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Base91

Base91 is a scheme that encodes binary data into 91 printable ASCII characters, with an expansion of about 23% — more compact than Base64.

Purpose

It maximizes alphabet utilization while staying entirely within printable ASCII, reducing encoding overhead.

Mechanism

Every 13 bits of input map to two base-91 characters (91^2 = 8281 > 2^13 = 8192). Bits accumulate from the input, and once at least 13 bits are available a character pair is emitted; a final short group yields a single character.

Use cases

  • Compact binary-to-text representation
  • Binary embedding in data URIs
  • High-density data encoding in QR codes
  • Log and debug output

Examples

  • Input 
    Hello World
    Output 
    >OwJh>Io0Tv!lE

    About 10% shorter than Base64

Common errors

Assuming Base91 is a standardized encoding (it is a non-RFC proposal)
Make sure both sides use the same implementation and alphabet

Related terms

Base64
Base64 maps arbitrary binary data onto 64 printable ASCII characters (A-Z, a-z, 0-9, +, /) so that binary data can travel over text-only channels.
GLOSSARY
Ascii85
Ascii85 (Base85) maps every 4 bytes (32 bits) to 5 ASCII characters, an expansion of about 25% — lower than Base64's 33% — and is used in PDF and PostScript.
GLOSSARY
Base58
Base58 is a human-oriented binary-to-text encoding using 58 characters (dropping the easily confused 0, O, I, l, +, and / from Base64), best known for cryptocurrency addresses.
GLOSSARY

Related tools

Base91 Encode
Encode text to Base91. Free, private, 100% client-side.
TOOL
Base91 Decode
Decode Base91 encoded text. Free, private, 100% client-side.
TOOL

References