Character frequency analysis is one of the oldest techniques in cryptanalysis. When a substitution cipher replaces each letter with a fixed substitute, the frequency distribution of the ciphertext mirrors the distribution of the original language. In English, E is the most common letter (~12.7%), followed by T, A, O, I, and N. Identifying the highest-frequency ciphertext character and guessing it represents E is often the first step in breaking a simple cipher.

Beyond cryptanalysis, frequency analysis appears in data compression — Huffman coding assigns shorter binary codes to more frequent characters, shrinking file size — and in game design. Scrabble tile counts and point values are derived directly from English letter frequency: rare letters like Q and Z score 10 points each because they appear so infrequently, while E scores only 1 point because it can be used in almost any word.

This tool counts every character in your input, including spaces, newlines, and punctuation, sorted by frequency with the most common first. The "Case sensitive" option controls whether uppercase and lowercase are counted separately — useful when analysing source code where capitalisation carries meaning, or when studying title-case patterns in a document.