What is the Base64 character set, and how many bits of entropy does each Base64 character provide?

Base64 uses 64 characters (A-Z, a-z, 0-9, +, /) providing exactly 6 bits per character.

The Base64 alphabet:

• A-Z (26 characters)
• a-z (26 characters)
• 0-9 (10 characters)
• + and / (2 characters)
• Total: $26 + 26 + 10 + 2 = 64$

Entropy per character:

$$\log_2(64) = 6 \text{ bits/character}$$

Padding: The = character is used for padding (not part of the 64) when the input length isn't divisible by 3 bytes.

Common uses: Email attachments (MIME), data URIs in HTML/CSS, JSON Web Tokens (JWT), cryptographic key representation.

Comparison to hex: Base64 is 50% more space-efficient than hexadecimal (6 bits/char vs 4 bits/char). A 256-bit key needs 64 hex characters but only ~43 Base64 characters.

Go deeper:

• Base64 — the full alphabet and padding rules.