Base64 Encoder / Decoder

About the Base64 Encoder / Decoder

Base64 (defined in RFC 4648) is a binary-to-text encoding that represents arbitrary bytes using only 64 printable ASCII characters: the uppercase letters A–Z, lowercase a–z, digits 0–9, and the symbols + and /, with = used as padding. It exists because many protocols — email, HTTP headers, JSON strings — can only safely carry text, so any binary payload must first be turned into bytes that fall inside the ASCII safe range.

How it works

The encoder treats the input as a stream of bits and reads it in groups of 6. Because three bytes hold 24 bits (3 × 8), they split exactly into four 6-bit indices, and each index selects one character from the 64-symbol Base64 alphabet. Those four characters form one output unit. When the input length is not a multiple of three bytes, the final group is padded: one leftover byte produces two characters plus ==, and two leftover bytes produce three characters plus =. This is why a single = means one byte was left over and == means two. The + and / symbols (and = padding) are not URL-safe, so URL-safe Base64 variants swap them for - and _.

A side effect of carrying 6 bits per character is that the output grows to roughly 133% of the input size (4 characters per 3 bytes), so Base64 is an encoding, not compression — and definitely not encryption. It is fully reversible: anyone with the string can decode it back to the original bytes. Treat it as a transport shape, never as a way to hide a secret.

Common use cases

• Embedding small images or fonts as data: URIs directly in HTML or CSS
• Encoding credentials for HTTP Basic Auth (user:pass → Authorization header)
• Carrying the three dot-separated segments of a JWT (header.payload.signature)
• Attaching binary files inside MIME email (Content-Transfer-Encoding: base64)
• Storing binary blobs in JSON or XML, which only accept text

Worked example

Encoding the 5-character string Hello:

Input text:  Hello
UTF-8 bytes: 48 65 6C 6C 6F  (5 bytes)
Base64:      SGVsbG8=

The first three bytes (H, e, l) map to SGVs, the next two (l, o) form an incomplete triple, so they become bG8= with one = pad. Decoding SGVsbG8= reverses every step and returns Hello exactly.