Convert letters into row-and-column coordinates on a Polybius square (H = 23, E = 15) and decode number pairs back into text. Use the classic 5×5 grid, a 6×6 grid with digits, or scramble it with a keyword, and see every step on the grid.
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Your Polybius square
Click a cell to add that letter to your text. Cells used by your message are highlighted.
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How to use the Polybius Square Cipher
Choose a direction: Text → numbers to encode, or Numbers → text to decode.
Pick the square: 5×5 (I and J share a cell) is the classic choice; 6×6 also covers digits.
Add a keyword if you want a keyed square. Sender and receiver must use the same keyword.
Type or paste your text. The result, the highlighted grid and the step-by-step lookup update instantly.
Copy the result, or copy the grid to share it as the key.
The standard 5×5 Polybius square
Read the row number first, then the column number. I and J share cell 24.
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
Example: CODE → C is row 1 column 3 (13), O is 34, D is 14 and E is 15, giving 13 34 14 15.
Building a keyed square
Write the keyword in capitals and remove repeated letters: SECRET MESSAGE becomes S E C R T M A G.
Fill the grid row by row with those letters.
Continue with the rest of the alphabet in order, skipping letters already placed (and J, if I/J share a cell).
A keyword changes almost every coordinate, but the grid is still a simple substitution, so long messages can be solved by letter frequency. For puzzles, a short keyword plus a hint is usually enough challenge.
Variants you may meet in puzzles
Variant
Grid
How it differs
Classic Polybius
5×5, I = J
Row then column, digits 1–5
Tap code
5×5, C = K
Pairs are sent as two groups of taps
6×6 square
A–Z and 0–9
No shared cells; digits can be sent
Lettered coordinates
5×5 with A–E labels
Pairs look like letters, e.g. H = BC
Nihilist and Bifid ciphers
Keyed 5×5
Use the square as the first step of a stronger cipher
Frequently asked questions
What is a Polybius square?
A Polybius square is a grid of letters where each letter is identified by its row and column number. The Greek historian Polybius described a signaling system based on such a grid in the 2nd century BC. In the common 5×5 version the first row holds A–E, so A = 11, B = 12, and H (row 2, column 3) = 23.
Why do I and J share a square?
A 5×5 grid has only 25 cells, but the English alphabet has 26 letters, so two letters must share one cell. Combining I and J is the usual convention; some puzzles combine C and K or simply leave out Q instead. When decoding, read the shared cell as whichever letter makes sense in the word. A 6×6 grid avoids the problem and adds digits 0–9.
How do I decode a Polybius cipher?
Split the numbers into pairs. The first digit is the row and the second is the column (unless the puzzle says otherwise), then look up the letter in the grid. For example 32 15 is row 3 column 2 (M) and row 1 column 5 (E): ME. If the result is nonsense, try column-first order, a keyword, or a 6×6 grid.
What is a keyed Polybius square?
In a keyed square, the letters of a keyword are written into the grid first, without repeats, followed by the remaining letters of the alphabet in order. With the keyword ZEBRAS the grid starts Z E B R A S, so the same message gives completely different numbers. Both people need the keyword to read the message.
Is the Polybius square the same as tap code?
Tap code used by prisoners is a Polybius square with C and K sharing a cell: you tap the row number, pause, then tap the column number. Choose the '5×5, C and K share a cell' option here to make tap code sequences, then tap each pair.
Is the Polybius square cipher secure?
No. On its own it is a simple substitution: each letter always gives the same pair, so frequency analysis breaks it quickly, even with a keyword. It is used in puzzles, classrooms and escape rooms, and historically as a building block of stronger ciphers such as ADFGVX and the Bifid cipher.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.