About Polybius Square
Polybius Square is a classical cipher that replaces each letter with a pair of digits. The alphabet is written into a five-by-five grid, and every letter is identified by its row number followed by its column number — A becomes 11, B becomes 12, and so on down to Z at 55. The tool performs that substitution in either direction, so you can turn a word into a string of number pairs or turn the number pairs back into letters.
A five-by-five grid has twenty-five cells, but the alphabet has twenty-six letters, so one letter has to share a cell. The convention, which this tool follows, is to merge I and J — a J in the input is treated as an I. That is the one detail that trips people up, because a decrypted word containing the shared cell can never tell you whether the original letter was an I or a J.
This cipher is a piece of history, not a security tool. It was devised in the ancient Mediterranean and survives today as a teaching example and a puzzle element, not as protection for anything that matters. Treat the output as a curiosity or a code for games; anyone who recognises the pattern can reverse it in seconds, so never use it for anything you actually need to keep secret.
How to use Polybius Square
- 1 Type the text you want to encrypt, or the number pairs you want to decrypt.
- 2 Choose whether to encrypt or decrypt.
- 3 Read the result — letters become space-separated number pairs, and number pairs become letters.
Frequently asked questions
Why does my J come back as an I?
Because the five-by-five grid has only twenty-five cells for twenty-six letters, so I and J share one cell. A J is encoded as I, and decryption can never distinguish the two, so the shared letter always reads back as I.
How do I read the number pairs?
Each pair is a row number followed by a column number in the grid. A is 11, B is 12, and the row and column each run from 1 to 5, so every pair is a two-digit number made only of the digits 1 through 5.
Is the Polybius square secure?
No. It is a classical substitution cipher from antiquity and is trivially broken once the pattern is recognised. It is useful for puzzles, teaching and games, but not for protecting anything that actually needs secrecy.
What happens to punctuation and spaces when I encrypt?
Non-alphabetic characters are dropped, because the grid only holds letters. The encryption keeps only A through Z (with J folded into I), so any spaces, digits or punctuation are simply not represented in the output.
Free, online, and private
Free to use
Polybius Square is completely free. There is no account to create, no login, and no hidden charge or usage limit.
Runs online in your browser
It is a free online tool that runs entirely in your browser. Nothing to download or install — open the page and start using it right away.
Your data stays private
All processing happens locally on your device. Your input is never uploaded, stored, or tracked, so it never leaves your browser.
How Polybius Square behaves in detail
These are the specifics of this implementation — the exact ranges, defaults and fixed settings you are working with.
| Parameter | Value |
|---|---|
| Alphabet | the 25-letter key `ABCDEFGHIKLMNOPQRSTUVWXYZ` — J is merged into I, giving a 5 by 5 grid |
| Encoding | letters only; each is mapped to a two-digit coordinate, row first then column, values 1 to 5 |
| Row / column derivation | row is floor(index / 5) + 1, column is (index mod 5) + 1 |
| Separator | coordinates are joined by a single space, so HELLO becomes `23 15 31 31 34` |
| Decoding | non-overlapping pairs are matched with /[1-5]{2}/g and mapped back through the same 25-letter square |
| Case | input is upper-cased before encoding; decoding always returns upper case |
| Default | HELLO in encrypt mode |
Compiled from the implementation of this tool and checked against it by itoolnet.
What Polybius Square does not do
Knowing the edges of a tool saves more time than another list of its features. These are the cases where it will not do what you might expect.
- Every character outside the 25-letter square is dropped. Digits, punctuation, spaces and accents are filtered out by the encoder rather than encoded or flagged, so a message loses anything that is not a plain A to Z letter and no warning appears to say how much was discarded.
- Letters are lossy at the source because J and I share a cell. The encoder rewrites every J to I before looking it up, and the decoder has no way to tell the two apart, so a round trip turns JASON into IASON permanently.
- Decoding returns upper case only and cannot restore the original letter case, spacing or word boundaries. The square stores no case information, so `hello world` encodes to coordinates that decode back as HELLOWORLD with the space gone.
- The decoder is lenient about malformed input. It scans for any two consecutive digits in the range 1 to 5 and silently ignores everything else, so text with stray characters decodes out of order and a single missing digit can shift every pair after it without producing an error.
Tested and documented by itoolnet, the team that builds and maintains this site.