizes when related to bytes, they are SI units (10^3) or IEC units (2^10). There is also JEDEC but their method is ambiguous and confusing. I noticed the other examples have errors such as using KB instead of kB to represent a kilobyte so I decided to write a function that will solve each of these cases using the range of currently accepted units of measure.

There is a formatting bit at the end that will make the number look a bit better (at least to my eye) feel free to remove that formatting if it doesn't suit your purpose.

Enjoy.

// pBytes: the size in bytes to be converted.
// pUnits: 'si'|'iec' si units means the order of magnitude is 10^3, iec uses 2^10

function prettyNumber(pBytes, pUnits) {
    // Handle some special cases
    if(pBytes == 0) return '0 Bytes';
    if(pBytes == 1) return '1 Byte';
    if(pBytes == -1) return '-1 Byte';

    var bytes = Math.abs(pBytes)
    if(pUnits && pUnits.toLowerCase() && pUnits.toLowerCase() == 'si') {
        // SI units use the Metric representation based on 10^3 as a order of magnitude
        var orderOfMagnitude = Math.pow(10, 3);
        var abbreviations = ['Bytes', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
    } else {
        // IEC units use 2^10 as an order of magnitude
        var orderOfMagnitude = Math.pow(2, 10);
        var abbreviations = ['Bytes', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB'];
    }
    var i = Math.floor(Math.log(bytes) / Math.log(orderOfMagnitude));
    var result = (bytes / Math.pow(orderOfMagnitude, i));

    // This will get the sign right
    if(pBytes < 0) {
        result *= -1;
    }

    // This bit here is purely for show. it drops the percision on numbers greater than 100 before the units.
    // it also always shows the full number of bytes if bytes is the unit.
    if(result >= 99.995 || i==0) {
        return result.toFixed(0) + ' ' + abbreviations[i];
    } else {
        return result.toFixed(2) + ' ' + abbreviations[i];
    }
}

You can use the filesizejs library.

Just slightly modified the code of @zayarTun answer to include an extra parameter representing the number of decimals in the result (also if the decimals are zero then no need to show a result like 15.00 KB, instead 15 KB is sufficient, that's why I wrapped the result value in parseFloat())

  bytesForHuman(bytes, decimals = 2) {
    let units = ['B', 'KB', 'MB', 'GB', 'TB', 'PB']

    let i = 0
    
    for (i; bytes > 1024; i++) {
        bytes /= 1024;
    }

    return parseFloat(bytes.toFixed(decimals)) + ' ' + units[i]
  }

Utility method to format bytes into the most logical magnitude (KB, MB, or GB)

Number.prototype.formatBytes = function() {
    var units = ['B', 'KB', 'MB', 'GB', 'TB'],
        bytes = this,
        i;
 
    for (i = 0; bytes >= 1024 && i < 4; i++) {
        bytes /= 1024;
    }
 
    return bytes.toFixed(2) + units[i];
}

let a = 235678; //bytes

console.log(a.formatBytes()); //result is 230.15KB

Using bitwise operation would be a better solution. Try this

function formatSizeUnits(bytes)
{
    if ( ( bytes >> 30 ) & 0x3FF )
        bytes = ( bytes >>> 30 ) + '.' + ( bytes & (3*0x3FF )) + 'GB' ;
    else if ( ( bytes >> 20 ) & 0x3FF )
        bytes = ( bytes >>> 20 ) + '.' + ( bytes & (2*0x3FF ) ) + 'MB' ;
    else if ( ( bytes >> 10 ) & 0x3FF )
        bytes = ( bytes >>> 10 ) + '.' + ( bytes & (0x3FF ) ) + 'KB' ;
    else if ( ( bytes >> 1 ) & 0x3FF )
        bytes = ( bytes >>> 1 ) + 'Bytes' ;
    else
        bytes = bytes + 'Byte' ;
    return bytes ;
}

This works for me.

bytesForHuman(bytes) {
    let units = ['B', 'KB', 'MB', 'GB', 'TB', 'PB']

    let i = 0
    
    for (i; bytes > 1024; i++) {
        bytes /= 1024;
    }

    return bytes.toFixed(1) + ' ' + units[i]
}

This is not about converting bytes to other units, but it helps to correctly display the number and the unit for the current locale:

bytes.toLocaleString(undefined, {
  style: 'unit',
  unit: 'gigabyte',
})

More options and details can be found here: https://v8.dev/features/intl-numberformat#units

According to Aliceljm's answer, I removed 0 after decimal:

function formatBytes(bytes, decimals) {
    if(bytes== 0)
    {
        return "0 Byte";
    }
    var k = 1024; //Or 1 kilo = 1000
    var sizes = ["Bytes", "KB", "MB", "GB", "TB", "PB"];
    var i = Math.floor(Math.log(bytes) / Math.log(k));
    return parseFloat((bytes / Math.pow(k, i)).toFixed(decimals)) + " " + sizes[i];
}

This solution builds upon previous solutions, but takes into account both metric and binary units:

function formatBytes(bytes, decimals, binaryUnits) {
    if(bytes == 0) {
        return '0 Bytes';
    }
    var unitMultiple = (binaryUnits) ? 1024 : 1000; 
    var unitNames = (unitMultiple === 1024) ? // 1000 bytes in 1 Kilobyte (KB) or 1024 bytes for the binary version (KiB)
        ['Bytes', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB']: 
        ['Bytes', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
    var unitChanges = Math.floor(Math.log(bytes) / Math.log(unitMultiple));
    return parseFloat((bytes / Math.pow(unitMultiple, unitChanges)).toFixed(decimals || 0)) + ' ' + unitNames[unitChanges];
}

Examples:

formatBytes(293489203947847, 1);    // 293.5 TB
formatBytes(1234, 0);   // 1 KB
formatBytes(4534634523453678343456, 2); // 4.53 ZB
formatBytes(4534634523453678343456, 2, true));  // 3.84 ZiB
formatBytes(4566744, 1);    // 4.6 MB
formatBytes(534, 0);    // 534 Bytes
formatBytes(273403407, 0);  // 273 MB

I originally used @Aliceljm's answer for a file upload project I was working on, but recently ran into an issue where a file was 0.98kb but being read as 1.02mb. Here's the updated code I'm now using.

function formatBytes(bytes){
  var kb = 1024;
  var ndx = Math.floor( Math.log(bytes) / Math.log(kb) );
  var fileSizeTypes = ["bytes", "kb", "mb", "gb", "tb", "pb", "eb", "zb", "yb"];

  return {
    size: +(bytes / kb / kb).toFixed(2),
    type: fileSizeTypes[ndx]
  };
}

The above would then be called after a file was added like so

// In this case `file.size` equals `26060275` 
formatBytes(file.size);
// returns `{ size: 24.85, type: "mb" }`

Granted, Windows reads the file as being 24.8mb but I'm fine with the extra precision.

Here is a solid Effective way to convert bytes . Only thing you need is to install mathjs library for accurate calculation . Just copy paste .

import { multiply, divide, round } from "mathjs";

class Size {
  constructor(value, unit) {
    this.value = value;
    this.unit = unit.toUpperCase();
  }
}

async function byteToSize(bytes) {
  const B = 1;
  const KB = multiply(B, 1024);
  const MB = multiply(KB, 1024);
  const GB = multiply(MB, 1024);
  const TB = multiply(GB, 1024);
  const PB = multiply(TB, 1024);

  if (bytes <= KB) {
    // @returns BYTE

    const result = round(divide(bytes, B));
    const unit = `B`;

    return new Size(result, unit);
  }

  if (bytes <= MB) {
    // @returns KILOBYTE

    const result = round(divide(bytes, KB));
    const unit = `KB`;

    return new Size(result, unit);
  }

  if (bytes <= GB) {
    // @returns MEGABYTE

    const result = round(divide(bytes, MB));
    const unit = `MB`;

    return new Size(result, unit);
  }

  if (bytes <= TB) {
    // @returns GIGABYTE

    const result = round(divide(bytes, GB));
    const unit = `GB`;

    return new Size(result, unit);
  }

  if (bytes <= PB) {
    // @returns TERABYTE

    const result = divide(bytes, TB).toFixed(2);
    const unit = `TB`;

    return new Size(result, unit);
  }

  if (bytes >= PB) {
    // @returns PETABYTE

    const result = divide(bytes, PB).toFixed(2);
    const unit = `PB`;

    return new Size(result, unit);
  }
}

const byteConversion = (bytes: number, decimals = 2) => {
if (bytes === 0) return '0 B';

const kiloByte = 1000;
const decimal = decimals < 0 ? 0 : decimals;
const sizes = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];

const i: number = Math.floor(Math.log(bytes) / Math.log(kiloByte));

return `${parseFloat((bytes / kiloByte ** i).toFixed(decimal))} ${sizes[i]}`;

};

You can use pretty-bytes library.

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'

I am updating @Aliceljm answer here. Since the decimal place matters for 1,2 digit numbers, I am round off the first decimal place and keep the first decimal place. For 3 digit number, I am round off the units place and ignoring the all decimal places.

getMultiplers : function(bytes){
    var unit = 1000 ;
    if (bytes < unit) return bytes ;
    var exp = Math.floor(Math.log(bytes) / Math.log(unit));
    var pre = "kMGTPE".charAt(exp-1);
    var result = bytes / Math.pow(unit, exp);
    if(result/100 < 1)
        return (Math.round( result * 10 ) / 10) +pre;
    else
        return Math.round(result) + pre;
}

I just wanted to share my input. I had this problem so my solution is this. This will convert lower units to higher units and vice versa just supply the argument toUnit and fromUnit

export function fileSizeConverter(size: number, fromUnit: string, toUnit: string ): number | string {
  const units: string[] = ['B', 'KB', 'MB', 'GB', 'TB'];
  const from = units.indexOf(fromUnit.toUpperCase());
  const to = units.indexOf(toUnit.toUpperCase());
  const BASE_SIZE = 1024;
  let result: number | string = 0;

  if (from < 0 || to < 0 ) { return result = 'Error: Incorrect units'; }

  result = from < to ? size / (BASE_SIZE ** to) : size * (BASE_SIZE ** from);

  return result.toFixed(2);
}

I got the idea from here

I use Recursive and assign level variable for proper Unit.

function getReadableByte(count, decimal=0, level=0) {
    let unitList = ["Bytes", "KB", "MB", "GB", "TB", "PT"];

    if (count >= 1024.0 && (level+1 < unitList.length)) {
        return getReadableByte(count/1024, decimal, ++level)
    }
    return `${decimal ? (count).toFixed(decimal) : Math.round(count)}${unitList[level]}`
}

console.log(getReadableByte(120, 2))

I made a algorithm 7 times faster (works like a ninja):

function rafaelFormatBytes(number){
  if(number == null || number === undefined || number <= 0) {
    return '0 Bytes';
  }
  var scaleCounter = 0;
  var scaleInitials = [' Bytes',' KB',' MB',' GB',' TB',' PB',' EB',' ZB',' YB'];
  while (number >= 1024 && scaleCounter < scaleInitials.length - 1){
    number /= 1024;
    scaleCounter++;
  }
  if(scaleCounter >= scaleInitials.length) scaleCounter = scaleInitials.length - 1;
  var compactNumber = number.toFixed(2)
                              .replace(/\.?0+$/,'')
                              .replace(/\B(?=(\d{3})+(?!\d))/g, ",");
  compactNumber += scaleInitials[scaleCounter];
  return compactNumber.trim();
}
var testNum = 0;

var startTime, endTime;

function start() {
  startTime = new Date();
};

function end() {
  endTime = new Date();
  var timeDiff = endTime - startTime; //in ms
  // strip the ms
  timeDiff /= 1000;

  // get seconds 
  var seconds = Math.round(timeDiff, 5);
  console.log(timeDiff + " seconds");
}

function formatBytes(a,b=2,k=1024){with(Math){let d=floor(log(a)/log(k));return 0==a?"0 Bytes":parseFloat((a/pow(k,d)).toFixed(max(0,b)))+" "+["Bytes","KB","MB","GB","TB","PB","EB","ZB","YB"][d]}}

console.log(formatBytes(1000000000000000000000000000));
console.log(rafaelFormatBytes(1000000000000000000000000000));

start();
for(i=0; i<100000; i++){
    formatBytes(1000000000000000);
}
end();
start();
for(i=0; i<100000; i++){
    rafaelFormatBytes(1000000000000000);
}
end();

... and the output:

827.18 YB
827.18 YB
0.293 seconds
0.039 seconds

Jesus Christ!!!!

more flexible and considered max pow in sizes list (upgraded l2aelba answer)

function formatBytes(bytes, decimals = 2, isBinary = false) {
      const sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB']; // or ['B', 'KB', 'MB', 'GB', 'TB']
    
      if (!+bytes) {
        return `0 ${sizes[0]}`;
      }
    
      const inByte = isBinary ? 1024 : 1000;
      const dm = decimals < 0 ? 0 : decimals;
    
      const pow = Math.floor(Math.log(bytes) / Math.log(inByte));
      const maxPow = Math.min(pow, sizes.length - 1);
    
      return `${parseFloat((bytes / Math.pow(inByte, maxPow)).toFixed(dm))} ${
        sizes[maxPow]
      }`;
    }

This worked for me with react, typescript.

export const FormatBytes = (bytes: number) => {
  const units = ['b', 'kb', 'mb', 'gb', 'tb'];

  let i = 0;

  for (i; bytes >= 1024 && i < 4; i++) {
    bytes /= 1024;
  }

  return `${bytes.toFixed(2)} ${units[i]}`;
};

This is how a byte should be shown to a human:

function bytesToHuman(bytes, decimals = 2) {
  // https://en.wikipedia.org/wiki/Orders_of_magnitude_(data)
  const units = ["bytes", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB"]; // etc

  let i = 0;
  let h = 0;

  let c = 1 / 1023; // change it to 1024 and see the diff

  for (; h < c && i < units.length; i++) {
    if ((h = Math.pow(1024, i) / bytes) >= c) {
      break;
    }
  }

  // remove toFixed and let `locale` controls formatting
  return (1 / h).toFixed(decimals).toLocaleString() + " " + units[i];
}

// test
for (let i = 0; i < 9; i++) {
  let val = i * Math.pow(10, i);
  console.log(val.toLocaleString() + " bytes is the same as " + bytesToHuman(val));

}

// let's fool around
console.log(bytesToHuman(1023));
console.log(bytesToHuman(1024));
console.log(bytesToHuman(1025));

function bytes2Size(byteVal){
    var units=["Bytes", "KB", "MB", "GB", "TB"];
    var kounter=0;
    var kb= 1024;
    var div=byteVal/1;
    while(div>=kb){
        kounter++;
        div= div/kb;
    }
    return div.toFixed(1) + " " + units[kounter];
}

ONE-LINER

const b2s=t=>{let e=Math.log2(t)/10|0;return(t/1024**(e=e<=0?0:e)).toFixed(3)+"BKMGP"[e]};

console.log(b2s(0));
console.log(b2s(123));
console.log(b2s(123123));
console.log(b2s(123123123));
console.log(b2s(123123123123));
console.log(b2s(123123123123123));

Same answer from @Aliceljm , but in a "more didactic" way. Thanks! =D

function formatBytes(numBytes, decPlaces) {
    /* Adjust the number of bytes informed for the most appropriate metric according
    to its value.

    Args:
        numBytes (number): The number of bytes (integer);
        decPlaces (Optional[number])): The number of decimal places (integer). If
            it is "undefined" the value "2" will be adopted.

    Returns:
        string: The number adjusted together with the most appropriate metric. */

    if (numBytes === 0) {
        return "0 Bytes";
    }

    // NOTE: 1 KB is equal to 1024 Bytes. By Questor
    // [Ref(s).: https://en.wikipedia.org/wiki/Kilobyte ]
    var oneKByte = 1024;

    // NOTE: Treats if the "decPlaces" is "undefined". If it is "undefined" the value
    // "2" will be adopted. By Questor
    if (decPlaces === undefined || decPlaces === "") {
        decPlaces = 2;
    }

    var byteMtrcs = ["Bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];
    // Byte Metrics

    // NOTE: Defines the factor for the number of bytes and the metric. By Questor
    var mtrcNumbFactor = Math.floor(Math.log(numBytes) / Math.log(oneKByte));
    // Metrics Number Factor

    return parseFloat((numBytes / Math.pow(oneKByte, mtrcNumbFactor)).
            toFixed(decPlaces)) + " " + byteMtrcs[mtrcNumbFactor];
}

I had a problem of meta data on files returning from the server with different size units. I used @Alicejim response and tried to do it more general. sharing the code here, maybe it will help someone.

enum SizeUnits {
   Bytes = 'Bytes',
   KB = 'KB',
   MB = 'MB',
   GB = 'GB',
   TB = 'TB',
   PB = 'PB',
   EB = 'EB',
   ZB = 'ZB',
   YB = 'YB'
}
function convertSizeUnites(size: number, sourceSizeUnits: SizeUnits, targetSizeUnits: SizeUnits) {
    const i = Object.keys(SizeUnits).indexOf(sourceSizeUnits);
    const sizeInBytes = size * Math.pow(1024, i);
    const j = Object.keys(SizeUnits).indexOf(targetSizeUnits);
    return sizeInBytes / Math.pow(1024, j);
}
function formatSize(size: number, measureUnit: SizeUnits, decimals = 2) {
    if (size === 0) return '0 Bytes';
    const sizeInBytes = convertSizeUnites(size, measureUnit, SizeUnits.Bytes);
    const dm = decimals < 0 ? 0 : decimals;
    const i = Math.floor(Math.log(sizeInBytes) / Math.log(1024));
    return parseFloat((sizeInBytes / Math.pow(1024, i)).toFixed(dm)) + ' ' + 
    Object.keys(SizeUnits)[i];
}