What is Base64 encoding?

Why this matters

Base64 shows up constantly in web development — embedded images in CSS, authentication headers, JWT tokens — understanding it helps you recognize and decode it instead of treating it as a mystery string.

Definition

Base64 encoding converts binary data into a string built from 64 characters (A–Z, a–z, 0–9, +, /), with `=` used for padding at the end. It is an encoding, not an encryption — anyone can decode Base64 back to the original data; it provides no confidentiality.

How it works

The encoder takes the binary data 3 bytes (24 bits) at a time and re-groups those bits into four 6-bit chunks, each mapped to one of the 64 allowed characters. This is why Base64 output is always about 33% larger than the original binary data — 3 bytes of input become 4 characters of output.

Step by step

1. 1Take 3 bytes of input. The encoder reads the binary data in 3-byte (24-bit) groups.
2. 2Split into 6-bit chunks. Each 24-bit group is split into four 6-bit values (0–63).
3. 3Map to characters. Each 6-bit value maps to one of 64 printable characters.
4. 4Pad if needed. If the input isn't a multiple of 3 bytes, `=` padding is added at the end.

Common mistakes

• Assuming Base64 is a form of encryption or security — it is not; it is trivially reversible by anyone.
• Forgetting that Base64-encoded data is roughly 33% larger than the original, which matters for embedding large files.

Best practices

• Use Base64 when binary data needs to travel through a text-only channel (like JSON, or an HTML attribute) — not as a security measure.
• Encrypt data separately first if confidentiality is actually required; Base64 alone provides none.

Related guides

• What is JSON?
• URL encoding explained

Sources

• RFC 4648 — The Base16, Base32, and Base64 Data Encodings