Index of Coincidence Calculator
Measure how uneven a text's letter distribution is — tell a monoalphabetic cipher from a polyalphabetic one and estimate a Vigenère key length. Runs entirely in your browser, no server, no sign-up.
What this tool does
The Index of Coincidence (IC) is the probability that two letters picked at random from a text are the same. It is one of the oldest tools in cryptanalysis, introduced by William F. Friedman in 1922. Because every language has a characteristic letter distribution, the IC barely changes when you scramble a text with a monoalphabetic cipher (a simple substitution or a transposition), but it collapses toward the random value when you use a polyalphabetic cipher like Vigenère. That single number tells you a lot about how a piece of text was produced — all without knowing the key.
This calculator runs entirely in your browser. Nothing you paste is sent to a server, it works offline once loaded, and there is no sign-up.
How to read the result
The tool reports the IC two ways:
| Form | English plaintext | Uniform / random text |
|---|---|---|
| Normalized (×26) | ≈ 1.73 | ≈ 1.00 |
| Raw (probability) | ≈ 0.0667 | ≈ 0.0385 |
A high IC (near 1.73 normalized) means the letter distribution is uneven — the text is plaintext, or enciphered with a method that keeps the distribution, such as a Caesar shift, a simple substitution, or a columnar transposition. A low IC (near 1.0) means the distribution is flat, which points to a long-key polyalphabetic cipher, a one-time pad, or already-random data.
Estimating a Vigenère key length
Set Estimate key length up to to a number greater than 0 and the tool also
runs a period analysis. For each candidate key length p, it splits the text
into p columns (taking every p-th letter) and averages the IC of those
columns. When p equals the true key length, each column was enciphered with a
single Caesar shift, so its IC jumps back up toward the plaintext value. The
period with the highest average column IC is the likely key length — the same
idea Friedman used, and a companion to the Kasiski examination.
Examples
| Input | Normalized IC | Reading |
|---|---|---|
| A paragraph of English prose | ≈ 1.7–1.9 | Monoalphabetic / plaintext |
| The same text with a Caesar shift | ≈ 1.7–1.9 | Unchanged — substitution preserves IC |
| Vigenère with a 5-letter key | ≈ 1.0–1.2 | Polyalphabetic |
| Output of a one-time pad | ≈ 1.0 | Random |
What gets counted
Only the 26 Latin letters A–Z are counted, and case is ignored — A and a
are the same letter. Digits, spaces, punctuation, and non-Latin characters are
skipped, following the classical cryptanalytic convention. Turn on Show
per-letter frequency table to see the count and percentage of each letter.
FAQ
Is it free and private?
Yes. Your text never leaves your device, and the page keeps working offline once it has loaded.
Why two numbers?
The raw IC is the literal probability (≈ 0.0667 for English). Many references quote the normalized form (raw × 26 ≈ 1.73) because it is easy to compare against 1.0 for random text. Both describe the same thing.
How much text do I need?
A few hundred letters give a reliable IC; key-length estimation wants more, ideally several times the key length per column.
Does it break the cipher?
No — the IC and the period estimate tell you the kind of cipher and the likely key length. You still recover the key and plaintext separately (e.g. with frequency analysis on each column).
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool index-of-coincidence "Paste ciphertext or plaintext here…"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/index-of-coincidence/?text=Paste%20ciphertext%20or%20plaintext%20here%E2%80%A6&max_period=0&show_counts=trueMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.