Polybius Square Cipher
A Polybius square cipher replaces each letter with its row and column in a grid of the alphabet, so BAT becomes 12 11 44. This tool encodes and decodes with the standard I/J square, the C/K square, a 6×6 square with digits, or a square reordered by a key, and decodes unkeyed messages by trying every square and order, entirely in your browser.
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I/J | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
- 5×5, I and J share a cell · row, then columnBest match
ATTACKATDAWN
- 5×5, I and J share a cell · column, then row
ATTALWATQAKN
- 5×5, C and K share a cell · row, then column
ATTACJATDAWN
- 6×6, letters and digits · row, then column
AVVACKAVDAZO
- 5×5, C and K share a cell · column, then row
ATTALWATQAJN
- 6×6, letters and digits · column, then row
AVVAMZAVSAKO
The square isn't written in the cipher, so every square and both coordinate orders are tried, using your key if you give one. Spaces aren't encoded, so the text comes back run together.
About Polybius Square Cipher
The Greek historian Polybius described the device in the 2nd century BC, crediting it to Cleoxenus and Democleitus: the alphabet laid out in a grid, so that any letter could be signalled with two numbers — in their case, by raising torches. With the Latin alphabet the grid is 5×5, so two letters have to share a cell: usually I and J, sometimes C and K, as in the tap code. A 6×6 grid fits all 26 letters and the ten digits with nothing merged. On its own the square is a fixed substitution, and anyone who knows the convention can read it. What makes it useful is what it feeds: the Bifid and Nihilist ciphers build on it, the German army's ADFGX and ADFGVX ciphers of 1918 used it with letters for coordinates, and the tap code is a Polybius square sent by knocking. A key makes it a little stronger by reordering the grid: the key's letters go first, without repeats, then the rest of the alphabet. The hard part of decoding is that the ciphertext doesn't say which convention it used. This tool decodes every square — I/J, C/K and 6×6 — with coordinates read row-first and column-first, and ranks the six readings by how English their letter pairs look; give it the key and it uses that too. Digits can be spaced, run together or comma-separated. Encoding shows the square you're using, skips spaces, and reports any characters the square has no cell for rather than dropping them silently.
What Polybius Square Cipher does
- Encodes and decodes with I/J, C/K and 6×6 squares
- Reorders the square with a key, using the standard keyed-square convention
- Decodes without knowing the square: all squares and both orders, best match first
- Reads digits spaced, run together or comma-separated
- Shows the square in use and reports characters it can't encode
When to reach for Polybius Square Cipher
- Solving a CTF or puzzle-hunt clue made of two-digit numbers
- Checking a Polybius step inside a Bifid, Nihilist or ADFGVX puzzle
- Teaching how fractionating ciphers turn letters into coordinates
How to use Polybius Square Cipher
- 01
Paste the numbers
Two digits per letter, spaced, run together or comma-separated.
- 02
Add the key if there is one
A keyed square puts the key's letters first. Without a key the standard square is used.
- 03
Read the best match
Every square and both coordinate orders are tried and ranked by how English they look. Spaces aren't encoded, so words run together.
- 04
Encode your own
Switch to Encode, pick the square and order, and copy the numbers.
When to use Polybius Square Cipher vs alternatives
| Alternative | Use Polybius Square Cipher when… | Use the alternative when… |
|---|---|---|
| A single-square online decoder | you don't know whether the puzzle merged I/J or C/K, used a 6×6 square, or wrote column before row. | you already know the exact square and just need the lookup. |
| Decoding by hand from a printed grid | the message is more than a few letters, or you need to try several squares. | you want to understand the cipher by working it yourself. |