Index Of Coincidence Calculator


The Index of Coincidence (IC) is a statistical measure used in cryptography to determine how likely it is that two randomly selected letters from a text will be the same. This measure is particularly useful for analyzing the frequency distribution of letters in a text and can help in identifying whether the text is in a certain language or if it has been encrypted using a specific cipher.

How to Use

Using the Index of Coincidence calculator is simple. Enter your text into the input field, click the "Calculate" button, and the calculator will compute the IC value for your text. The result will help you understand the frequency characteristics of the text you are analyzing.


The formula to calculate the Index of Coincidence is as follows:


  • fi‚Äč is the frequency of the i-th letter.
  • N is the total number of letters in the text.

This formula measures the likelihood that two randomly selected letters from the text are identical.

Example Solve

Let's consider an example text: "HELLO WORLD". To compute the Index of Coincidence:

  1. Count the frequency of each letter.
  2. Use the formula to calculate the IC.


  • H: 1, E: 1, L: 3, O: 2, W: 1, R: 1, D: 1
  • Total letters (N) = 10

Plugging the values into the formula:


What is the purpose of the Index of Coincidence?

The IC helps determine the frequency characteristics of a text and can indicate whether a text is encrypted.

Can the Index of Coincidence identify the language of a text?

Yes, different languages have different characteristic IC values, which can help in language identification.

Is the Index of Coincidence used in modern cryptography?

While IC was more crucial in classical cryptography, it still finds use in cryptanalysis and frequency analysis today.

What does a high IC value indicate?

A high IC value typically indicates that the text has a non-random distribution of letters, possibly suggesting a natural language or a simple substitution cipher.

Can IC be used to crack modern encryption algorithms?

IC is less effective against modern encryption algorithms which are designed to produce a uniform letter distribution, making IC less useful for breaking such ciphers.


The Index of Coincidence calculator is a valuable tool for cryptographers and enthusiasts alike, providing insight into the letter frequency distribution of a text. By understanding and utilizing this measure, one can gain a deeper appreciation for the intricacies of text analysis and cryptography.

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