Character Frequency: What It Is and How to Count It

What is character frequency?

Character frequency is a count of each character in a piece of text, often shown as a percentage of the total. Letter frequency is the same idea limited to letters.

Why it matters

Frequency analysis breaks simple substitution ciphers, because each plaintext letter always maps to the same cipher letter and keeps its frequency. Compression methods such as Huffman coding give shorter codes to common characters. Linguists use frequency to compare languages, and developers use it to inspect data.

English letter order

In ordinary English text, the most common letters are roughly E, T, A, O, I, N, S, H and R. Exact figures vary with the sample, so treat any table as an average.

How to count it

By hand, write each character once and add a tally mark every time you see it. In code, a counter does the job:

from collections import Counter
text = "hello world"
print(Counter(text).most_common())

The result lists each character with its count, most common first.

Common uses in practice

• Breaking classical ciphers and solving puzzles
• Detecting the language of a text
• Checking data for unexpected characters
• Designing compression codes

Tips for accurate results

Use a large sample, decide in advance whether case and spaces matter, and clean the text if it contains markup. Remember that frequencies describe a sample, not a rule for every sentence.

Try it

Count your own text with the character frequency counter, compare against English with the letter frequency counter, or view an A to Z table with the by-letter page.