Readability Index Calculator
Analyze text readability with Flesch-Kincaid Grade Level, Flesch Reading Ease, Gunning Fog Index, and SMOG Index. Get grade level recommendations.
Readability Index Calculator
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Formula: Flesch Ease = 206.835 − 1.015 × (words/sentences) − 84.6 × (syllables/words)
Worked example — Flesch Ease: 100 (Very Easy) | FK Grade: ~1 | Elementary level
Formula
Flesch Ease = 206.835 − 1.015 × (words/sentences) − 84.6 × (syllables/words)
The Flesch Reading Ease score is computed from average sentence length and average syllables per word. The Flesch-Kincaid Grade Level, Gunning Fog, and SMOG use similar inputs but different coefficients to estimate the school grade level required for comprehension.
Worked Examples
Example 1: Simple Web Content
Problem:Analyze: 'The cat sat on the mat. It was a warm day. The sun was bright and the sky was blue. The cat liked to nap in the sun.'
Solution:Words: 30 | Sentences: 4 | Avg words/sentence: 7.5 Avg syllables/word: 1.07 Flesch Ease: 206.835 - 1.015(7.5) - 84.6(1.07) = 108.8 → capped at 100 FK Grade: 0.39(7.5) + 11.8(1.07) - 15.59 = 0.0
Result:Flesch Ease: 100 (Very Easy) | FK Grade: ~1 | Elementary level
Example 2: Academic Text Analysis
Problem:Analyze a passage with 200 words, 8 sentences, avg 25 words per sentence, avg 1.8 syllables per word, and 40 polysyllabic words.
Solution:Flesch Ease: 206.835 - 1.015(25) - 84.6(1.8) = 28.9 FK Grade: 0.39(25) + 11.8(1.8) - 15.59 = 15.6 Gunning Fog: 0.4 × (25 + 20) = 18.0 SMOG: 1.043 × √(40 × 30/8) + 3.13 = 15.9
Result:Flesch Ease: 28.9 (Difficult) | FK Grade: 15.6 | Graduate level
Frequently Asked Questions
What is the Flesch Reading Ease score?
The Flesch Reading Ease score rates text on a 0-100 scale, where higher scores mean easier to read. Scores of 60-70 are considered standard (understandable by 13-15 year olds). Scores above 80 are easy for most readers. Scores below 30 are best understood by university graduates. The formula considers average sentence length and average syllables per word.
What is the Flesch-Kincaid Grade Level?
The Flesch-Kincaid Grade Level translates text difficulty into a U.S. school grade level. A score of 8.0 means an eighth grader can understand the text. Most popular novels score between 7-9, newspapers aim for 6-8, and academic papers often score 12+. The U.S. military requires documents to score at grade level 8 or below.
What is the Gunning Fog Index?
The Gunning Fog Index estimates years of formal education needed to understand text on first reading. It considers sentence length and percentage of complex words (3+ syllables). A Fog Index of 12 requires college-level reading ability. Ideal business writing scores 8-10. Texts scoring above 17 are extremely difficult for general audiences.
What is the SMOG Index?
SMOG (Simple Measure of Gobbledygook) predicts the grade level needed to comprehend a text. It focuses specifically on polysyllabic words (3+ syllables) and is considered one of the most accurate readability formulas for health-related materials. SMOG typically gives scores 1-2 grades higher than other formulas for the same text.
What readability score should I target?
For general audiences, aim for a Flesch Reading Ease of 60-70 (Flesch-Kincaid grade 7-8). For web content, grade 6-8 works best. For academic papers, grade 12+ is acceptable. For health communications, the CDC recommends grade 6-8. For legal or technical documents, grade 10-12 is typical. Always consider your specific audience's reading level.
Why do the four indices disagree on the same passage?
They use different inputs and were calibrated against different criteria. Flesch Reading Ease and Flesch-Kincaid weigh syllables per word, Gunning Fog counts the percentage of words with three or more syllables, and SMOG counts polysyllabic words within a fixed number of sentences. SMOG was calibrated against complete comprehension of a passage rather than partial comprehension, which tends to push its grade up relative to the others, though not on every passage: the second worked example on this page returns a higher Gunning Fog than SMOG. A spread of one or two grades is normal; read it as a range rather than picking the flattering number.
How does a sentence detection error affect all four scores at once?
Every one of these formulas divides words by sentences, so the sentence count is a shared input and any error propagates into all of them together. Headings and bullet points without terminal punctuation get absorbed into the following sentence and inflate the average length, while abbreviations such as Fig. and et al. can be read as sentence ends and deflate it. Because the indices then move in the same direction, agreement between them is not evidence that the text was parsed correctly.
How should I sample a long document rather than scoring it whole?
Score representative extracts of continuous prose instead of the entire file, which usually contains headings, tables, captions and references the formulas were never designed for. The SMOG procedure is a useful template: take ten sentences from near the beginning, ten from the middle and ten from the end, and analyse those thirty. Sampling across the document also exposes variation, such as a plain introduction followed by a dense technical section, which a single whole-document average conceals.
Which index suits instructions, forms and other non-narrative content?
None of them fit well, because all four assume flowing prose with ordinary sentence punctuation. Step-by-step instructions, form labels and tables are read by scanning and locating rather than straight through, so an average sentence length describes them poorly. For that kind of content, score any explanatory prose separately and judge the rest on structural criteria instead: one action per step, consistent labelling, plain verbs in the imperative, and information placed where the reader needs it.
What do readability formulas fail to measure?
They measure surface features, so everything above the level of the sentence is invisible to them. Coherence, logical ordering, signposting, worked examples, whether the reader already knows the subject, and layout choices such as headings, white space and type size all shape comprehension, and none of them appear in the arithmetic. Two passages with the same score can differ enormously in how easy they are to follow, so use the number as a screening signal and rely on a real reader for the verdict.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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