How is Base91 different from Base64?

Base64 maps 3 bytes to 4 characters, a 33% size increase. basE91 squeezes about 13 bits into 2 characters, so the overhead drops to roughly 23%. It uses 91 printable ASCII characters instead of 64.

Which characters does the alphabet use?

It uses 91 printable ASCII characters and deliberately excludes the dash, backslash and apostrophe-style quoting hazards, making the output safe to embed in source code and many text formats.

Is Base91 a standard like Base64?

It is a de-facto standard defined by Joachim Henke's reference implementation rather than an IETF RFC. This tool follows that canonical bit-packing algorithm so output is interoperable with other basE91 libraries.

Can I encode binary files?

This in-browser tool operates on text input encoded as UTF-8 bytes. The underlying algorithm handles arbitrary bytes, but the input box is text, so use it for strings and UTF-8 payloads.

Why does my decoded output look garbled?

Decoding produces raw bytes. If those bytes are not valid UTF-8 text — for example, they came from a non-text binary — the tool cannot render them as readable characters and reports an error.

Base91 Encoder/Decoder

What basE91 does

basE91 (sometimes written Base91) encodes arbitrary bytes into a string of printable ASCII characters so binary data can travel safely through text-only channels. Its appeal over Base64 is efficiency: it carries more information per output character, so encoded payloads are noticeably shorter.

How it works

The encoder treats the input as a stream of bits and reads them into a bit buffer. Whenever the buffer holds enough bits it pulls a value of either 13 or 14 bits — choosing 14 bits when the low 13 bits would exceed 88, which keeps every value below 8281 (that is 91 * 91). Each value is then written as two characters: the low digit is value mod 91 and the high digit is value / 91, both indexed into the 91-character alphabet.

v = buffer & 0x1FFF        // try 13 bits
if v > 88:  shift out 13 bits
else:       v = buffer & 0x3FFF, shift out 14 bits
emit alphabet[v % 91], alphabet[v / 91]

Decoding reverses this: characters are read in pairs, recombined as low + high * 91, and the bits are streamed back out as bytes.

Tips and notes

basE91 is ideal when you need binary-safe text but want less bloat than Base64 — for instance embedding small blobs in JSON or source files. Because the alphabet avoids the backslash and dash, the output rarely needs further escaping. Remember that decoding yields raw bytes; this tool then interprets them as UTF-8, so feeding it the output of a non-text binary will produce a decode error rather than readable text.