The Gunning Fog Index, explained: what the score means and how to lower it

Some prose reads like it was written to be understood. Other prose reads like it was written to survive a committee. The Gunning Fog Index is a 1952 attempt to measure the difference with a single number — and unlike a vague note that a paragraph "feels heavy", the score points straight at the two levers you can actually pull.

Robert Gunning built the index for newspapers and corporate trainers who needed to know, fast, whether a piece of writing matched its audience. His insight was that difficulty comes from two sources working together: how long your sentences run and how many long words you pack into them. The Gunning Fog Index calculator computes both and hands you back a US school-grade estimate. This piece walks through what that number means, exactly how it is produced, and what to change when it comes back too high.

What the number is telling you

The Fog Index is expressed as a grade level: the years of formal education a reader needs to understand the text on the first reading. A score of 8 means a typical eighth-grader should follow it without re-reading. A score of 16 means you are writing at the level of a university graduate.

The calculator translates the raw score into a band so you do not have to memorise the cutoffs. Roughly: grade 6 and under reads as easy, plain English; grade 6–8 is standard adult writing that most people handle comfortably; grade 9–12 is "fairly difficult" to "difficult" — late-high-school register; and anything past grade 16 is flagged as postgraduate, specialist text. For most general audiences — marketing copy, news, internal documents people actually need to act on — the sweet spot is grade 6 to 9. Drift much above that and you are quietly shedding readers.

The formula, in plain terms

The Gunning Fog Index is one of the few readability formulas you can run in your head. It is:

0.4 × ( average words per sentence + percentage of complex words )

Two ingredients, added together, then scaled by 0.4. The first ingredient is your average sentence length — total words divided by total sentences. The second is the share of complex words, expressed as a percentage of all words. So a document whose sentences average 18 words and where 12% of words are "complex" scores 0.4 × (18 + 12) = 12 — late high school.

What makes the formula honest is that both terms are things you control directly. You cannot argue with arithmetic: cut your sentences in half or swap a cluster of long words for short ones, and the score moves. That is the whole point of measuring it.

What actually counts as a "complex" word

This is where most people misread their own Fog score, so it is worth being precise about what the calculator treats as complex. The base rule is simple: a word with three or more syllables is complex. But Gunning's original method carved out some sensible exceptions, and this tool applies the two that matter most:

• Common inflections do not count. A word that only reaches three syllables because you added an ordinary -es, -ed, or -ing ending is not treated as complex if its stem is under three syllables. "Created", "processes", and "deciding" all clear three syllables, but their roots ("create", "process", "decide") do not, so the tool leaves them alone. This stops the formula from punishing you for ordinary verb tenses.
• Hyphenated compounds do not count. A word joined by a hyphen — "well-meaning", "decision-making" — is excluded, because the components are usually familiar even when the compound is long.

One nuance worth knowing: the classic textbook version of Gunning Fog also excludes capitalised proper nouns, on the logic that a long name like "Mississippi" is not really a difficulty. This calculator counts words by syllable regardless of capitalisation, so a passage thick with long proper names will read slightly foggier here than the strict textbook formula would suggest. If you are scoring a biography or a report full of multi-syllable place names, read the number with that in mind — the people and places are inflating it, not your prose.

How to bring a foggy score down

Because the formula has exactly two inputs, lowering it is mechanical. Attack whichever term is larger.

If your sentences are long, split them. Average sentence length is usually the cheaper win. A 40-word sentence joined by three commas and a "however" almost always wants to be two or three sentences. The meaning rarely lives in the connective tissue; it lives in the clauses. Breaking one long sentence into two short ones can pull a whole paragraph down a grade without changing a single word of substance. The calculator's own guidance flags this first: shorten sentences over 20 words.

If your vocabulary is dense, swap the long words. The percentage-of-complex-words term rewards Anglo-Saxon plainness. "Utilise" becomes "use"; "facilitate" becomes "help"; "in order to" becomes "to". Each swap drops your complex-word ratio and, more importantly, makes the sentence land harder. Watch for the corporate-Latinate register — "operationalise", "incentivise", "leverage" — because that is exactly the writing Gunning designed the index to expose.

What you should not do is game the number. Because the formula only sees length and syllables, you can technically lower it by chopping every sentence into a staccato fragment, or by replacing a precise technical term with a vague short one. The score will improve and the writing will get worse. The Fog Index is a smoke detector, not a chef: it tells you something is burning, but it cannot taste the meal.

Where the Fog Index fits — and where it doesn't

Gunning Fog is at its best on business writing, journalism, financial summaries, and any prose that tends to accumulate unconscious jargon, because it specifically penalises the long-word density that jargon produces. It is less suited to technical or medical content where long words are unavoidable and genuinely necessary — "photosynthesis" is not the problem in a biology text. For health and patient-facing material, the SMOG index is calibrated more strictly. For a quick, intuitive single number on general copy, Flesch Reading Ease is the familiar choice. And because every formula has blind spots, the smartest move is rarely to trust one in isolation — running the same text through a readability consensus that averages several formulas (Fog among them) smooths out the quirks any single measure introduces.

When you want to see where your own writing sits, paste it into the Gunning Fog Index calculator. It will show you the grade, break out your average sentence length and complex-word percentage so you know which lever to pull, and tell you in plain terms whether the text matches the audience you are writing for — leaving you to do the one thing no formula can, which is decide what you actually meant to say.