Why is a base 64 encoded file 33% larger than the original?

Bharat Kalluri / 2022-05-09

Base 64 is a binary to text encoding scheme which represents binary data in ASCII string format as six, four bit strings. All base 64 characters are between A-Z, a-z & 0-9.

How does base64 work?

Let’s take an example of a simple string cat

In binary, cat is 01100011 01100001 01110100. A byte is a collection 8 bits. A bit is a 0 or 1. So, cat is essentially 3 bytes.

But for base64, the rules are different. Binary should be broken down into 4 chunks of 6 bits each.

01100011 01100001 01110100 becomes 011000 110110 000101 110100

Six bits can represent 64 states (2 ^ 6). In base64, this is A-Z , a-z, 0-9, +, / and = (for padding). That is 26 capitals + 26 smalls + 9 digits + 1 + + 1 /, that’s 63 states. Finally = completes this set to 64. The base64 RFC has a handy table for all these states

Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y

Let’s go back to our cat example, now that the basics are out of the way.

Using the table as a reference, cat would be

011000 110110 000101 110100
| | | |
Y 2 F 0

(Since 011000 in decimal is 24 and 24 maps to Y and so on)

cat being 24 bits was no coincidence ;) If the data is not 24 bits, then = is used as padding to make it into a multiple of 24 bits. So ca encoded in base64 would be

011000 110110 000100 (padding)
| | |
Y 2 E =

Reason behind increase in size

Instead of one character representing 256 (2^8) as it does in normal encoding, each character represents only 64 (2^6) states in base64.

To send three bytes of information (cat for example) we would have to actually send out four bytes in base64 (Y2F0). The base64 version is 4/3rd larger.

So, 33 % more storage is used than normal.

Why is base64 used then?

Base64 is used when there was a need to encode binary data so that it can be stored and transferred over mediums that primarily designed to deal with ASCII text. E-Mail attachments are sent out as base64 encoded strings.

Base64 is useful to send over any kind of data over the wire as a plain text string. API endpoints which have JSON payloads sometimes take the file as base64 in the JSON payload.

Data sent over the wire, between the server and the browser is usually compressed with algorithms like gzip, brotli etc. The encoding size overhead is not that significant (typically around 2-5%) between the compressed original and compressed encoded files. So, the space overhead is not a deal-breaker for sending files over web.


The base64 implementation shown above covers the core concept. To dive deeper into the details of the specification, refer RFC4648

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