Readability Analyzer — Score Text Complexity
Analyze text readability with Flesch-Kincaid, Gunning Fog, SMOG, ARI, and Coleman-Liau formulas. Color-coded sentence heatmap shows which sentences are hardest to read.
How to Use
Analyze the readability of any text in three steps:
- Paste or type your text — Enter text into the editor area. The tool begins analyzing as you type, with a brief debounce to avoid unnecessary computation. Sample text is provided to demonstrate the scoring system immediately.
- Review your scores — Six readability formulas run simultaneously: Flesch Reading Ease (0–100 scale, higher is easier), Flesch-Kincaid Grade Level, Gunning Fog Index, SMOG Index, Coleman-Liau Index, and Automated Readability Index. Each formula measures complexity from a different angle — syllable density, word length, or sentence structure — so comparing them gives a well-rounded assessment.
- Inspect the sentence heatmap — Every sentence is color-coded from green (easy) through yellow (moderate) and orange (hard) to red (very hard). Click any sentence to see its individual metrics: word count, average syllables per word, and average word length. This pinpoints exactly which sentences need simplification.
The statistics ribbon at the top provides an at-a-glance summary: total words, sentences, paragraphs, characters, average words per sentence, average syllables per word, estimated reading time (at 200 words per minute), and speaking time (at 150 words per minute).
About This Tool
How Readability Formulas Work
Readability formulas quantify text complexity using measurable features of prose: word count per sentence, syllable count per word, character count per word, and the proportion of polysyllabic words. Each formula was calibrated through empirical studies correlating these features with reading comprehension scores across specific populations. No single formula is definitive — each has strengths and known limitations, which is why this tool presents all six simultaneously.
The Six Formulas Explained
Flesch Reading Ease (Rudolf Flesch, 1948) produces a 0–100 score where higher values indicate easier text. It uses the ratio of syllables to words and words to sentences: 206.835 − 1.015(words/sentences) − 84.6(syllables/words). Scores above 60 are suitable for general audiences; scores below 30 indicate graduate-level difficulty. This formula is mandated for U.S. Department of Defense documents and many insurance policies.
Flesch-Kincaid Grade Level (Kincaid et al., 1975) converts the same inputs into a U.S. school grade level. A score of 8.0 means an eighth-grader should understand the text. The U.S. military uses this formula to assess technical manuals. The formula is 0.39(words/sentences) + 11.8(syllables/words) − 15.59.
Gunning Fog Index (Robert Gunning, 1952) emphasizes polysyllabic words — those with three or more syllables. The formula is 0.4[(words/sentences) + 100(complex words/words)]. A Fog index above 12 typically signals that the text requires a college education to comprehend comfortably.
SMOG Index (G. Harry McLaughlin, 1969) is considered one of the most accurate formulas for health literacy assessment. It counts polysyllabic words and uses a square root transformation: 3 + √(polysyllables × 30/sentences). It requires at least 30 sentences for reliable results, though this tool provides estimates for shorter texts. The SMOG is the gold standard for healthcare materials per the AMA and NIH guidelines.
Coleman-Liau Index (Coleman and Liau, 1975) uniquely avoids syllable counting, using character counts instead: 0.0588L − 0.296S − 15.8 where L is average letters per 100 words and S is average sentences per 100 words. This makes it computationally simpler and more stable across different texts.
Automated Readability Index (Smith and Senter, 1967) also uses characters instead of syllables: 4.71(characters/words) + 0.5(words/sentences) − 21.43. It was designed for real-time readability monitoring on electric typewriters and remains popular for its simplicity and speed.
Sentence Difficulty Heatmap
Beyond aggregate scores, this tool analyzes each sentence individually. Sentence difficulty is determined by word count and average syllable density: sentences with over 35 words or average syllable counts above 2.5 are marked "very hard," while sentences under 15 words with average syllables below 1.5 are "easy." The heatmap visualization lets you scan an entire document at a glance and immediately identify problem areas. Clicking a sentence shows its exact metrics so you can decide how to simplify it — breaking it into shorter sentences, replacing multisyllabic words with simpler alternatives, or restructuring for clarity.
Why Use This Tool
Target Your Audience
Different audiences require different reading levels. For general consumer content, the American Medical Association and the National Institutes of Health recommend writing at a 6th-to-8th-grade level. Blog posts and marketing copy typically target grades 8–10. Technical documentation and academic papers naturally score higher, but even within those domains, lower readability scores correlate with better comprehension and retention.
Common Use Cases
- Content marketing: Optimize blog posts, landing pages, and email campaigns for your target demographic. Posts scoring Flesch Reading Ease 60–70 tend to perform best for general audiences.
- Healthcare communications: Patient-facing materials must be understandable at a 6th-grade level per AMA and NIH guidelines. Use the SMOG Index specifically for this purpose.
- Legal plain language: Many jurisdictions require consumer contracts to meet specific readability thresholds. The Flesch Reading Ease score is legally mandated for insurance policies in several U.S. states.
- Technical writing: API documentation, user guides, and README files benefit from sentence-level analysis. The heatmap reveals where jargon-heavy sentences need simplification.
- Academic writing: Dissertation advisors and journal reviewers value clear prose. Analyze your drafts to identify unnecessarily complex passages before submission.
- UX writing: Interface copy, error messages, and onboarding flows should score easy on all formulas. Short sentences with common words reduce cognitive load and improve task completion rates.
Privacy-First Analysis
All analysis runs locally in your browser using pure JavaScript. Your text is never transmitted to any server — the formulas execute entirely on your device. This makes the tool safe for confidential documents, unpublished manuscripts, legal drafts, and patient records. Unlike cloud-based readability tools like Hemingway Editor or Grammarly, there is no data retention or third-party access. Related tools like Word Counter, Text Diff, and Case Converter follow the same privacy-first approach.