This free readability checker analyzes your text using five established readability score formulas. All calculations run in your browser — your content stays completely private.
How the Readability Scores Work
Readability scores quantify how easy or difficult a text is to read. Every formula uses sentence length and word complexity — measured in syllables — as its core inputs, because both strongly predict the cognitive effort required to process a piece of writing.
Flesch Reading Ease scores text on a scale from 0 to 100. The formula is: 206.835 − (1.015 × words per sentence) − (84.6 × syllables per word). Higher scores mean easier reading:
- 90–100: Very easy — understandable by a 5th grader
- 70–80: Easy — plain English, accessible to most adults
- 60–70: Standard — 7th–8th grade, the target for general web content
- 50–60: Fairly difficult — 10th–12th grade
- 30–50: Difficult — college level
- 0–30: Very difficult — professional or academic audiences
Flesch-Kincaid Grade Level converts the same inputs into a US school grade. A score of 8 means an 8th grader can understand the text. Most US newspapers target grade 6–8. Legal documents often score above grade 14 — which is one reason they are notoriously hard to understand.
Gunning Fog Index estimates the years of formal education a reader needs to understand the text on the first pass. It penalizes "complex words" — those with three or more syllables. A score of 12 corresponds to a high school diploma; 17 corresponds to a college degree. Most business communication targets a Fog Index of 10–12.
Coleman-Liau Index differs from the other formulas by using character count per word instead of syllable count. This makes it useful for languages where counting syllables programmatically is less reliable, and it tends to correlate well with Flesch-Kincaid in practice.
SMOG Index (Simple Measure of Gobbledygook) counts complex words across 30 sentences. It tends to produce higher (harder) scores than Flesch and is the preferred formula for healthcare and public health literacy assessments, where accuracy of difficulty estimation is critical.
Example: Rewriting Patient Instructions to Grade 6
A healthcare organization wants their post-discharge patient instructions accessible to virtually all adult patients, targeting a Flesch-Kincaid Grade Level of 6 or below. Their first draft scores:
- Flesch Reading Ease: 34 (difficult)
- Flesch-Kincaid Grade Level: 12.3
- Gunning Fog Index: 14.8
The main culprits are long, compound sentences and medical terminology. A sentence like "Patients should be advised to contact their primary care physician or specialist in the event that any of the following symptoms present themselves within 72 hours of discharge" runs 33 words with multiple embedded clauses. Medical terms like "anticoagulation therapy" and "thromboembolism prophylaxis" carry three to six syllables each and penalize all the syllable-based formulas heavily.
After one revision pass — breaking long sentences, replacing jargon with plain equivalents ("blood thinners" instead of "anticoagulation therapy"), and switching from passive to active voice — the scores become:
- Flesch Reading Ease: 68 (standard)
- Flesch-Kincaid Grade Level: 6.4
- Gunning Fog Index: 8.1
The instructions now meet the organization's accessibility standard. Clinical accuracy was not compromised — only the phrasing changed.
Key Factors in Readability
Sentence length is the most powerful lever available. Cutting a 35-word sentence into two 17-word sentences improves every readability score simultaneously. Aim for an average sentence length of 15–20 words. Vary lengths for rhythm — a run of identically-sized sentences feels mechanical — but keep the average below 20.
Word syllable count is the second major input. Replacing long words with shorter synonyms makes a measurable difference: "use" instead of "utilize," "buy" instead of "purchase," "end" instead of "terminate." Technical and specialized domains often require longer words, but every unnecessary syllable raises the difficulty score.
Passive voice tends to produce longer, more complex sentences even when the underlying idea is simple. "The report was submitted by the team on Friday" (active: "The team submitted the report on Friday") is shorter, cleaner, and reads faster. Active constructions are almost always a readability improvement.
Readability formulas have limits. A short sentence can still be confusing if it contains domain jargon, double negatives, or abstract nouns stacked together. The scores here are a useful diagnostic first pass — not a substitute for editing with a real reader in mind, and not a guarantee that a high score means the writing is clear.