What is a base64 length in bytes?

Question

I have a file encoded in Base64 with a certain file length. How do I get the size in bytes?

Example:

var size= canvas.toDataURL();   

console.log(resizeCanvasURL.length);


// -> 132787  base64 length

You mean the length of the file after you decode it from base64? You have to decode it first. — Robert Harvey, Jul 01 '13 at 16:41
Take a look at: http://stackoverflow.com/questions/1533113/calculate-the-size-to-a-base-64-encoded-message and (http://stackoverflow.com/questions/4715415/base64-what-is-the-worst-possible-increase-in-space-usage) (you will need to reverse those formulas) — Anthony Accioly, Jul 01 '13 at 16:48

score 10 · Accepted Answer · answered Jul 01 '13 at 16:47

10

Each symbol in Base64 encoding holds 6 bits of information. Each symbol in normal file hold 8 bits. Since after decoding you have the same amount of information you need:

normal file size - 1000 bytes
base64 file size - 1333 bytes

answered Jul 01 '13 at 16:47

mishik

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Yep, for some short period I thought that 64 is 5th power of 2 :) – mishik Jul 01 '13 at 16:51
and how do I calculate: 132787 //base64 length – daisy Jul 01 '13 at 16:54
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If you have a file of 132787 size in base64, then normal size will be 132787 * 0.75 – mishik Jul 01 '13 at 16:55
your are totally right, sorry I didn't sleep for 2 days due to my stupid exams – daisy Jul 01 '13 at 17:18

score 8 · Answer 2 · answered Sep 22 '16 at 13:12

8

The answer from mishik is wrong.

Base64 is filled up with =-chars (to have the right amount of bits). So it should be

    var byteLength = parseInt((str).replace(/=/g,"").length * 0.75));

answered Sep 22 '16 at 13:12

Vincent Hoch-Drei

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I've found a better solution: Convert the Base64 string into a "normal" String and then read the length: var str = atob(base64); var byteLength = str.length; – Vincent Hoch-Drei Sep 26 '16 at 11:29
I'm not sure if this solution is accurate. I'm getting back bad results. – jdmdevdotnet Jan 23 '18 at 22:04
2

correct code example, should be: `var byteLength = parseInt((str).replace(/=/g,"").length * 0.75);` – GomatoX Mar 02 '18 at 16:01
You're right. There's one closing bracket too much. – Vincent Hoch-Drei Mar 05 '18 at 08:46
isn't it Math.floor(parseInt((str).replace(/=/g,"").length * 0.75))) ? – Antoan Elenkov Feb 05 '21 at 11:07

score 0 · Answer 3 · answered Oct 21 '20 at 06:58

const stringLength = resizeCanvasURL.length 
const sizeInBytes = 4 * Math.ceil(stringLength / 3) * 0.5624896334383812;
const sizeInKb = sizeInBytes / 1000;

OR

 const stringLength = resizeCanvasURL.length 
 const sizeInBytes = stringLength * (3 / 4) - 2;
 const sizeInKb = sizeInBytesNew / 1000;

What is a base64 length in bytes?

3 Answers3

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