What is Base64 Encoding and How Does It Work?

Table of Contents

• Why Base64 Exists
• The Base64 Alphabet
• How Encoding Works
• Padding
• Base64 vs Base64URL
• When to Use It

Why Base64 Exists

Binary data (images, files, keys) can contain any byte value (0–255). Many protocols — email (SMTP), HTTP headers, JSON — are designed for text and can't safely carry arbitrary binary.

Base64 solves this by encoding binary into a subset of ASCII characters that are safe to transmit anywhere.

The Base64 Alphabet

Base64 uses 64 characters: A–Z (26), a–z (26), 0–9 (10), + and / (2). Each character represents 6 bits of data. Three bytes (24 bits) → four Base64 characters.

How Encoding Works

Take the string Man:

1. ASCII bytes: M=77, a=97, n=110
2. Binary: 01001101 01100001 01101110
3. Split into 6-bit groups: 010011 010110 000101 101110
4. Map to alphabet: indices 19, 22, 5, 46 → T, W, F, u → TWFu

btoa("Man")   // "TWFu"
atob("TWFu")  // "Man"

Padding

If the input isn't divisible by 3 bytes, = padding is added:

• 1 leftover byte → 2 Base64 chars + ==
• 2 leftover bytes → 3 Base64 chars + =

Base64 vs Base64URL

Standard Base64 uses + and / which are special in URLs. Base64URL replaces them:

JWTs use Base64URL encoding.

When to Use It

• Embedding images in HTML/CSS (data URIs)
• Encoding binary in JSON APIs
• Email attachments (MIME)

Note: Base64 is encoding, not encryption. Anyone can decode it.

Try the Base64 Encoder/Decoder to convert data instantly.