Readability Checker
Measure Flesch Reading Ease, Flesch-Kincaid Grade, and Gunning Fog index.
About the Readability Checker
The Readability Checker scores your text using three of the most widely cited readability formulas: Flesch Reading Ease, Flesch-Kincaid Grade Level, and Gunning Fog Index. These scores estimate how difficult your writing is to understand and what grade level a reader needs to comprehend it. The results help you calibrate your writing for your target audience — whether that is a general audience, technical specialists, or young students.
Readability matters because it directly affects comprehension, engagement, and conversion. Research from the Nielsen Norman Group shows that web users scan rather than read, and text written at a 7th-8th grade level (Flesch 60-70) tends to perform best for general audiences. Legal and medical content often scores much higher (college level) because of specialized vocabulary — which is why the Plain Writing Act of 2010 requires US federal agencies to write at a 9th grade level or below. For more context, see our readability and SEO ranking article.
The three formulas were developed between 1948 and 1975 and remain the most widely used readability metrics. They are not perfect (they cannot measure conceptual difficulty), but they provide a quick, reproducible signal about sentence length, word length, and syllable complexity.
How It Works
The analyzer tokenizes your text into words (\S+) and sentences ([.!?]+). For each word, it counts syllables using a heuristic that counts vowel groups, adjusts for silent e at the end of words, and floors words of three characters or fewer at one syllable. This heuristic is approximately 85-90% accurate for English — enough for readability scoring but not for linguistic analysis.
Flesch Reading Ease (Flesch, 1948, Journal of Applied Psychology 32(3):221-233) uses 206.835 − 1.015 × (words/sentences) − 84.6 × (syllables/words). Scores range 0-100; higher is easier. The score maps to grade levels: 90-100 is 5th grade, 60-70 is 8th-9th grade, 0-30 is college graduate.
Flesch-Kincaid Grade Level (Kincaid et al., 1975, US Navy Report 8-75) uses 0.39 × (words/sentences) + 11.8 × (syllables/words) − 15.59. The result is a US grade level — 8.0 means an 8th grader could read it comfortably. It was developed for the US Navy to assess technical manuals.
Gunning Fog Index (Gunning, 1952, The Technique of Clear Writing) uses 0.4 × [(words/sentences) + 100 × (complex words/words)] where complex words have three or more syllables. The result is also a US grade level.
Worked Examples
The default sample text (“The cat sat on the mat…”) produces a Flesch Reading Ease score around 90 (5th grade, very easy), Flesch-Kincaid grade around 2-3, and Gunning Fog around 4-5. This is appropriate for children's books or very simple content. The short sentences and simple words keep all three scores low.
Contrast this with a sentence like: “The implementation of quantum cryptographic protocols necessitates a comprehensive understanding of quantum superposition, entanglement phenomena, and post-quantum cryptographic algorithms.” This sentence has 18 words, approximately 45 syllables, and 5 complex words. Flesch drops to roughly 20 (college graduate), Flesch-Kincaid rises to roughly 18, and Gunning Fog to roughly 17. The long sentence and polysyllabic vocabulary make it very difficult.
For a blog post targeting a general audience, aim for Flesch 60-70 (8th-9th grade). This means sentences averaging 15-20 words and mostly one- or two-syllable words. If your score is below 50, consider splitting long sentences, replacing jargon with common words, and using active voice.
When to Use This Tool
- Calibrating blog posts for a general audience (aim for Flesch 60-70).
- Writing government or legal content that must meet plain-language requirements (9th grade or below).
- Creating educational materials targeted at specific grade levels.
- Evaluating the accessibility of technical documentation before publishing.
- Editing press releases for maximum media pickup (journalists prefer shorter, simpler text).
- Writing product descriptions for e-commerce (shorter sentences convert better).
- Assessing whether your content matches the reading level of your target persona.
Limitations & Disclaimer
The syllable counter uses a heuristic that is approximately 85-90% accurate for English; results may vary slightly from tools that use dictionary-based syllable counting. The formulas were developed for English and may not be accurate for other languages. Complex words in the Gunning Fog formula are defined as three or more syllables, but the tool does not apply the exceptions for proper nouns, jargon, and verb-form suffixes specified by Gunning. Readability scores measure surface features only — they cannot assess conceptual difficulty, prior knowledge requirements, or clarity of explanation. See our disclaimer for full terms and our readability and SEO ranking article for deeper context.
Frequently Asked Questions
Which readability formula should I use?
Flesch Reading Ease gives a 0-100 score that is easy to interpret. Flesch-Kincaid gives a US grade level, useful for educational content. Gunning Fog is popular for business writing. Microsoft Word uses Flesch Reading Ease in its built-in checker.
How accurate is the syllable counter?
The heuristic is approximately 85-90% accurate for English. It struggles with words like "every" (counted as 3, should be 2) and "simile" (counted as 2, should be 3). For most prose, the error averages out across many words.
What is a good Flesch Reading Ease score for web content?
Aim for 60-70 (8th-9th grade) for general audiences. The Nielsen Norman Group recommends this range because users scan rather than read. Scores above 70 are fine for children's content or ESL audiences. Scores below 50 should be reserved for expert audiences.
Do readability scores affect SEO?
Indirectly, yes. Google's helpful content system rewards content that is clear and accessible. High readability scores correlate with better user engagement (longer dwell time, lower bounce rate), which are positive UX signals. Google has never confirmed readability as a direct ranking factor.
Can readability formulas be gamed?
Yes. You can lower your grade level by splitting long sentences and using shorter words, even if the content is conceptually complex. The formulas measure surface features, not conceptual difficulty. A text about quantum physics written at a 6th grade level is still about quantum physics.
Is my text uploaded anywhere?
No. All analysis happens in your browser. Your unpublished drafts and proprietary content never leave your device.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team