Index of Coincidence Calculator

You have a block of ciphertext and no idea where to start. Caesar? Simple substitution? Vigenere with a five-letter key? Brute-forcing every option wastes hours, and the index of coincidence exists for exactly that first triage: a single number derived from letter counts that separates single-alphabet ciphers from ones that rotate through several alphabets.

IC is the probability that two letters drawn from the text without replacement are identical: the sum of nᵢ(nᵢ−1) divided by N(N−1). Monoalphabetic substitution only relabels letters and preserves the distribution, so it keeps the language IC, near 0.072 for Portuguese and 0.067 for English. A polyalphabetic key flattens frequencies and drags the value toward 1/26, that is 0.0385. The sample text loaded on the page scores 0.0675 across 79 letters.

IC needs volume: below about a hundred letters it swings too much to support any conclusion, and the ranges the page prints are guidance rather than a verdict. The tool computes one IC for the whole text; it does not test periods or estimate key length. To do that, slice the ciphertext into columns for candidate periods 2, 3, 4 and run each column through here: when the columns start reading near 0.07, you have found the period. Accented letters and the cedilla are discarded before counting, not folded.