How does the Polybius square work?

Letters are placed in a 5×5 grid filled A to Z with I and J sharing one cell. Each letter is replaced by its row and column number, so A=11 and Z=55.

Why do I and J share a cell?

A 5×5 grid has 25 cells but the alphabet has 26 letters, so I and J are combined into a single cell. When decoding, a result of I may stand for J depending on context.

How do I format text to decode?

Enter two-digit pairs using digits 1 to 5, such as 22 15 42 11, with spaces between pairs. The tool also accepts runs of digit pairs joined together within a token.

What happens to spaces, digits and punctuation?

When encoding, characters that are not letters are passed through unchanged. When decoding, tokens that are not valid coordinate pairs are left as they are.

Is the Polybius square secure?

No. On its own it is a simple substitution that preserves letter frequencies and is trivial to break. Its real value is as a building block inside stronger ciphers like Bifid and ADFGVX.

Polybius Square Cipher Encoder

The Polybius square is an ancient device that turns each letter into a pair of numbers based on its position in a 5×5 grid. Described by the Greek historian Polybius over two thousand years ago, it later became the foundation for more advanced fractionating ciphers. This tool encodes text into coordinate pairs and decodes them back, entirely in your browser.

How it works

Fill a 5×5 grid with the alphabet A to Z, merging I and J into one cell so that 25 letters fit exactly. Each letter is then represented by two digits: the first is its row number (1–5) and the second is its column number (1–5). So A sits at row 1, column 1 and becomes 11, while Z sits at row 5, column 5 and becomes 55.

Decoding reverses the process: every pair of digits is read as a row and column and mapped back to the letter at that cell. Because I and J share a cell, a decoded I may need to be interpreted as a J from context.

Example

Encode the word GERA:

G is at row 2, column 2 → 22
E is at row 1, column 5 → 15
R is at row 4, column 2 → 42
A is at row 1, column 1 → 11

So GERA becomes 22 15 42 11, and decoding those pairs returns the original letters.

Notes

The square is most useful as a component of stronger systems. Bifid and ADFGVX ciphers both start by converting letters to Polybius coordinates and then fractionate or transpose those numbers, which spreads each letter’s influence and defeats simple frequency analysis.