Polybius Square Cipher

Encode letters as row/column coordinates in a 5×5 grid.

Plaintext

Encoded

23 15 31 31 34  /  52 34 42 31 14

Polybius Square (I and J share position 24)

	1	2	3	4	5
1	A	B	C	D	E
2	F	G	H	I	K
3	L	M	N	O	P
4	Q	R	S	T	U
5	V	W	X	Y	Z

How the Polybius Square Works

The Polybius square is a 5×5 grid containing the alphabet (I and J share one cell). Each letter is encoded as its row number followed by its column number — so A = 11, B = 12, Z = 55. To decode, split the digits into pairs and look up the row/column. It is one of the building blocks of more complex ciphers like the ADFGVX cipher used in WWI.

Built and maintained by Meet Shah · Last updated 25 July 2026

What this tool is used for

Working through a classical cipher exercise based on a coordinate grid.
Solving a puzzle whose ciphertext is pairs of digits.
Demonstrating how a substitution can double the message length.
Reproducing a historical cipher for a talk or a lesson.
Producing a numeric ciphertext for a treasure hunt.

Frequently Asked Questions

How does the Polybius square work?: Each letter is located in a 5×5 grid and replaced by its row and column numbers, so A becomes 11 and Z becomes 55. It is a substitution cipher whose output is digits, which is what made it useful for signalling by torches, knocks or taps.
Why do I and J share a cell?: Because the alphabet has 26 letters and a 5×5 grid has 25 cells. Merging I and J is the traditional resolution — they were interchangeable in Latin — so decoding always produces I and the reader restores J from context. Some variants merge C and K instead.
What is the tap code?: The same square signalled as knocks: one group of taps for the row, a pause, then the column. It was used by prisoners of war in Vietnam to communicate through cell walls, and its variant drops the K rather than the J.
Is it secure?: No. It is a simple substitution, so letter frequencies survive intact in the digit pairs and a frequency count breaks it quickly. Its historical value was as a signalling encoding — reducing an alphabet to two symbols out of five — rather than as protection.
What is it used for today?: As a building block. The Nihilist and ADFGVX ciphers both start with a Polybius square and then add transposition or a key, which is what turns a trivially breakable substitution into something that required real effort in its era.

Common errors and gotchas

Forgetting that I and J share a cell, so the plaintext needs a human to disambiguate.
Reading the coordinates in the wrong order, which produces a different letter entirely.
Expecting punctuation and digits to encode, when the grid holds twenty-five letters.
Assuming the grid is standard, when a keyword can reorder it completely.
Treating it as secure, since it is a simple substitution once the grid is known.