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The Polybius square is one of the oldest known ciphers, dating back to ancient Greece. It converts each letter into a pair of numbers based on its position in a 5x5 grid.
This tool encodes text into coordinate pairs, or decodes coordinate pairs back into text.
How it works
The 26 letters of the alphabet are arranged in a 5x5 grid (I and J share one cell, since 26 letters can't fit evenly into 25 cells), giving each letter a row and column number from 1 to 5. To encode, each letter is replaced with its row and column digits (e.g. A becomes 11). To decode, each pair of digits is looked up to find the matching letter.
- Enter mode.
- Enter text or Coordinates.
- Click Calculate to see your results.
Examples
Encoding "HELLO"
"HELLO" becomes "23 15 31 31 34" using the standard 5x5 grid (I/J combined).
Who should use it
- Puzzle and escape-room style word games.
- Teaching how classical grid-based ciphers work, as a stepping stone to Bifid or Trifid.
Industry applications
- Cryptography education
- Games and puzzles
Advantages
- Simple, visual, and easy to do by hand with a drawn grid.
- The foundation for more advanced classical ciphers like Bifid and ADFGVX.
Limitations
- Offers no real security — a direct, fixed substitution with no key.
Common mistakes to avoid
- Forgetting to separate coordinate pairs with spaces when decoding, which makes them impossible to split correctly.
- Expecting J to decode separately from I — in the standard grid, they share one cell.
Best practices
- Use the Polybius square for puzzles, teaching, or as a building block for more advanced ciphers like Bifid and Trifid, which use it internally.
Tips
- This tool's Bifid and Trifid ciphers build directly on the Polybius square idea — try them next if you want to see how it can be combined with transposition for extra security.