What this calculator does
All three of the common readability formulas measure the same two things: how long your sentences are and how long your words are. Nothing else. A passage of plain narrative with twelve words a sentence and barely a word over two syllables scores in the nineties on Flesch Reading Ease and around grade three on Flesch-Kincaid. The same content written in committee prose scores below zero.
That is worth saying plainly, because the scores get treated as a measure of quality and they are not. They cannot tell whether the writing is accurate, well organised or worth reading, and a text can be gibberish and still score 90. What they are good for is catching the specific failure where sentences have quietly grown to thirty words and every noun has turned into a four-syllable abstraction.
The formula
Flesch Reading Ease is 206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words), and higher is easier. Flesch-Kincaid converts the same two ratios into a US school grade level with 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59. Gunning Fog uses words of three or more syllables instead of a syllable average: 0.4 × (words per sentence + 100 × complex words ÷ words). Syllables are counted with the usual vowel-group heuristic, since English has no exact rule, and the counts are shown so you can check them.
| Term | Meaning |
|---|---|
| Flesch Reading Ease | Nominally 0 to 100, higher is easier. The formula is not actually bounded at either end. |
| Flesch-Kincaid grade | The same inputs expressed as a US school year. |
| Gunning Fog | Years of education needed, driven by long words rather than an average. |
| Complex words | Three syllables or more. The Fog index counts these as the hard part. |
The inputs explained
| Field | What to enter |
|---|---|
| Text | Paste the text to score. A few hundred words gives a steadier result than one paragraph, and headings and bullet fragments will drag the sentence length down. |
When to use it
Checking a page before it goes live
Paste the body copy and look at words per sentence first. If it is over twenty-five, splitting sentences will move every score more than any word substitution.
Writing to a required level
Some public-sector and insurance copy has a mandated grade level. Flesch-Kincaid is usually the one specified, so work to that and treat the others as supporting evidence.
Comparing two drafts
Score both and compare the two ratios rather than the headline. The ratios tell you which lever moved, and the headline only tells you that something did.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do real passages score?
The same calculator run over five different texts.
| Sample | Flesch Reading Ease | How that reads | Flesch-Kincaid grade level | Gunning Fog index |
|---|---|---|---|---|
| The tide was out… | 96.1 | very easy | 1.8 | 4.8 |
| Put the kettle on… | 109.7 | very easy | 0.3 | 3.7 |
| The council has agreed… | 52.1 | fairly difficult | 9.3 | 11.3 |
| The committee determined that… | -28.0 | very difficult | 22.4 | 27.4 |
| Notwithstanding the aforementioned considerations,… | -55.4 | very difficult | 27.2 | 30.8 |
Which matters more, long sentences or long words?
The same statement, rewritten from plain to dense.
| The same idea | Flesch Reading Ease | Words per sentence | Syllables per word | Flesch-Kincaid grade level |
|---|---|---|---|---|
| Rates went up. Our… | 117.5 | 4.7 | 1.00 | -2.0 |
| The rate went up… | 106.0 | 16.0 | 1.00 | 2.4 |
| Because the interest rate… | 55.2 | 17.0 | 1.59 | 9.8 |
| Consequently, the escalation in… | -29.8 | 23.0 | 2.52 | 23.1 |
Questions
Why did my text score above 100 or below zero?
Because the formula is unbounded. The 0 to 100 range is a convention, not a limit: very short sentences made of one-syllable words run past 100, and dense academic prose goes negative. Both are correct outputs rather than errors.
How accurate is the syllable count?
Good but not exact, because English has no reliable rule. The heuristic counts "prices" as one syllable and "business" as three, so errors run in both directions and largely cancel over a few hundred words. The counts are shown above so you can sanity-check them.
How are sentences counted?
By full stops, question marks and exclamation marks, with a trailing fragment counting as one sentence. That means abbreviations inflate the count: "Dr. Smith went home. He was tired." reads as three sentences rather than two.
What score should I aim for?
For general web copy, 60 to 70 on Reading Ease and grade 8 to 9 is a common target. For a technical audience reading by choice, lower is fine. The number only matters against the audience you actually have.
Does a good score mean good writing?
No. These formulas count sentence and word length and nothing else, so they cannot see accuracy, structure or sense. Treat a bad score as a prompt to look at the sentences, not as a verdict on the text.
For the rest of the on-page measurements there is keyword density, text-to-HTML ratio and meta description length. Reading time turns a word count into minutes.