Readability Calculator
Score any text for readability with Flesch Reading Ease, Flesch–Kincaid Grade Level and the Gunning Fog Index.
How to use this calculator
- 1Paste the text you want to score — at least 100 words, since these are averages and short samples are unstable.
- 2Read the Reading Ease band first for a plain-language verdict, then the grade levels if you need a specific target.
- 3To improve a score, shorten sentences before hunting for shorter words — sentence length is the lever both formulas respond to most.
How the calculation works
Reading Ease = 206.835 − 1.015·ASL − 84.6·ASW Flesch–Kincaid = 0.39·ASL + 11.8·ASW − 15.59 Gunning Fog = 0.4·(ASL + 100 × complex ÷ words)- ASL
- Average sentence length — total words ÷ total sentences
- ASW
- Average syllables per word — total syllables ÷ total words
- complex
- Words of three or more syllables, for Gunning Fog only
All three formulas use only two things: how long sentences are, and how long words are. Nothing about vocabulary difficulty, sentence structure, logical flow or subject matter enters the calculation — which is both why they are so easy to compute and why they are so easy to game.
Reading Ease runs high-is-easy on a nominal 0–100 scale, while the two grade-level scores run low-is-easy in US school years. They therefore move in opposite directions on the same text, which is a frequent source of confusion.
Reading Ease is unbounded despite the 0–100 presentation. Extremely short simple sentences can exceed 100, and very dense prose can go below 0.
Worked example
A short passage mixing simple and dense sentences
- 1.The passage has 5 sentences. Two are very short, one is long and heavily polysyllabic.
- 2.Average sentence length and average syllables per word are computed across the whole passage.
- 3.Reading Ease = 206.835 − 1.015 × ASL − 84.6 × ASW.
- 4.Flesch–Kincaid = 0.39 × ASL + 11.8 × ASW − 15.59, expressed in US school grades.
- 5.Gunning Fog = 0.4 × (ASL + percentage of words with 3+ syllables), which the long middle sentence pushes up sharply.
Result: See the result panel — it recalculates live as you edit the text
What these formulas actually measure
Every classic readability formula rests on the same two proxies: sentence length and word length. Longer sentences take more working memory to parse; longer words are, on average, rarer and more abstract. Both correlate with reading difficulty across large samples, which is why formulas built on them have survived since the 1940s despite their obvious crudeness.
What they do not measure is anything about meaning. Vocabulary familiarity, sentence structure, logical organisation, how much background knowledge a reader needs — none of it enters the arithmetic. A formula cannot tell the difference between a clear explanation and incoherent nonsense, provided both use short words in short sentences.
Three scores, two directions
The scores here point opposite ways, which trips people up constantly.
- Flesch Reading Ease — higher is easier, on a nominal 0–100 scale. Around 60–70 is the plain-English target most style guides aim for. Rudolf Flesch published it in 1948.
- Flesch–Kincaid Grade Level — lower is easier, expressed in US school grades. Developed for the US Navy in 1975 to check that technical manuals matched enlisted reading levels — which is why it is still embedded in a great deal of government and defence procurement.
- Gunning Fog Index — lower is easier, in years of formal education. Robert Gunning published it in 1952 for business writing. It differs from the Flesch pair by counting the *share* of words that are long rather than the average length, so a text with a few very long words scores differently than one where every word is moderately long.
The syllable problem
Every formula on this page takes syllable counts as an input, and English syllable counting cannot be done exactly by algorithm. There is no reliable rule: "fire" is one syllable or two depending on the speaker, "business" is two despite looking like three, and names and acronyms defeat any heuristic entirely.
Practical implementations therefore use vowel-group heuristics with a handful of corrections for silent "e", "-le" endings and similar cases, which is exactly what this calculator does. Accuracy on ordinary prose is within a few percent; accuracy on technical vocabulary, proper nouns and abbreviations is considerably worse. This is why the same text scored in Word, in an online checker and here will produce slightly different numbers — none of them is wrong, they simply guess syllables differently.
Using them without being fooled
The formulas are genuinely useful as a rough check and genuinely misleading as a target. Because only two variables feed them, both are trivially gameable: chopping every sentence at the nearest comma improves every score without improving comprehension at all, and can actively harm it by stripping out the connectives that show how ideas relate.
- 1Use them to find problems, not to prove quality — a grade level far above your audience is a real signal worth investigating. A good score is not evidence that the writing is clear.
- 2Fix sentence length first — it is the variable both formulas weight most heavily, and shortening genuinely overlong sentences almost always helps real readers too.
- 3Do not chase word length blindly — the precise technical term is often clearer than a longer plain-language circumlocution, even though the formula prefers the latter.
- 4Test with actual readers where it matters — for anything high-stakes — medical consent, legal notices, safety instructions — no formula substitutes for checking that people understood.
What this assumes, and where it stops
Assumptions
- Sentences are delimited by full stops, question marks and exclamation marks. Abbreviations ending in a full stop will be miscounted as sentence boundaries.
- Syllables are estimated by a vowel-group heuristic, which is approximate — see the limitations below.
- The text is English. These formulas were calibrated on English and do not transfer to other languages.
Limitations
- Syllable counting is heuristic and cannot be exact. Expect small differences against other tools scoring the same text, particularly on names, acronyms and technical vocabulary.
- Abbreviations, decimals and ellipses can be mistaken for sentence endings, which shortens the apparent average sentence length and flatters the score.
- Measures only sentence and word length. It says nothing about clarity, organisation, accuracy or whether the text makes sense.
- Scores are unstable on short samples. Under about 100 words a single long sentence can move the grade level by several years.
Common questions
What Flesch Reading Ease score should I aim for?
For a general audience, 60–70 is the usual plain-English target — roughly 8th to 9th grade. Many government plain-language guidelines and insurance regulations specify something in that range. Technical documentation written for specialists reasonably scores lower, and there is nothing wrong with that if the audience genuinely is specialists.
Why does this give a different score than Microsoft Word?
Almost always because of syllable counting, which no tool can do exactly. Different implementations use different heuristics and disagree on words like "fire", "business" or any proper noun. Sentence detection differs too — how a tool handles abbreviations, bullet lists and headings changes the sentence count and therefore the average. The formulas are identical; the inputs are estimated slightly differently.
Can I just shorten every sentence to improve the score?
You can, and the score will improve, which is precisely the weakness of these formulas. They measure only sentence and word length, so chopping sentences at every comma raises the score without helping the reader — and often hurts, since the connectives that show how ideas relate are the first casualties. Use the score to find genuinely overlong sentences, not as a target to optimise.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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