How It Works

Base64 is an encoding that represents binary data with 64 printable ASCII characters (A-Z, a-z, 0-9, +, /). It expresses 3 bytes as 4 characters, which means it causes roughly a 33% size increase.

What is it used for?

Carrying binary data in HTTP headers (e.g. Basic Auth).
Attachment encoding in email (MIME).
Data URLs (data:image/png;base64,...).
The header and payload parts of a JWT (with the URL-safe variant).

The URL-safe variant

Standard base64 uses the + and / characters, but these have special meaning in URLs. The URL-safe variant (RFC 4648 §5):

+ → -
/ → _
= padding is usually dropped

Protocols like JWT and OAuth use the URL-safe variant.

A note on Unicode

Base64 works on bytes, not on characters. JavaScript’s btoa/atob functions, on the other hand, misleadingly assume Latin-1: the threshold is U+00FF. Characters above it — ğ, ı, İ, ş and every emoji — throw an InvalidCharacterError when handed straight to btoa. The more dangerous case is the one below it: ç (U+00E7), ö (U+00F6) and ü (U+00FC) sit in the Latin-1 Supplement block, so btoa does not throw — it silently produces the wrong bytes. btoa("çöp") returns 5/Zw, which is Latin-1, not UTF-8. Silent corruption is more dangerous than an exception.

This tool first converts the text to UTF-8 bytes and then base64-encodes it — so Turkish characters and emoji work safely.

Privacy

Encoding and decoding run entirely in your browser. The text you paste never leaves your browser.