What is the size of the file if it is encoded in 450 KB base64 encoding?

Question

Is it possible to calculate the size of the data if I know its size with base64 encoding?

I have a 450 KB file with base64 encoding, but what size is it unpacked?

Is there a way to find the size of the output without first unpacking the file?

Answer 1

I have a 450 KB file with base64 encoding, but what size is it unpacked?

In fact, you are not “unpacking”, you are decoding. The result will be less than encoded data.

Since Base 64 encoding requires ~ 8 bits for each 6 bits of source data (or 4 bytes to store 3), the math is simple:

Encoded Decoded 450KB / 4 * 3 = ~ 337KB 

The overhead between Base64 and the decoded string is almost constant, 33.33%. I say "almost" just because padding bytes at the end ( = ), which make the string length a multiple of 4. See a few examples:

 String Encoded Len B64 Pad Space needed A QQ== 1 2 2 400.00% AB QUI= 2 3 1 200.00% ABC QUJD 3 4 0 133.33% ABCD QUJDRA== 4 6 2 200.00% ABCDEFGHIJKLMNOPQ QUJDREVGR0hJSktMTU5PUFE= 17 23 1 140.00% ( 300 bytes ) ( 400 bytes ) 300 400 0 133.33% ( 500 bytes ) ( 668 bytes ) 500 666 2 133.60% ( 5000 bytes ) ( 6668 bytes ) 5000 6666 2 133.36% ... tends to 133.33% ... 

Calculation of space for unregistered data:

Let me get the value QUJDREVGR0hJSktMTU5PUFE= mentioned above.

• The encoded value has 24 bytes.

• Let it be calculated 24/4 * 3 => the result is 18.

• Let me count the number = at the end of the encoded value: In this case, 1 (we only need to check the last two bytes of the encoded data).

• Having received 18 (obtained in step 2) - 1 (obtained in step 3), we obtain 17

So, we need 17 bytes to store the data.

Answer 2

base64 adds about a third of its original size, so your file should be larger or smaller .75 * 450 KB.