Readability Calculator (Flesch-Kincaid, SMOG)
Text
Readability formulas turn 'is this writing hard to read?' into a number. They count words per sentence and syllables (or characters) per word and combine them into a grade level or ease score. This tool runs your text through six well-known formulas at once so you can see how they agree (or don't), plus the underlying stats — characters, words, sentences, syllables, complex words (3+ syllables), and averages. Flesch Reading Ease is on a 0-100 scale where higher is easier (60-70 is conversational, 90+ is grade-school). The five grade-level scores (Flesch-Kincaid, Gunning Fog, SMOG, Automated Readability Index, Coleman-Liau) all output approximate years of US schooling needed to understand the passage on first reading — plain-language guidelines for healthcare, government, and consumer-facing copy typically target 6-8. The combined band averages the grade scores so you have one headline number. Computation is entirely local; the text never leaves your browser.
Text statistics
Readability scores
How the formulas work
Flesch Reading Ease = 206.835 − 1.015 × (words/sentences) − 84.6 × (syllables/words). Higher is easier. Flesch-Kincaid Grade = 0.39 × (words/sent) + 11.8 × (syl/words) − 15.59. Gunning Fog = 0.4 × (words/sent + 100 × complexWords/words). SMOG = 1.0430 × √(complexWords × 30/sentences) + 3.1291. ARI = 4.71 × (chars/words) + 0.5 × (words/sent) − 21.43. Coleman-Liau = 0.0588 × L − 0.296 × S − 15.8, where L = chars per 100 words and S = sentences per 100 words. All computation is local. The text never leaves your browser.
How to use
- Paste the text you want to evaluate, or click Load sample.
- Read the stats row — anything under 15 words per sentence and 1.5 syllables per word usually scores well.
- Check the Flesch Reading Ease card — green/lime = anyone can read it, amber = standard adult prose, red = academic/technical.
- Compare the five grade-level cards. They use different inputs (syllables vs characters, all words vs polysyllabic), so a single outlier is usually a quirk of the formula.
- Use the combined grade summary as your headline number for the audience-fit conversation.
Frequently asked questions
- Which formula should I trust?
- All six are heuristics from the 1940s–1970s, calibrated against English passages of known difficulty. Flesch Reading Ease and Flesch-Kincaid are the most cited in writing guides; Gunning Fog is preferred in business journalism; SMOG was designed for healthcare patient materials; ARI and Coleman-Liau use character counts instead of syllables, so they're robust to syllable-counting errors. If the formulas disagree by more than 2 grade levels, look at the source — a single very long sentence or one polysyllabic word can move SMOG noticeably.
- How are syllables counted?
- By a common heuristic: lowercase the word, strip non-letters, drop a trailing silent 'e' or 'ed/es', strip a leading 'y', then count contiguous vowel groups. It's English-only and gets about 95% of common words right. Names, foreign loanwords, and technical jargon ("ophthalmologist", "sphygmomanometer") may be off by one — for a paragraph or longer, those errors average out. Works that's under 4 letters always count as 1 syllable.
- What counts as a complex word?
- Three or more syllables, by the heuristic above. Gunning Fog and SMOG both use this definition, but Gunning Fog traditionally excludes proper nouns and common '-ed/-es' inflections. This tool keeps the simpler 'all 3+ syllable words' rule for consistency with most online calculators. Expect Gunning Fog to read 0.5-1 grade higher here than in a strict implementation.
- Does this work for non-English text?
- The English syllable heuristic is built around English vowel patterns, so it'll produce garbage for languages with different orthography (German, French, Spanish all read differently). The character-based scores (ARI, Coleman-Liau) are language-agnostic in spirit but still calibrated to English grade levels. For non-English, use language-specific tools — LIX for Swedish/Danish, Fernández Huerta for Spanish, Kandel-Moles for French.
- What's a good Flesch score?
- It depends on audience. Reader's Digest averages around 65, Time around 50, Harvard Law Review around 30. Plain-language guidelines (PlainLanguage.gov, NHS) recommend Flesch ≥ 60 / Flesch-Kincaid grade ≤ 8 for general-public materials. Marketing copy and consumer apps often shoot for 70+. Technical documentation, legal text, and academic papers can run 30-50 and still be appropriate for their audience.
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