What is Base64? Encoding Explained with Examples

The one-sentence explanation

Base64 converts binary data (bytes) into a string of 64 printable characters so it can be safely embedded anywhere that only accepts plain text — email, JSON, HTML, URLs, and HTTP headers.

Why does Base64 exist?

Many older protocols and systems were designed for 7-bit ASCII text. Binary data (images, files, cryptographic keys) contains byte values that these systems interpret as control characters, causing corruption or truncation. Base64 sidesteps this by mapping every 3 bytes of binary data into 4 printable ASCII characters — none of which are control characters.

The 64 characters used are: A–Z, a–z, 0–9, +, / (with = as padding). URL-safe Base64 replaces + with - and / with _.

How encoding works

Base64 takes 3 bytes (24 bits) at a time and splits them into four 6-bit groups. Each 6-bit group maps to one of the 64 characters in the alphabet.

If the input length isn't divisible by 3, = padding characters are appended to make the output length a multiple of 4.

Common real-world uses

• Email attachments (MIME) — email was designed for 7-bit text. SMTP encodes attachments as Base64 so binary files survive transit.
• Data URIs in HTML/CSS — embed images directly in HTML without a separate HTTP request: src="data:image/png;base64,iVBORw0KGgo..."
• JSON Web Tokens (JWT) — a JWT is three Base64url-encoded sections separated by dots: header.payload.signature. The payload is not encrypted — just encoded — so never put secrets in a JWT payload.
• Basic HTTP authentication — the Authorization: Basic header sends username:password as Base64.
• Cryptographic keys and certificates — PEM format (the -----BEGIN CERTIFICATE----- files) is Base64-encoded DER binary.

Base64 is not encryption

Size overhead

Base64 output is always approximately 33% larger than the input (3 bytes become 4 characters). A 1 MB image becomes roughly 1.37 MB as Base64. This is the trade-off for text-safe transport.

Base64 vs hex encoding