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Readability
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About readability formulas
These 8 formulas are heuristics that estimate complexity from sentence and word length. They don't measure understanding, audience fit, or the quality of the writing itself. Use the scores as a starting point for revision, not absolute rules.
SMOG works best with 30+ sentences. Dale-Chall uses a curated list of ~1,400 common English words; obscure-but-common technical terms may count as "hard". LIX is language-agnostic and works on any Latin-script text.
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How it works
The Readability Checker scores your text against 8 industry-standard readability formulas in parallel, surfaces a per-sentence color-coded breakdown so you can see exactly which sentences are dragging the score down, and offers free per-sentence AI rewrites for any sentence flagged Hard or Very hard. The audit itself is deterministic PHP (no AI, no upstream cost); only the optional sentence rewrites use AI tokens.
The 8 formulas:
- Flesch Reading Ease (0-100, higher = easier).
- Flesch-Kincaid Grade (US grade level).
- Gunning Fog (US grade level; weights "complex" 3+ syllable words).
- Coleman-Liau (US grade level; uses characters per word, language-agnostic).
- ARI (US grade level; characters per word + words per sentence).
- SMOG (US grade level; polysyllabic-word counting, needs 30+ sentences).
- Dale-Chall (4-10+ score; penalizes words off a curated ~1,400-word common-English list).
- LIX (0-100+, language-agnostic; long-word ratio).
Per-sentence complexity classifier. Each sentence is scored on word count, average syllables per word, passive voice presence, and complex-word ratio. The classifier emits one of four levels (Easy, Medium, Hard, Very hard) and the sentence breakdown panel highlights Hard and Very hard sentences with yellow/red backgrounds. Click any highlighted sentence to get 3 free AI rewrite variants.
Audience-fit assessment. Pick a target audience and the score panel grades your text against the typical Flesch range for that audience (e.g. General public targets 60-70; Academic 30-50). The audience pill goes green/amber/red.
CEFR estimate (A1-C2). Derived from Flesch RE plus Dale-Chall hard-word percentage. Useful for ESL writers and language teachers.
Honest framing. Readability formulas are heuristics that measure sentence and word complexity. They don't measure understanding, audience fit, factual accuracy, or quality of the writing. Use them as a starting point for revision, not absolute rules. SMOG is unreliable on short text (under 30 sentences). Dale-Chall flags obscure-but-common technical terms as "hard". LIX works on any Latin-script text but can't see grammatical complexity.
Plan caps. The audit itself is free (no upstream cost). Each AI rewrite call consumes a portion of your shared daily AI token budget; the budget is shared across all AI tools on this site (Free 50/day; Bronze 100; Silver 200; Gold 400; Platinum 1,000). Word caps per submission: Free 2,000; Bronze 5,000; Silver 10,000; Gold 25,000; Platinum 50,000.
What you will see
- Input modes (Paste / URL / Upload)
- Three ways to give the tool source text. URL fetches a page through a proxy; Upload accepts PDF or .txt files up to 10 MB.
- Target audience
- What kind of reader you're writing for. The audience-fit pill on the results panel grades your text against the typical readability range for that audience.
- Document Quality score circle
- The big circle is the Flesch Reading Ease score (0-100). The band below it (Very easy / Easy / Plain / Fairly difficult / Difficult / Very difficult) labels what that score means.
- Summary pills
- CEFR estimate (A1-C2), audience-fit pill (good / close / too simple / too complex), and grade-level pill.
- The 8-formula grid
- Each of the 8 formulas as its own small card with the score, label, and a short note on what the score means in plain English.
- Sentence breakdown
- Your text re-rendered with every sentence wrapped in a span and color-coded by complexity (Easy / Medium / Hard / Very hard). Hard and Very hard sentences are clickable, click any one for 3 free AI rewrite variants.
- Document stats
- Word count, sentence count, paragraph count, average words per sentence, average syllables per word, passive-voice percentage. Useful for spotting structural issues at a glance.
- Recent audits
- The last 10 audits, kept locally in your browser for 30 days.
Frequently asked questions
How do I check the readability of my text?
Pick an input mode (Paste, URL, or Upload), drop your content in, pick a target audience (General, 8th grade, High school, College, Medical, Legal, Academic, Marketing, Blog), and click Check Readability. You get scores from 8 formulas in parallel, a per-sentence color-coded breakdown that highlights every hard sentence, a CEFR English-level estimate (A1-C2), an audience-fit pill, and free per-sentence AI rewrites for any sentence flagged Hard or Very hard.
What is a good Flesch Reading Ease score?
Flesch Reading Ease runs from 0 (very difficult) to 100 (very easy). General convention:
- 90-100: 5th-grade level. Very easy.
- 80-89: 6th-grade. Easy.
- 70-79: 7th-grade. Fairly easy.
- 60-69: 8th-9th-grade. Plain English (the most common target for public-facing content).
- 50-59: 10th-12th-grade. Fairly difficult.
- 30-49: College. Difficult.
- 0-29: College graduate. Very difficult.
Aim for 60-70 for blog posts and marketing copy; 50-60 for B2B technical content; 30-50 is appropriate for academic or scientific writing. The audience-fit pill on the score card grades your text against typical norms for the audience you picked.
How does Flesch Reading Ease work?
The formula is: 206.835 - (1.015 × average words per sentence) - (84.6 × average syllables per word). Longer sentences and more multi-syllable words drive the score down. The formula is a heuristic, not a comprehension test: it doesn't know whether your content is well-organized, factually accurate, or relevant to your audience. It only measures sentence and word complexity. Use it as a complexity gauge, not as a quality measure.
How can I improve the readability of my writing?
- Shorten sentences. Most of the readability formulas penalize sentence length more than anything else. A 30-word sentence and a 15-word sentence rate very differently even with identical vocabulary.
- Cut unnecessary multisyllabic words. "Use" instead of "utilize"; "help" instead of "facilitate"; "show" instead of "demonstrate".
- Switch passive to active voice. "The report was reviewed by the team" -> "The team reviewed the report".
- Break up dense paragraphs. A wall of text is harder to read than the same content in smaller chunks, even when sentence-level readability is identical.
- Use the per-sentence breakdown. Click any sentence flagged Hard or Very hard for a free AI rewrite that simplifies it without changing the meaning.
What is a Flesch-Kincaid grade level of 14?
FK Grade Level 14 means the text reads at the comprehension level of a US-educated person with 14 years of formal education, roughly a second-year college student. Higher than 14 is graduate-school level; 10-12 is high school; 6-8 is middle school. The most common target for general public writing is 7-9 (the level that comfortably reaches the typical adult reader, who reads at around an 8th-grade level on average in the US). Your blog's ideal target depends on your audience: B2B SaaS marketing copy commonly lands at 10-12; consumer health content typically targets 6-8; academic papers run 14-18.
What is the difference between the 8 readability formulas?
- Flesch Reading Ease, 0-100, higher = easier. Sentence length + syllables.
- Flesch-Kincaid Grade, US school grade level. Same inputs as Flesch RE, different mapping.
- Gunning Fog, US grade level. Sentence length + percentage of "complex" (3+ syllable) words.
- Coleman-Liau, US grade level. Uses characters per word instead of syllables (easier to compute, language-agnostic).
- ARI (Automated Readability Index), US grade level. Characters per word + words per sentence.
- SMOG, US grade level. Counts polysyllabic words in 30-sentence samples. Works best on longer documents.
- Dale-Chall, raw score 4-10+. Penalizes words not on a curated list of ~1,400 common English words.
- LIX, 0-100+, language-agnostic. Sentence length + percentage of long (7+ character) words.
Are AI rewrites of hard sentences really free?
Yes. Click any sentence flagged Hard or Very hard in the per-sentence breakdown, and the tool generates 3 simpler rewrite variants for that sentence using a large language model. The rewrites preserve meaning and shorten the sentence or reduce syllable load. Hemingway charges for this feature; we include it free. Each call consumes a portion of your shared daily AI token budget; the quota chip shows your real-time remaining count.
How long can the input be?
The audit pass itself is local PHP (no upstream cost), so it scales well. Word caps per submission: Free 2,000; Bronze 5,000; Silver 10,000; Gold 25,000; Platinum 50,000. Long-form content over Free's cap should split into chapters or sections. SMOG specifically works best with 30+ sentences (it samples 30); on shorter texts SMOG can produce noisy results and the tool surfaces a warning.