B64
Encoding

Base64 Encoding Reference

Base64 alphabet, padding rules, URL-safe variant, encoding/decoding examples in multiple languages.

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What Is Base64?

  • › Encodes binary data as ASCII text using 64 printable characters
  • › 3 bytes of binary → 4 Base64 characters (33% size increase)
  • › Used in: email attachments (MIME), data URLs, JWT tokens, HTTP Basic Auth
  • › Not encryption — easily decoded, no security value
  • › Standard alphabet: A-Z, a-z, 0-9, +, / with = padding

Alphabet (64 characters + padding)

Value Char Value Char Value Char Value Char
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
10 K 26 a 42 q 58 6
63 / pad =

Padding Rules

Input bytes Output chars Padding
1 byte 2 chars + == e.g. "A" → "QQ=="
2 bytes 3 chars + = e.g. "AB" → "QUI="
3 bytes 4 chars e.g. "ABC" → "QUJD"

Standard vs URL-Safe

Variant Extra chars Padding Use case
Standard (RFC 4648) + / = required MIME, general
URL-Safe (RFC 4648 §5) - _ = omitted URLs, JWT, filenames

Code Examples 

# Python
import base64
encoded = base64.b64encode(b'hello').decode()
decoded = base64.b64decode(encoded)

# URL-safe
base64.urlsafe_b64encode(b'hello')

# JavaScript
btoa('hello')          // encode
atob('aGVsbG8=')       // decode

// URL-safe (manual)
btoa(str).replace(/\+/g,'-').replace(/\//g,'_')

Free Online Tools for Encoding

B64 Base64 Encode/Decode → 🖼 Image to Base64 Converter →

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