Readability Checker
Score text with Flesch Reading Ease, Flesch-Kincaid grade, Gunning Fog, SMOG, Coleman-Liau and ARI, and see hard sentences, passive voice and wordy phrases.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the Readability Checker?
Paste a draft and the Readability Checker scores it with six classic formulas: Flesch Reading Ease, Flesch-Kincaid Grade Level, Gunning Fog, SMOG, Coleman-Liau and the Automated Readability Index. Pick a target reading level and it tells you whether the text is on target and by how many grades it misses.
Scores alone do not tell you what to fix, so the text is highlighted. Long, dense sentences are shaded amber or red, passive constructions are underlined, -ly adverbs and wordy phrases such as "in order to" or "utilize" are marked, and hovering a phrase suggests a plainer word from the US Federal Plain Language Guidelines.
Everything runs in your browser. The same scoring is available to AI assistants through the RedBison MCP server as the readability tool.
- Flesch Reading Ease and Flesch-Kincaid Grade Level
- Gunning Fog, SMOG, Coleman-Liau and Automated Readability Index
- Hard and very hard sentence highlighting
- Passive voice, -ly adverb and wordy phrase detection with plainer alternatives
- Target reading level check and copyable score summary
- Private: scoring runs in your browser
How to use the Readability Checker
- Paste at least a paragraph of English text. A hundred words or more gives stable scores; SMOG expects 30 sentences.
- Read the six scores. Reading Ease runs from about 0 to 100 (higher is easier); the other five are US school grades.
- Choose a target level. Grade 8 suits most web pages; grade 6 is the usual advice for health and public information.
- Work through the highlighted text. Split amber and red sentences, rewrite underlined passive verbs in the active voice, and swap green phrases for the suggestion shown on hover.
- Watch the scores update as you edit, then copy the summary for your brief or style review.
Worked examples
A short, plain passage
Input
We fixed the bug. The app is faster now. Tell us if you see a problem.
Output
Reading Ease about 106, Flesch-Kincaid grade about 0
Five words per sentence and almost all one-syllable words. Reading Ease can go above 100 and grades below zero for very simple text; that is fine for instructions and alerts.
One bureaucratic sentence
Input
Notwithstanding the aforementioned considerations, organizational stakeholders should systematically evaluate the comprehensive implications of the proposed administrative restructuring initiative.
Output
Reading Ease about -103, Flesch-Kincaid grade about 32, Fog about 34, marked very hard
Eighteen words averaging almost three and a half syllables each. Splitting it and using everyday words ("Even so, staff should check what the reorganisation means for them") brings it under grade 8.
Hand check of the formulas
Input
100 words, 5 sentences, 130 syllables, 10 complex words, 470 letters
Output
FRE 76.6, FK grade 7.6, Fog 12.0, Coleman-Liau 10.4, ARI 10.7
FRE = 206.835 − 1.015 × 20 − 84.6 × 1.3 = 76.555. FK = 0.39 × 20 + 11.8 × 1.3 − 15.59 = 7.55. These are the numbers the tests for this tool check.
Background and insight
Readability formulas were built in the mid-twentieth century by regressing reading test results against simple text features. Rudolf Flesch published Reading Ease in 1948: 206.835 − 1.015 × (words per sentence) − 84.6 × (syllables per word), on a scale where 60 to 70 is plain English and below 30 is very difficult. In 1975 J. Peter Kincaid and colleagues re-fitted the same two inputs for US Navy enlisted personnel to give a school grade: 0.39 × (words per sentence) + 11.8 × (syllables per word) − 15.59. That is the grade Microsoft Word reports.
The others use different proxies for hard words. Robert Gunning's Fog index (1952) counts words of three or more syllables, excluding proper nouns, hyphenated compounds and words that only reach three syllables through -es, -ed or -ing. G. Harry McLaughlin's SMOG (1969) counts polysyllables per 30 sentences and is widely used for health materials because it was validated against 100% comprehension. Coleman-Liau (1975) and the Automated Readability Index (Smith and Senter, 1967) use letters per word instead of syllables, which made them easy for early computers and typewriter counters.
Every formula measures length, not meaning. Short words in short sentences score well even when the ideas are muddled, and a necessary technical term can push the grade up. The scores are best used to compare drafts of the same text and to find the sentences that need work. This tool estimates syllables with English spelling rules plus an exception list, which agrees with dictionary counts for about 95% of common words, so scores can differ by a point or so from other checkers.
Common use cases
Web and product copy
Keep help pages, onboarding and error messages around grade 6 to 8 so most visitors read them in one pass.
Health and public information
Check patient leaflets and consent forms against the grade 6 to 8 level that health literacy guidance recommends.
Plain language compliance
Find wordy phrases and passive voice when writing to the US Plain Writing Act or a house style guide.
Teaching and grading
Match passages to a class reading level, or show students how sentence length changes a score.
Policies, contracts and insurance
Measure Reading Ease where a regulator or client sets a minimum score for consumer documents.
Tips and limitations
- Sentence length moves the scores most. Splitting one 40-word sentence often lowers the grade more than replacing a dozen long words.
- Headings, bullet points and captions without full stops count as sentences when "Line breaks end sentences" is on. Turn it off for hard-wrapped plain text.
- Proper nouns and necessary technical terms raise syllable-based scores. Do not swap the correct term for a vaguer one just to hit a number.
- SMOG needs 30 sentences. With fewer it extrapolates and the tool flags the result.
- The passive and adverb highlights are pattern matches. "Was tired" or "only" are not flagged, but check each suggestion in context.
- The formulas were built for English. Scores for other languages are not meaningful.
Frequently asked questions
What is a good Flesch Reading Ease score?
For general audiences aim for 60 or higher, which Flesch called standard English, readable by 13 to 15 year olds. 70 to 80 is fairly easy. Academic and legal writing often scores 30 to 50, and below 30 is very difficult.
What grade level should a website be written at?
Grade 8 or lower suits most public websites, and grade 6 is common advice for health and government information. Specialist audiences can handle more, but plain text still gets read faster.
Why do the six formulas give different grades?
They measure difficulty differently. Flesch-Kincaid and SMOG count syllables, Fog counts three-syllable words with exceptions, and Coleman-Liau and ARI count letters. Expect a spread of two or three grades; the average is shown as well.
How accurate is the syllable count?
It uses spelling rules and an exception list, which matches dictionary syllable counts for about 95% of common English words. Odd names and rare words can be off by one, which moves a long text's score very little.
Does this tool find passive voice?
Yes. It underlines a form of "be" or "get" followed by a past participle, such as "was written" or "is being reviewed", allowing one adverb in between. Adjectives like "tired" or "interested" are skipped, but always read the sentence before rewriting.
Which sentences are marked hard?
A sentence with 14 or more words is hard when its own Flesch-Kincaid grade is 10 to 13.9 and very hard at 14 or above. Long sentences of short words can still be marked, because length alone raises the grade.
Is the Flesch-Kincaid score here the same as in Microsoft Word?
It uses the same Kincaid formula, but Word counts syllables and sentences with its own rules, so the two can differ slightly. For comparing drafts, use one tool consistently.
Is there an in-depth guide to the Readability Checker?
Yes. “Readability Formulas Explained: Flesch, Fog, SMOG and More” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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