Which readability formula is most accurate?

No single formula is universally most accurate — it depends on the text type. SMOG is most validated for healthcare material. Gunning Fog is standard in journalism. Coleman-Liau is preferred in computational linguistics. Use all three and take the average as your working grade level.

Can I use this for non-English text?

These formulas were designed for English. They will produce scores for other languages but the grade level mapping will not be meaningful. Syllable counting is approximated by vowel groups, which works reasonably for English but poorly for languages like Finnish or Japanese.

Why is SMOG marked as approximate for short texts?

McLaughlin's original research validated SMOG on texts with at least 30 sentences. The formula involves a square root of polysyllabic word counts, which is very sensitive to small sample sizes. Under 30 sentences, SMOG is indicative, not precise.

What readability score do top blogs target?

Analysis of top-ranking content marketing blogs (HubSpot, Buffer, Moz) shows an average Gunning Fog of 9-11 and Coleman-Liau of 8-10. News sites target Fog 8-10. Wikipedia averages Fog 12-14. Academic papers average Fog 16-20.

Readability Tool

Reading Level Checker

Calculate three professional readability formulas — Gunning Fog, SMOG, and Coleman-Liau — and see the US grade level for each. Color-coded for web suitability.

InputYour content· Paste 100+ words for reliable scores

OutputReadability scores· Awaiting input

Enter at least 10 words and 1 sentence to calculate scores.

Readability Formulas Explained

Three readability formulas every writer should understand

Different readability formulas measure different things. No single formula is definitive — each was developed for a specific context and audience. Using three together gives you a fuller picture of your text's complexity. For web content, aim for grade 6–8 across all three.

Gunning Fog Index

Developed by Robert Gunning in 1952. Counts syllables and complex words (3+ syllables). Formula: 0.4 × ((words/sentences) + 100 × (complex words/words)). Widely used in business writing. A Fog score of 12 means your reader needs a high school diploma. Score of 8 is ideal for web.

SMOG Index

Simple Measure of Gobbledygook, developed by G. Harry McLaughlin in 1969. Highly accurate for healthcare and medical writing. Counts polysyllabic words (3+ syllables) across 30 sentences. Requires at least 30 sentences for accuracy — results for shorter texts are approximate.

Coleman-Liau Index

Created by Meri Coleman and T.L. Liau in 1975. Unlike other formulas, it uses characters per word rather than syllables — making it more programmable and consistent. Formula: 0.0588 × (avg chars per 100 words) − 0.296 × (avg sentences per 100 words) − 15.8.

Grade 6–8: the web sweet spot

Grade 6 = 11-12 year olds. Grade 8 = 13-14 year olds. Major US newspapers (USA Today, NY Times) target grade 7–9. Most successful content marketing targets grade 6–8. Technical documentation for professionals can go to grade 12. Academic papers often exceed grade 16.

Why three formulas diverge

Gunning Fog and SMOG both use syllable counts and tend to agree closely. Coleman-Liau uses character counts and often diverges — especially for texts with many short words that have many letters (e.g., "through" is 7 chars but only 1 syllable). Both views are valid.

What grade level means in practice

Grade 6 (age 11): Simple sentences, common words. Grade 10 (age 15): More complex sentences, technical vocabulary OK. Grade 14 (college sophomore): Dense prose, specialist terms. Grade 17+ (postgraduate): Academic, assumes expert reader. Most web traffic lands at grade 7–8 reading ability regardless of education level.

Pro Tips

Use 300+ words for accuracy

SMOG requires 30 sentences minimum; Gunning Fog and Coleman-Liau work on shorter text but produce noisy results under 100 words. Paste at least 300 words for reliable grade level estimates across all three formulas.

Lower the grade: shorter sentences

The single fastest way to lower your readability grade level is to cut average sentence length. Long sentences inflate every formula. Split compound sentences at conjunctions. Target 15-20 words per sentence for grade 8.

Replace polysyllabic words

Gunning Fog and SMOG penalise words with 3+ syllables. Replace "utilise" with "use", "demonstrate" with "show", "approximately" with "about". Strunk & White rule: prefer the simple word. Your grade level will drop immediately.

Frequently Asked Questions

Which readability formula is most accurate?: No single formula is universally most accurate — it depends on the text type. SMOG is most validated for healthcare material. Gunning Fog is standard in journalism. Coleman-Liau is preferred in computational linguistics. Use all three and take the average as your working grade level.
Can I use this for non-English text?: These formulas were designed for English. They will produce scores for other languages but the grade level mapping will not be meaningful. Syllable counting is approximated by vowel groups, which works reasonably for English but poorly for languages like Finnish or Japanese.
Why is SMOG marked as approximate for short texts?: McLaughlin's original research validated SMOG on texts with at least 30 sentences. The formula involves a square root of polysyllabic word counts, which is very sensitive to small sample sizes. Under 30 sentences, SMOG is indicative, not precise.
What readability score do top blogs target?: Analysis of top-ranking content marketing blogs (HubSpot, Buffer, Moz) shows an average Gunning Fog of 9-11 and Coleman-Liau of 8-10. News sites target Fog 8-10. Wikipedia averages Fog 12-14. Academic papers average Fog 16-20.