Text Complexity Analyzer

What text complexity measures

Text complexity is shorthand for "how hard is this passage to read?" It's the same question Flesch-Kincaid, SMOG, Lexile, ATOS, and Coh-Metrix all answer with different formulas. The analyzer on this page runs three of the most widely-cited formulas in real time on whatever you paste. See also: Syllable Counter. See also: Sentence Counter.

The metrics:

• Flesch-Kincaid Grade Level. The classic readability formula. Returns a grade level (4.5 = mid 4th-grade reading level; 12 = high-school senior; 16 = college junior). Formula: 0.39 × (words/sentence) + 11.8 × (syllables/word) − 15.59.
• SMOG Index. Simple Measure of Gobbledygook. Designed for healthcare materials. Counts polysyllabic words (3+ syllables) in 30 sentences. Better than F-K for short texts.
• Average sentence length. Number of words per sentence. Above 20 starts feeling academic; above 30 starts feeling unreadable.
• Average syllables per word. The other lever of complexity. Below 1.5 is plain English; above 1.7 is academic or technical.
• Type-token ratio (TTR). Lexical diversity. Number of unique words divided by total words. Higher = more varied vocabulary.

Reading the difficulty band

The single Difficulty band combines the formulas into one of four labels: Easy, Moderate, Difficult, Very difficult. It uses Flesch-Kincaid as the primary signal because F-K has the most established empirical mapping to U.S. grade levels.

Rough alignment:

• F-K < 6 = Easy. Elementary school reading level. Plain English. Most content for general audiences should target this.
• F-K 6-9 = Moderate. Middle school to early high school. Standard for newspapers, blog posts, marketing copy.
• F-K 9-13 = Difficult. High school to early college. Standard for serious journalism, business writing.
• F-K 13+ = Very difficult. Academic, technical, or legal writing. Reserved for specialist audiences.

These bands are guidelines, not absolutes. A short passage with one very long sentence can spike F-K artificially; a long passage with one short sentence can deflate it.

What the formulas miss

F-K, SMOG, and their cousins all use the same two levers: sentence length and word length (in syllables or character count). They were designed in the 1940s through 1970s and don't account for:

• Vocabulary difficulty beyond syllable count. "Apple" and "axle" are both two syllables but vary wildly in word-frequency familiarity.
• Sentence structure. A 30-word sentence can be perfectly clear (parataxis) or impenetrable (deeply nested subordinate clauses). Word count alone misses this.
• Background knowledge required. A short, simple sentence about quantum mechanics is harder than a long sentence about cooking.
• Coherence and cohesion. Whether sentences connect to each other logically (the focus of Coh-Metrix and more modern measures).

For content QA - "is this readable for my audience?" - F-K and SMOG are still useful coarse signals. For high-stakes accessibility, pair with manual review or with tools that use richer linguistic features.

Use cases for content teams

Where text-complexity analyzers earn their keep: See also: Essay Word Counter.

• Content QA before publish. Many marketing teams keep blog posts under F-K 10. Run the analyzer in the editing pass to flag passages drifting above target.
• ESL / EFL grading. Match passages to student reading levels in language-learning curricula.
• Accessibility audit. WCAG 2.1 doesn't mandate a specific reading level, but plain-language guidelines (gov.uk, US Plain Language Act) recommend targeting middle-school reading level (F-K 6-8) for public-facing content.
• Compare drafts. Did your edit make the passage easier or harder? Run before and after.
• Pre-publish AI rewrites. AI rewrites often spike complexity unintentionally. Run the analyzer on both the original and the rewrite.