正在加载，请稍候…

Base64 Alphabet Table: The 64 Characters and Index Reference

The full Base64 character set — the 64 index-to-character mappings (A–Z, a–z, 0–9, + /), how padding with = works, and the Base64url variant for URLs.

What the Base64 Alphabet Is

Base64 encodes binary data using 64 printable characters, so every 6 bits of input map to exactly one character. The standard alphabet (RFC 4648) is, in index order:

A-Z  →  0–25
a-z  →  26–51
0-9  →  52–61
+    →  62
/    →  63

Full Index Table

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

How Padding Works

Base64 processes input in 3-byte (24-bit) groups, producing 4 characters. When the input isn't a multiple of 3 bytes, = pads the output:

Input bytes Output Padding
3 4 chars none
2 3 chars + = one =
1 2 chars + == two =

So Man → TWFu (no padding), Ma → TWE=, M → TQ==.

Base64url: The URL-Safe Variant

+ and / are unsafe in URLs and filenames, so the base64url variant swaps them:

Standard Base64url
+ (62) -
/ (63) _
= padding usually omitted

JWTs, for example, use base64url for their header and payload — which is why you see - and _ but never + or / in a token.

Frequently Asked Questions

What are the 64 characters in Base64? A–Z (0–25), a–z (26–51), 0–9 (52–61), + (62), and / (63).

What does the = sign mean in Base64? It is padding. One = means the final group had 2 input bytes; == means it had 1. It keeps the output length a multiple of 4.

What is the difference between Base64 and Base64url? Base64url replaces + with - and / with _ so the result is safe in URLs and filenames, and usually drops the = padding.

Encode or decode any text with the Base64 converter, and see how it compares to other encodings in Base64 vs Hex Encoding.