Fixed 5 by 5 coordinate cipher

Polybius Square Encoder and Decoder

Turn ASCII letters into strict row-column pairs, or decode validated pairs with the classic I/J cell shown in the square.

01 / Workspace

Encode or decode

Direction

Encode ASCII letters with the row digit first and the column digit second.

This option applies only in Decode mode because coordinate 24 cannot identify the original letter.

0 / 5,000 code points

Use ASCII A-Z in either case. Unicode whitespace separates words. Punctuation, digits, accents, non-Latin letters, and emoji are rejected.

Keyboard shortcut: Ctrl+Enter or Command+Enter runs the selected direction.

Ready to encode.

02 / Square

Reference grid

Rows and columns are numbered 1 through 5. Read the row first, then the column.

Fixed English Polybius square with I and J sharing coordinate 24
Row / col12345
1A11B12C13D14E15
2F21G22H23I/J24K25
3L31M32N33O34P35
4Q41R42S43T44U45
5V51W52X53Y54Z55

This tool has no keyword or alternate grid. Both directions always use the square shown here.

03 / Result

Result

One ASCII space separates coordinate pairs. A standalone slash separates words.

04 / Convention

How this version works

  1. Use the fixed square. The alphabet fills the grid in order, with I and J sharing the fourth cell of row 2.
  2. Read row, then column. A is 11, H is 23, I and J are 24, and Z is 55.
  3. Mark words explicitly. Encoding places one space between pairs and a standalone / between words. Runs of Unicode whitespace become one word boundary.
  4. Validate every symbol. Encoding rejects anything except ASCII letters and whitespace. Decoding accepts only whitespace-separated pairs from 11 through 55 and correctly placed slash tokens.

Known-answer example

HELLO WORLD encodes to 23 15 31 31 34 / 52 34 42 31 14.

JIG encodes to 24 24 22. Standard decoding returns IIG; choosing J returns JJG.

Limits and security

  • Decoded output is uppercase. Original case, whitespace runs, punctuation, and the distinction between I and J cannot be recovered.
  • Input is limited to 5,000 Unicode code points and 10,000 UTF-16 code units. Browser text fields normalize CR and CRLF line endings to LF.
  • Polybius Square is a classical substitution cipher for learning and puzzles. It provides no modern security and must not protect sensitive information.