Base64 decoder

What Base64 is

Base64 writes arbitrary bytes using 64 safe characters (A–Z, a–z, 0–9, + and /), so binary data can travel through systems built for text: email attachments (MIME), data URLs, JSON fields, HTTP Basic authentication headers and JWT tokens all use it. Every 3 bytes become 4 characters, so encoded data is about 33% larger than the original. The = signs at the end are padding that makes the length a multiple of four.

The URL-safe variant, Base64url, swaps + and / for - and _ and usually drops the padding, so the result can sit in a URL or filename. JWTs use it. This decoder accepts both alphabets automatically.

Worked example

SGVsbG8sIFV0aWxOZXN0IQ== decodes to Hello, UtilNest!. Group the characters in fours: SGVs → Hel, bG8s → lo, and so on. Each character carries 6 bits, so four characters carry exactly three bytes.

When decoding fails

If the result is not valid UTF-8, the input probably encodes binary data such as an image or a compressed file rather than text. A length that leaves one character over in the last group of four, or characters like $ or %, mean the text was truncated or is not Base64.

The page follows RFC 4648: it removes whitespace, maps - and _ back to + and /, restores the = padding, and converts each group of four characters (24 bits) into three bytes. The bytes are then read as UTF-8; if they are not valid UTF-8 the page says so rather than showing garbled characters. Switch to Encode to go the other way. Everything runs in this tab, and nothing you paste is sent anywhere.