Gunning Fog Readability Index Calculator
**The Gunning Fog Index**, introduced by Robert Gunning in *The Technique of Clear Writing* (1952), combines average sentence length and the percentage of complex words (≥ 3 syllables) into a single number that approximates the US grade level needed to read the passage on first hearing: under 8 = elementary, 8–12 = popular press, 12–14 = high school, 14–17 = college, 17+ = academic / postgraduate. It is widely used in technical documentation, news editing and textbook publishing. Paste an English passage; the tool returns the index plus detailed metrics (word count, sentence count, complex words, average sentence length, % complex) so writers can adjust sentence length and vocabulary to hit a target audience.
Paste at least one complete English sentence.
Fog index
12.0
High school (12–13.9)
Index bands (US grade level)
- < 8 · Elementary (< 8)
- 8 – 11.9 · Middle school / popular (8–11.9)
- 12 – 13.9 · High school (12–13.9)
- 14 – 16.9 · College (14–16.9)
- ≥ 17 · Graduate / academic (≥ 17)
- Words
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- Sentences
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- Complex words (≥ 3 syll.)
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- Avg sentence length
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- % complex words
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Formula: Fog = 0.4 × (avg sentence length + 100 × pct complex words) — Gunning, "The Technique of Clear Writing" (1952); DuBay, "Principles of Readability" (2004). The score approximates the US grade level needed to read the passage on first hearing.
Formula
Fog = 0.4 × ((words / sentences) + 100 × (complex_words / words)) A "complex" word has ≥ 3 syllables, EXCLUDING: · proper nouns (capitalised mid-sentence) · hyphenated compounds of common words · suffix -es / -ed / -ing that does not add a syllable
- · **Index band reference**: < 6 children's magazines (e.g. *Highlights for Children*); 6–8 popular press (*USA Today*, Reader's Digest); 8–10 popular fiction and online news (*Time*, *BBC News*); 10–12 *The Economist*, US presidential inaugural addresses; 12–14 academic-journal abstracts and technical manuals; 14–17 legal contracts and policy documents; 17+ academic research papers.
- · **Target by writing type**: technical documentation aim 9–12 (close to popular press); annual reports and editorials 11–14; research papers 14–17; business email should stay 8–10 or recipients skim. **The US average reading level is ~ 8th grade** (NAAL 2003 survey) — write for the general public aiming for ≤ 12.
- · **Techniques to lower the Fog index**: (1) split long sentences (target 12–18 words each on average); (2) substitute simple Anglo-Saxon words for Latinate ones — "utilise → use", "approximately → about", "demonstrate → show"; (3) prefer active voice over passive; (4) unpack nested clauses and trim of-phrase chains; (5) use bullets and sub-headings instead of long paragraphs.
- · **Limitations**: (1) Fog only weighs word length and sentence length — it ignores actual vocabulary difficulty (rare vs common), sentence structure complexity (subordinate clauses) and abstract vs concrete reference; (2) it does not work on poetry, dialogue or spoken language; (3) it under-estimates difficulty for non-native English speakers (cognates from L1 are not modelled); (4) it over-estimates difficulty for jargon-rich domains (medicine, law, code) where specialist readers find the terminology easier than its syllable count suggests.
- · **Related indices**: Flesch Reading Ease (0–100, higher = easier; 60–70 = plain English); Flesch–Kincaid Grade Level (similar to Fog but uses syllable count, not complex-word percentage); SMOG (samples 30 sentences for complex-word density); Automated Readability Index (uses character count — no syllable analysis); Dale–Chall (uses a 3 000-word familiar-word list). They correlate but are not interchangeable.
- · **References**: (1) Gunning, R., *The Technique of Clear Writing* (McGraw-Hill, 1952); (2) DuBay, W. H., *The Principles of Readability* (Impact Information, 2004); (3) Klare, G. R., *The Measurement of Readability* (Iowa State University Press, 1963); (4) US Plain Language Action and Information Network (plainlanguage.gov).
Frequently asked
Why does pasting Chinese text give 0? Does this tool support Chinese?
**Gunning Fog is English-specific — it does not work for Chinese**. Reasons: (1) Chinese is logographic (one character ≈ one syllable), so "3+ syllables = complex" makes no sense; (2) Chinese has no whitespace-delimited words — segmentation requires a dictionary (Jieba, CKIP); (3) the "average sentence length × complex-word %" recipe has no Chinese equivalent — Chinese readability is driven by character frequency, stroke count and sentence structure. **Chinese alternatives**: (1) Chinese Readability Tools (e.g. Da Jun Wang 2008) combining common-character ratio, average sentence length and stroke count; (2) HSK vocabulary level (1–6) — the standard for Chinese-as-a-foreign-language; (3) Zipf-based frequency analysis. A dedicated Chinese readability tool may come later. For English writing (tech articles, business email, academic abstracts), this tool is the right fit.
My business email scores Fog = 15. Is that too high? How do I bring it down?
**Fog = 15 is too high for business email (target 8–10) — recipients will skim**. 15 corresponds to "college level" — readers must work to understand it. **5 fast techniques to lower Fog**: (1) **Split sentences**: target 12–18 words each. "The implementation of the optimisation strategy will significantly improve our quarterly performance, particularly in the Asian markets." (24 words) becomes "We're changing how we optimise. This should boost Q3 — especially in Asia." (two sentences, 8 words avg). Fog drops from ~ 18 to ~ 8. (2) **Anglo-Saxon substitution**: utilise → use, approximately → about, demonstrate → show, implement → do, commence → start. Each cuts a syllable. (3) **Active voice**: "The report was reviewed by the team" → "The team reviewed the report" — fewer words, cleaner structure. (4) **Unpack of-chains**: "the analysis of the performance of the system" → "the system's performance analysis". (5) **Bullets instead of paragraphs**: split three points into three bullets — each becomes its own "sentence". **Target**: 8–10 for internal mail; 10–12 for external customers; > 14 belongs only in contracts or academic writing.
How does Fog compare to Flesch / SMOG, and why might they disagree?
**All three measure reader difficulty but use different proxies; mild disagreement is normal**. **Flesch Reading Ease** (0–100, higher = easier): uses syllable count, very sensitive to vocabulary difficulty; intuitive bands (< 30 academic, 60–70 plain English, > 90 5th-grade). **Flesch–Kincaid Grade Level**: converts Flesch Ease to a US grade; usually within ± 2 grades of Gunning Fog. **Gunning Fog**: uses the percent of complex words, so it spikes on jargon-heavy text — a medical paper might score Fog 25 but Flesch–Kincaid only 15. **SMOG (Simple Measure of Gobbledygook)**: samples 30 sentences and counts complex words; more stable than Fog but needs ≥ 30 sentences (no good for short text). **Picking one**: (1) **news / blog editing** → Flesch Ease (intuitive, industry standard); (2) **textbooks, patient-education leaflets** → SMOG (McLaughlin's recommendation for medical text); (3) **legal contracts, technical manuals** → Gunning Fog (highlights jargon load); (4) **academic writing** → report both Flesch–Kincaid + SMOG and average. In practice: report all three; if they diverge by > 3 grades, dive in and inspect which word class is driving the spread.
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