Readability Score Calculator — Flesch, Fog and Coleman–Liau

Background.

Readability formulas turn visible features of prose into repeatable indices. They do not read the text or test a reader; they count sentence length, word length, syllables or complex words and apply coefficients fitted to earlier samples. This calculator reports four established formulas side by side from counts you have already verified.

Flesch Reading Ease uses words per sentence and syllables per word. Higher values generally correspond to shorter sentences and shorter words. Flesch–Kincaid Grade uses the same two ratios with different coefficients and returns a grade-like index. The number is a scale position, not a promise that every reader at that school year will understand the material.

Gunning Fog combines words per sentence with the percentage of complex words, commonly words of three or more syllables under a declared convention. Coleman–Liau avoids syllables and uses letters and sentences per hundred words. Because the formulas weight different features, their results need not agree. Do not average them into a supposedly more accurate score.

The page asks for counts instead of pretending English tokenisation is trivial. Abbreviations, decimals, headings, hyphenation, proper nouns and quotations can change sentence and word counts. Syllable algorithms disagree on silent letters, names and technical terms. A transparent count from the same sample is easier to audit than an unexplained automatic parser.

The worked sample has 100 words, 5 sentences, 150 syllables, 12 complex words and 500 letters. It averages 20 words per sentence and 1.5 syllables per word. The resulting Reading Ease is 59.635, Flesch–Kincaid Grade 9.91, Fog 12.8 and Coleman–Liau 12.12.

Scores are deliberately not clamped. Very short or artificial text can produce Reading Ease above 100 or a grade below zero, and changing the value would no longer be the published formula. Small samples are unstable: one long heading or sentence can move an index sharply, so analyse a representative passage rather than a slogan.

No universal pass band appears. A legal notice, children's story, safety instruction and specialist paper serve different audiences and purposes. Required terminology can raise an index while careful definitions improve real comprehension. Tables, diagrams, layout, motivation and prior knowledge are invisible to every formula here.

Use the results diagnostically. If all formulas flag long sentences, inspect sentence structure. If only syllable-based formulas rise, check unavoidable technical terms and explain them. Then test the revised material with actual intended readers or a domain review instead of writing solely to a score. Run the same counting method before and after revision so a change reflects the prose rather than a changed tokenisation rule.

Every count must be a whole number. Words, sentences, syllables and letters are positive; complex words may be zero. Syllables and letters cannot be fewer than words, and complex words cannot exceed words. Decimal.js carries all divisions and each output rounds once to ten decimal places.

Readability is one part of clear communication. Organisation, headings, active structure, concrete examples, accessible design and accurate content remain essential. A lower grade estimate can coexist with misleading or incomplete prose, so the formula must never replace editorial judgment.

What is readability score calculator?

A readability score is a mathematical index based on surface features of a text sample. Flesch formulas use sentence length and syllable density; Fog adds complex-word share; Coleman–Liau uses letters and sentences. They estimate linguistic difficulty, not comprehension, accuracy or suitability by themselves.

How to use this calculator.

1. 01Choose one representative text sample and count every field with one consistent convention.
2. 02Enter words, sentences, syllables, complex words and letters from that same sample.
3. 03Compare the four scores and inspect which underlying ratio drives each difference.
4. 04Revise structure or explanations where the counts reveal a real obstacle.
5. 05Validate with intended readers and subject experts rather than targeting a score alone.

The formula.

Compute average words per sentence and syllables per word for the Flesch formulas. Fog adds complex words as a percentage. Coleman–Liau converts letters and sentences to rates per 100 words. None of the published formulas is clamped.

For 100 words, 5 sentences and 150 syllables, W/S=20 and Y/W=1.5. Reading Ease is 206.835−1.015×20−84.6×1.5=59.635. Grade is 0.39×20+11.8×1.5−15.59=9.91. With 12% complex words, Fog is 0.4×32=12.8; with 500 letters and 5 sentences per 100 words, Coleman–Liau is 12.12.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Kincaid et al. (1975), Navy Research Branch Report 8-75, reports the Flesch–Kincaid coefficients and related formulas.
2. [2]Flesch (1948), A New Readability Yardstick, is the original Reading Ease calibration.
3. [3]CDC Clear Communication Index evaluates communication features beyond readability formulas.
4. [4]U.S. Plain Language guidelines cover audience, organisation, wording and design.
5. [5]CMS Written Materials Toolkit warns against treating a readability formula as a complete communication assessment.