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Flesch Reading Ease and Other Readability Scores Explained
Six readability formulas side by side, the same three texts scored by each, what a Flesch Reading Ease score means, and why syllable counting makes tools disagree.
By TooleemPublished Facts checked
Short answer
Readability formulas estimate how hard a text is from two things only: how long its sentences are and how long its words are (in syllables or letters). Flesch Reading Ease gives a score where higher is easier (60 to 70 is plain English); Flesch-Kincaid, Gunning Fog, SMOG, Coleman-Liau and ARI give a US school grade instead. On the same news-style paragraph they ranged from grade 7.6 to 10.3. None of them reads meaning: a short sentence full of jargon scores as easy, and syllable counting alone can move a score by several points.
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English Readability Score Checker
Paste your English text and get its Flesch Reading Ease score and Flesch-Kincaid grade, with the word, sentence and syllable counts behind them.
On this page
The six formulas side by side
| Name | Uses | Returns | Formula |
|---|---|---|---|
| Flesch Reading Ease (1948) | words per sentence, syllables per word | score, higher = easier | 206.835 − 1.015 × W/S − 84.6 × Sy/W |
| Flesch-Kincaid Grade (1975) | words per sentence, syllables per word | US grade | 0.39 × W/S + 11.8 × Sy/W − 15.59 |
| Gunning Fog (1952) | words per sentence, share of 3+ syllable words | US grade | 0.4 × (W/S + 100 × complex/W) |
| SMOG (1969) | 3+ syllable words per 30 sentences | US grade | 1.043 × √(complex × 30/S) + 3.1291 |
| Coleman-Liau (1975) | letters and sentences per 100 words | US grade | 0.0588 × L − 0.296 × S100 − 15.8 |
| Automated Readability Index (1967) | letters per word, words per sentence | US grade | 4.71 × letters/W + 0.5 × W/S − 21.43 |
Coleman-Liau and ARI count letters instead of syllables, so they are easier for software to compute consistently. Flesch-Kincaid is the one most word processors show, and it was originally fitted on US Navy training material.
Measured: three texts, six scores
| Measure | A: a child’s diary | B: local news | C: contract clause |
|---|---|---|---|
| Words / sentences | 69 / 10 | 80 / 5 | 71 / 3 |
| Words per sentence | 6.9 | 16.0 | 23.7 |
| Syllables per word | 1.06 | 1.44 | 2.20 |
| Flesch Reading Ease | 110.3 | 69.0 | −3.1 |
| Flesch-Kincaid Grade | −0.4 | 7.6 | 19.6 |
| Gunning Fog | 2.8 | 8.4 | 25.2 |
| SMOG | 3.1 | 8.2 | 20.6 |
| Coleman-Liau | −0.4 | 10.3 | 20.3 |
| ARI | −2.2 | 8.9 | 20.3 |
- Scores leave their nominal range at both ends: the diary scores 110 on a 0 to 100 scale and a negative grade; the contract scores below zero.
- On ordinary prose the grade formulas disagree by two to three grades (7.6 to 10.3 on text B). Pick one formula and track it over time instead of comparing numbers from different formulas.
- Gunning Fog reacts most to long words: text C has 28 words of three or more syllables in 71, and Fog puts it at grade 25.
What a Flesch Reading Ease score means
| Score | Description | US school level in Flesch’s table |
|---|---|---|
| 90 to 100 | very easy | 5th grade |
| 80 to 89 | easy | 6th grade |
| 70 to 79 | fairly easy | 7th grade |
| 60 to 69 | standard | 8th and 9th grade |
| 50 to 59 | fairly difficult | 10th to 12th grade |
| 30 to 49 | difficult | college |
| below 30 | very difficult | college graduate |
Why two tools give different scores for the same text
Most tools count syllables with a rule (groups of vowels, minus a silent final e) rather than a dictionary. The rule is right for most words and wrong for some, and each tool uses slightly different rules. The heuristic used for the table above gives:
| Word | Rule says | Dictionary |
|---|---|---|
| created | 1 | 3 |
| area | 2 | 3 |
| business | 3 | 2 |
| queue | 2 | 1 |
| readability | 5 | 5 |
Each wrong syllable in a 100-word text moves Flesch Reading Ease by 0.85 points (84.6 ÷ 100). Errors partly cancel, but a text full of words like "created" and "area" can score several points easier than it is. Sentence splitting differs too: some tools treat a colon, a semicolon or a line break as a sentence end.
Common mistake
What the formulas cannot see
"The yacht’s jib luffed." is four short words in one sentence and scores as very easy. The formulas know nothing about unfamiliar words, logic, layout or whether the reader knows the subject. Use them to catch long sentences and heavy words, then read the text aloud. They were built for English; other languages need their own formulas.
Do the calculation
English Readability Score Checker
Paste your English text and get its Flesch Reading Ease score and Flesch-Kincaid grade, with the word, sentence and syllable counts behind them.