About this tool
The Polybius Square Cipher Encoder / Decoder is a free, in-browser tool for one of the oldest coordinate ciphers, described by the Greek historian Polybius over two thousand years ago. The 25 letters of the alphabet are arranged in a 5×5 grid whose rows and columns are labelled 1–5. Each letter is then replaced by the two digits giving its row and column — for a standard square A is 11, B is 12, and so on. Because a 5×5 grid holds only 25 cells, two letters must share one: by default I and J are merged, with an option to merge C and K instead.
You can seed the square with a keyword: its unique letters are placed first, in order, and the remaining letters of the alphabet fill the rest of the grid. This changes every coordinate and produces a keyed Polybius square. The row and column labels default to 12345 but can be set to any five distinct characters. All computation runs locally in your browser, and the current square is drawn out so you can see exactly how each letter maps.
Encoding walks through your text, skips non-letters, and emits a row-then-column pair for each letter. Decoding strips everything except the label characters, groups them into pairs, and looks each pair up in the square. The Polybius square is the foundation of several later systems, including the tap code used by prisoners of war (where the coordinates are tapped out) and the ADFGX/ADFGVX field ciphers, making it a cornerstone of both cryptographic history and modern puzzle culture.