How many bytes in a JavaScript string?

I have a javascript string which is about 500K when being sent from the server in UTF-8. How can I tell its size in JavaScript?

I know that JavaScript uses UCS-2, so does that mean 2 bytes per character. However, does it depend on the JavaScript implementation? Or on the page encoding or maybe content-type?



String values are not implementation dependent, according the ECMA-262 3rd Edition Specification, each character represents a single 16-bit unit of UTF-16 text:

4.3.16 String Value

A string value is a member of the type String and is a finite ordered sequence of zero or more 16-bit unsigned integer values.

NOTE Although each value usually represents a single 16-bit unit of UTF-16 text, the language does not place any restrictions or requirements on the values except that they be 16-bit unsigned integers.


Try this combination with using unescape js function:

const byteAmount = unescape(encodeURIComponent(yourString)).length

Full encode proccess example:

const s  = "1 a ? ? @ ®"; //length is 11
const s2 = encodeURIComponent(s); //length is 41
const s3 = unescape(s2); //length is 15 [1-1,a-1,?-2,?-3,@-1,®-2]
const s4 = escape(s3); //length is 39
const s5 = decodeURIComponent(s4); //length is 11

Note that if you're targeting node.js you can use Buffer.from(string).length:

var str = "\u2620"; // => "?"
str.length; // => 1 (character)
Buffer.from(str).length // => 3 (bytes)

UTF-8 encodes characters using 1 to 4 bytes per code point. As CMS pointed out in the accepted answer, JavaScript will store each character internally using 16 bits (2 bytes).

If you parse each character in the string via a loop and count the number of bytes used per code point, and then multiply the total count by 2, you should have JavaScript's memory usage in bytes for that UTF-8 encoded string. Perhaps something like this:

      getStringMemorySize = function( _string ) {
        "use strict";

        var codePoint
            , accum = 0

        for( var stringIndex = 0, endOfString = _string.length; stringIndex < endOfString; stringIndex++ ) {
            codePoint = _string.charCodeAt( stringIndex );

            if( codePoint < 0x100 ) {
                accum += 1;

            if( codePoint < 0x10000 ) {
                accum += 2;

            if( codePoint < 0x1000000 ) {
                accum += 3;
            } else {
                accum += 4;

        return accum * 2;


getStringMemorySize( 'I'    );     //  2
getStringMemorySize( '?'    );     //  4
getStringMemorySize( '????'   );     //  8
getStringMemorySize( 'I?????' );     // 14

The answer from Lauri Oherd works well for most strings seen in the wild, but will fail if the string contains lone characters in the surrogate pair range, 0xD800 to 0xDFFF. E.g.

// URIError: URI malformed

This longer function should handle all strings:

function bytes (str) {
  var bytes=0, len=str.length, codePoint, next, i;

  for (i=0; i < len; i++) {
    codePoint = str.charCodeAt(i);

    // Lone surrogates cannot be passed to encodeURI
    if (codePoint >= 0xD800 && codePoint < 0xE000) {
      if (codePoint < 0xDC00 && i + 1 < len) {
        next = str.charCodeAt(i + 1);

        if (next >= 0xDC00 && next < 0xE000) {
          bytes += 4;

    bytes += (codePoint < 0x80 ? 1 : (codePoint < 0x800 ? 2 : 3));

  return bytes;


// 3

It will correctly calculate the size for strings containing surrogate pairs:

bytes(String.fromCharCode(55555, 57000))
// 4 (not 6)

The results can be compared with Node's built-in function Buffer.byteLength:

Buffer.byteLength(String.fromCharCode(55555), 'utf8')
// 3

Buffer.byteLength(String.fromCharCode(55555, 57000), 'utf8')
// 4 (not 6)

The size of a JavaScript string is

  • Pre-ES6: 2 bytes per character
  • ES6 and later: 2 bytes per character, or 5 or more bytes per character

Always 2 bytes per character. UTF-16 is not allowed because the spec says "values must be 16-bit unsigned integers". Since UTF-16 strings can use 3 or 4 byte characters, it would violate 2 byte requirement. Crucially, while UTF-16 cannot be fully supported, the standard does require that the two byte characters used are valid UTF-16 characters. In other words, Pre-ES6 JavaScript strings support a subset of UTF-16 characters.

ES6 and later
2 bytes per character, or 5 or more bytes per character. The additional sizes come into play because ES6 (ECMAScript 6) adds support for Unicode code point escapes. Using a unicode escape looks like this: \u{1D306}

Practical notes

  • This doesn't relate to the internal implemention of a particular engine. For example, some engines use data structures and libraries with full UTF-16 support, but what they provide externally doesn't have to be full UTF-16 support. Also an engine may provide external UTF-16 support as well but is not mandated to do so.

  • For ES6, practically speaking characters will never be more than 5 bytes long (2 bytes for the escape point + 3 bytes for the Unicode code point) because the latest version of Unicode only has 136,755 possible characters, which fits easily into 3 bytes. However this is technically not limited by the standard so in principal a single character could use say, 4 bytes for the code point and 6 bytes total.

  • Most of the code examples here for calculating byte size don't seem to take into account ES6 Unicode code point escapes, so the results could be incorrect in some cases.


These are 3 ways I use:

  1. TextEncoder()

    (new TextEncoder().encode("myString")).length)

  2. Blob

    new Blob(["myString"]).size)

  3. Buffer

    Buffer.byteLength("myString", 'utf8'))


A single element in a JavaScript String is considered to be a single UTF-16 code unit. That is to say, Strings characters are stored in 16-bit (1 code unit), and 16-bit is equal to 2 bytes (8-bit = 1 byte).

The charCodeAt() method can be used to return an integer between 0 and 65535 representing the UTF-16 code unit at the given index.

The codePointAt() can be used to return the entire code point value for Unicode characters, e.g. UTF-32.

When a UTF-16 character can't be represented in a single 16-bit code unit, it will have a surrogate pair and therefore use two code units( 2 x 16-bit = 4 bytes)

See Unicode encodings for different encodings and their code ranges.


This function will return the byte size of any UTF-8 string you pass to it.

function byteCount(s) {
    return encodeURI(s).split(/%..|./).length - 1;


JavaScript engines are free to use UCS-2 or UTF-16 internally. Most engines that I know of use UTF-16, but whatever choice they made, it’s just an implementation detail that won’t affect the language’s characteristics.

The ECMAScript/JavaScript language itself, however, exposes characters according to UCS-2, not UTF-16.



If you're using node.js, there is a simpler solution using buffers :

function getBinarySize(string) {
    return Buffer.byteLength(string, 'utf8');

There is a npm lib for that : (from yours faithfully)


You can use the Blob to get the string size in bytes.

  new Blob(['????']).size,                             // 4
  new Blob(['????']).size,                             // 4
  new Blob(['????????']).size,                           // 8
  new Blob(['????????']).size,                           // 8
  new Blob(['I\'m a string']).size,                  // 12

  // from Premasagar correction of Lauri's answer for
  // strings containing lone characters in the surrogate pair range:
  new Blob([String.fromCharCode(55555)]).size,       // 3
  new Blob([String.fromCharCode(55555, 57000)]).size // 4 (not 6)


I'm working with an embedded version of the V8 Engine. I've tested a single string. Pushing each step 1000 characters. UTF-8.

First test with single byte (8bit, ANSI) Character "A" (hex: 41). Second test with two byte character (16bit) "?" (hex: CE A9) and the third test with three byte character (24bit) "?" (hex: E2 98 BA).

In all three cases the device prints out of memory at 888 000 characters and using ca. 26 348 kb in RAM.

Result: The characters are not dynamically stored. And not with only 16bit. - Ok, perhaps only for my case (Embedded 128 MB RAM Device, V8 Engine C++/QT) - The character encoding has nothing to do with the size in ram of the javascript engine. E.g. encodingURI, etc. is only useful for highlevel data transmission and storage.

Embedded or not, fact is that the characters are not only stored in 16bit. Unfortunally I've no 100% answer, what Javascript do at low level area. Btw. I've tested the same (first test above) with an array of character "A". Pushed 1000 items every step. (Exactly the same test. Just replaced string to array) And the system bringt out of memory (wanted) after 10 416 KB using and array length of 1 337 000. So, the javascript engine is not simple restricted. It's a kind more complex.


You can try this:

  var b = str.match(/[^\x00-\xff]/g);
  return (str.length + (!b ? 0: b.length)); 

It worked for me.


Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.