Readability formulas are not style critics. They count sentences, words and syllables, then feed those counts into an equation calibrated against reader comprehension tests. That makes them blunt but consistent, which is what you want when the question is whether a page has drifted into something dense enough to lose people.
Nothing you paste leaves the tab. There is no upload and no account, so an unpublished article or a patient information leaflet can be scored without it ever touching a server.
When a readability score checker earns its keep
Scores matter most where comprehension is a requirement rather than a preference. Public sector writing, health information, terms of service and support documentation all have a duty to be understood on the first pass, and a number gives an editorial argument something to point at.
They are also a drift alarm. A blog that once scored around 60 and now scores 35 has usually accumulated subordinate clauses one revision at a time, and nobody noticed because each change looked small.
- Check government, health and legal copy against a mandated reading grade level.
- Compare two drafts of the same page to see whether an edit genuinely simplified it.
- Sanity check help centre articles that support agents send to frustrated users.
- Skip the scores entirely for poetry, fiction and anything written for specialists.
How the Flesch Reading Ease formula works
Flesch Reading Ease is the oldest of the four and still the most quoted. The equation is 206.835 minus 1.015 times the average number of words per sentence, minus 84.6 times the average number of syllables per word. The result is normally between 0 and 100, although unusual text can fall outside that range in both directions.
Higher is easier. A score of 90 to 100 reads like text for a nine year old, 60 to 70 is standard plain English, 30 to 50 is the register of academic and business writing, and anything below 30 is genuinely difficult prose that most readers will have to reread. Only two things move the number: sentence length and syllable count per word. That is why splitting a long sentence in half raises a score immediately.
Flesch Kincaid, Gunning Fog and SMOG compared
The other three formulas report a school grade instead of a 0 to 100 scale, which makes them easier to explain to a stakeholder. Flesch Kincaid uses the same two inputs as Reading Ease, rearranged to output a grade. Gunning Fog weights complex words, meaning words of three or more syllables. SMOG uses only polysyllabic word counts and was designed for health materials.
Expect the three grades to disagree by a level or two on the same passage, because each was calibrated against a different reading test in a different decade. Treat the cluster as a range rather than trusting a decimal place.
What these scores cannot tell you
Every formula here is blind to meaning. Reorder a sentence into nonsense and the score barely moves, because the syllables and the sentence boundaries are unchanged. Short sentences and short words can still be vague or simply wrong.
Formulas also mistreat bullets, code samples, tables and headings, which produce sentence lengths the equations were never designed to see. Long proper nouns inflate the syllable average without making anything harder to read.
Use the number to find candidates for editing, then read the passage yourself. When you check readability of text without also reading it, you end up optimising for the equation rather than for the reader.
How this readability score checker counts syllables
Syllable counting is the only genuinely hard part of the calculation, because English spelling gives no reliable signal. This readability score checker uses the standard heuristic: group adjacent vowels into a single nucleus, drop a silent trailing e, guarantee a minimum of one syllable per word, and add a syllable back for endings such as le after a consonant.
That lands within one syllable of a dictionary for most English words, and errors in both directions largely cancel across a paragraph. It is weaker on names, loanwords and technical vocabulary, which is another reason to read the result as a band rather than a precise value.
Sentence detection uses the browser Intl.Segmenter API where available, falling back to a punctuation split. Segmenter handles abbreviations and decimals sensibly, so the words per sentence figure stays honest.
Writing to hit a target reading grade level
If a brief specifies grade eight, aim for an average of 15 to 18 words per sentence and keep three syllable words to a modest share of the total. This readability score checker reports both averages beside the scores, so you can see which of the two inputs is holding the number back.
Work in passes. Split every sentence over about 25 words, rescore, then replace the ten longest words and rescore again. Two passes usually move Flesch Reading Ease by 10 to 15 points.
| Score | Output | Easy end | Hard end |
|---|---|---|---|
| Flesch Reading Ease | 0 to 100, higher is easier | 70 and above | Below 30 |
| Flesch Kincaid | US school grade | Grade 6 | Grade 16 and above |
| Gunning Fog | US school grade | Grade 8 | Grade 17 and above |
| SMOG | US school grade | Grade 7 | Grade 15 and above |
How to check the readability of your writing
- 1
Paste or drop in your text
Paste the passage into the box, or drop a .txt or .md file onto the drop area. Everything stays on your device.
- 2
Choose whether to ignore short lines
Turn on the short line filter when the document contains bullets or headings, so fragments do not distort the sentence average.
- 3
Run the scores
Press check readability. Flesch Reading Ease, Flesch Kincaid, Gunning Fog and SMOG appear together with the counts behind them.
- 4
Edit and rescore
Shorten the longest sentences, simplify the longest words, then run the scores again to confirm the change moved the number.
Related tools worth bookmarking
Sources and further reading
- W3C: Understanding reading levelThe accessibility guideline that ties reading level to a measurable success criterion for web content.w3.org
- MDN: Intl.SegmenterThe browser API this tool uses to find sentence and word boundaries correctly across languages.developer.mozilla.org
- PlainLanguage.gov: federal plain language guidelinesOfficial United States guidance on sentence length, word choice and structure behind most readability targets.plainlanguage.gov
Frequently asked questions
Common questions about the readability score checker.
For general audience writing on the web, aim for 60 to 70, which is standard plain English understood by most adults without effort. Health and government material often targets 70 or above, while technical documentation for specialists commonly sits in the 30 to 50 band and that is acceptable.
Flesch Kincaid, Gunning Fog and SMOG were each calibrated against different reading comprehension tests using different assumptions about what makes a word hard. A spread of one to three grades on the same passage is completely normal, so read them as a range rather than picking whichever number you prefer.
At least 100 words gives a stable result, and 200 or more is better. SMOG in particular was designed around samples of 30 sentences, so on very short passages it becomes unreliable. Scoring a single sentence produces a number, but it is not meaningful.
No. All counting and arithmetic happens in JavaScript inside your browser tab. There is no upload request, no server side processing and no storage, which you can verify by opening the network panel of your developer tools while you run a check.
Use it as a diagnostic, not a target. The formulas cannot see meaning, structure or accuracy, so text can be chopped into short choppy sentences that score brilliantly and still fail the reader. Fix the sentences the score flags, then judge the result by reading it.