What this measures: classic readability formulas on English text. Your tool counts words, sentences, letters, syllables, and polysyllables (3+ syllables), then applies the formulas below. Reading time assumes ~200 words per minute.

Formulas & interpretations

Flesch Reading Ease (FRE)

FRE = 206.835 − 1.015 × (W ÷ S) − 84.6 × (Sy ÷ W)

Higher is easier. ~90–100 Very Easy, 80–89 Easy, 70–79 Fairly Easy, 60–69 Standard, 50–59 Fairly Difficult, 30–49 Difficult, 0–29 Very Confusing.

Flesch–Kincaid Grade Level (FKGL)

FKGL = 0.39 × (W ÷ S) + 11.8 × (Sy ÷ W) − 15.59

Approximate U.S. school grade needed to understand the text.

Gunning Fog Index (FOG)

FOG = 0.4 × [ (W ÷ S) + 100 × (PW ÷ W) ]

Emphasizes sentence length and polysyllabic words (PW = words with ≥3 syllables). Typical web copy targets ~8–12.

SMOG Index

SMOG = 1.043 × √( PW × (30 ÷ S) ) + 3.1291

Designed for shorter samples; uses polysyllable density to estimate grade level.

Coleman–Liau Index (CLI)

CLI = 0.0588 × L − 0.296 × S₁₀₀ − 15.8

Where L is letters per 100 words and S₁₀₀ is sentences per 100 words. Returns grade level.

Automated Readability Index (ARI)

ARI = 4.71 × (C ÷ W) + 0.5 × (W ÷ S) − 21.43

Uses characters per word and words per sentence to return grade level.

Exactly how this tool counts things

• Words (W): alphabetic tokens (letters A–Z) with optional internal apostrophes (e.g., don’t). Regex: [A-Za-z]+(?:'[A-Za-z]+)?
• Sentences (S): sequences ending in ., !, or ?.
• Letters: A–Z counts only ([A-Za-z]) for Coleman–Liau.
• Characters for ARI (C): alphanumeric only ([A-Za-z0-9]) as implemented in your script.
• Syllables (Sy): heuristic: silent endings trimmed (e/ed/es when appropriate), leading y handled; vowel groups [aeiouy]{1,2} counted. Minimum 1 per word.
• Polysyllables (PW): words with ≥3 syllables.
• Reading time: words ÷ 200 wpm (shown as minutes/seconds).
• Pre-processing: if enabled, HTML tags are stripped and whitespace is normalized.

Assumptions & limitations

• Designed for modern English prose. Other languages or heavy jargon will skew results.
• Syllable detection is approximate (dictionary-free heuristic). Expect small differences vs. dictionary-based tools.
• Sentence detection is punctuation-based; unusual punctuation or transcripts may reduce accuracy.
• Numbers, acronyms, code snippets, lists, and tables can distort grade levels.
• Different tools use slightly different tokenization and syllable rules, so scores may vary.

Tip: For more approachable copy, aim for higher Flesch scores (≥60) and lower grade levels (≈8–10) unless your audience is highly specialized.