Base64 Encoding and Decoding Explained

Base64 represents bytes; it does not hide them

Base64 is a reversible way to represent bytes with a restricted set of text characters. It is useful where a text channel must carry byte data, but it provides no confidentiality. Anyone with the encoded string can decode it; do not treat it as encryption or a way to protect secrets.

From text to Base64

The Base64 Converter first encodes entered text as UTF-8 bytes. Standard Base64 groups those bytes into 24-bit blocks and represents each block as four symbols, with each symbol carrying six bits. Its alphabet uses A–Z, a–z, 0–9, +, and /. An incomplete final block uses = padding: one remaining byte needs two = characters, while two remaining bytes need one.

For example, UTF-8 encodes Hello World as the ASCII-compatible bytes 48 65 6C 6C 6F 20 57 6F 72 6C 64 in hexadecimal. Standard padded Base64 gives SGVsbG8gV29ybGQ=; decoding that value gives exactly Hello World. For a shorter check, Man becomes TWFu with no padding, while M becomes TQ==.

Canonical form and decoding

RFC 4648 describes canonical Base64, including zero unused pad bits. Calzivo checks its decoded result by re-encoding the bytes and requiring the original string to match. This rejects noncanonical trailing bits as well as malformed padding. Decoded bytes must also form valid UTF-8; the tool is for text, not arbitrary binary files.

Standard Base64 versus base64url

Base64url is a distinct RFC 4648 alphabet that uses - and _ in place of + and /. Calzivo accepts only the standard + and / alphabet with required padding. It rejects URL-safe strings, omitted padding, embedded whitespace, and characters outside its exact grammar; it does not silently normalize variants. RFC 4648 defines multiple encodings—these rejections are this tool's chosen scope, not a claim that every application must reject every variant.

Calzivo limits

Raw input is limited to 100,000 UTF-16 code units. The conversion runs on entered text in the browser; there is no file upload, streaming binary conversion, encryption, or automatic line wrapping. A Unicode character can occupy multiple UTF-8 bytes and, for some characters, two UTF-16 code units, so character count is not always byte count.