Readability Checker
Measure readability scores including Flesch-Kincaid and Gunning Fog.
About the Readability Checker
The Readability Checker scores your text on three of the most widely used readability formulas: Flesch Reading Ease, Flesch-Kincaid Grade Level, and Gunning Fog Index. Together, these metrics estimate how difficult your writing is to understand and which grade level a reader needs to comprehend it. The tool also reports the underlying counts (words, sentences, syllables, complex words) so you can see exactly what is driving your score.
Readability matters because it directly affects whether your audience will finish reading what you wrote. Research from the Nielsen Norman Group shows that web users scan rather than read, and content scoring above an 8th-grade Flesch-Kincaid level loses a significant share of mobile readers. Government agencies (under the Plain Writing Act of 2010 in the US) are required to write at an 8th-grade level. News outlets like the BBC and Reuters target a Flesch Reading Ease of 60+ (Standard) for general audiences.
The three formulas approach the same question from slightly different angles. Flesch Reading Ease produces a 0-100 score (higher is easier). Flesch-Kincaid Grade Level maps that score to a US school grade. Gunning Fog Index emphasizes complex words (3+ syllables) and tends to score academic and technical writing more harshly. Comparing all three gives you a more robust picture than any single formula.
How It Works
The tool tokenizes your text into words (using /[a-zA-Z]+/g — numbers and punctuation are excluded from word counting) and sentences (using /[.!?]+/g for terminal punctuation). Syllable counting is the trickiest part because English syllable rules are irregular; the tool uses a well-known heuristic that strips common suffixes (-es, -ed, -e), removes leading Y, then counts vowel-group runs. This produces an estimate that is within 5-10% of accurate for most English prose.
Flesch Reading Ease is calculated as 206.835 - 1.015 × (words/sentences) - 84.6 × (syllables/words). Scores range from 0 to 100, with 60+ considered Standard (8th-9th grade) and 90+ considered Very Easy (5th grade). The formula was developed by Rudolf Flesch in 1948 and remains the most widely cited readability metric.
Flesch-Kincaid Grade Level is 0.39 × (words/sentences) + 11.8 × (syllables/words) - 15.59. The result is a US grade level: 8 means an 8th grader can read it, 12 means a high school senior, 14 means college sophomore. Gunning Fog Index is 0.4 × (words/sentences + 100 × complex_words/words), where complex words are those with 3 or more syllables. Fog tends to over-penalize technical writing because every jargon term with 3+ syllables counts against you.
Worked Examples
The default text (“The cat sat on the mat... Everyone was happy.”) scores around 90-95 on Flesch Reading Ease (Very Easy, 5th grade) because the sentences are short, the words are short (mostly one syllable), and there are no complex words. The Flesch-Kincaid grade would be around 3-4 and the Fog Index around 4-5. This is the readability of children's books.
Compare with a paragraph of academic prose: “The implementation of pedagogical frameworks necessitates a comprehensive understanding of cognitive development paradigms.” This single sentence scores below 30 on Flesch (Very Difficult) and above 18 on Flesch-Kincaid (graduate-level). The complex words (“implementation,” “pedagogical,” “comprehensive,” “understanding,” “cognitive,” “development,” “paradigms”) drive the Fog Index above 20.
To improve a difficult score, do three things: shorten sentences (aim for an average of 15-20 words), replace 3+ syllable words with shorter synonyms (“utilize” → “use,” “facilitate” → “help”), and break up paragraphs. Most web content should target a Flesch Reading Ease of 60+ and a Flesch-Kincaid grade of 8-10.
When to Use This Tool
- Writing website copy that mobile users will actually finish reading.
- Drafting government communications that comply with Plain Writing Act requirements.
- Editing technical documentation to be accessible to a broader audience.
- Writing children's books or educational materials at a specific grade level.
- Optimizing marketing copy for readability before A/B testing.
- Translating academic research into accessible summaries for lay audiences.
- Checking that internal communications (HR memos, policy docs) are understood by all employees.
Limitations & Disclaimer
The readability formulas were developed for English prose and use English-specific constants; applying them to other languages produces meaningless results. The syllable counter uses a heuristic that is approximately 90-95% accurate; unusual words (especially technical jargon) may be miscounted, skewing the grade-level estimates by 1-2 levels. The formulas do not account for content difficulty, prior knowledge requirements, or visual design (which strongly affects actual comprehension). A low grade level does not guarantee clarity — “See spot run. Run spot run.” scores beautifully but conveys little. Use these metrics as one signal among many. See our disclaimer for full terms.
Frequently Asked Questions
What is a good Flesch Reading Ease score?
For general web audiences, aim for 60+ (Standard, 8th-9th grade). For consumer-facing content like news or marketing, 70+ is better. For technical or academic writing, 30-50 is acceptable. Below 30 is difficult for most readers; above 90 is suitable for children.
Which formula should I trust if they disagree?
All three formulas use the same underlying counts (words, sentences, syllables) but weight them differently. Flesch Reading Ease is the most widely cited; Flesch-Kincaid is the standard for US government and education; Gunning Fog is preferred for business writing. If they disagree by more than 2 grade levels, your text likely has an unusual mix of long sentences and long words — aim to improve both.
How accurate is the syllable counter?
The tool uses a heuristic that is accurate within 5-10% for most English prose but can struggle with unusual words (e.g., “queue” counts as 4 syllables via vowel groups but is actually 1). For highly technical or scientific content, syllable counts may be off enough to skew the formulas by 1-2 grade levels.
Do these formulas work for non-English text?
No. The formulas were developed for English and use English-specific constants (the 206.835 in Flesch, the 0.4 in Fog). Applying them to Spanish, French, or German produces meaningless numbers. Separate formulas exist for other languages (e.g., the Huerta formula for Spanish, the Amstad formula for German).
Can I use this to improve my writing?
Yes — readability scores are diagnostic, not prescriptive. If your score is too low, the breakdown shows whether the problem is long sentences, long words, or complex words. Address the dominant factor first. Many great writers (Hemingway, Orwell) wrote at a Flesch Reading Ease of 70+.
Is my text uploaded anywhere?
No. All analysis happens in your browser using JavaScript. Your drafts are never sent to a server, which makes the tool safe for confidential or unpublished content.
Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team