Find out how many years of education your text needs

The Gunning fog index estimates how many years of formal education a reader needs to understand a text on the first reading[1]. A score of 12 matches a US high school senior, and a score of 8 matches an eighth grader. Robert Gunning, an American businessman who had worked in newspaper and textbook publishing, published the formula in 1952[1].

Gunning was working on clearer writing years before he had a formula. In 1944 he founded the first consulting firm that specialized in readability. Over the next few years he worked with more than 60 large city daily newspapers and with popular magazines, helping their writers and editors write for their readers[2].

The index rests on two numbers that a program can count without understanding the text: the average sentence length and the share of words with three or more syllables. The calculator above counts both as you type and lists the long words that push the score up.

Add the average sentence length to the percentage of complex words, then multiply the sum by 0.4[1]. The average sentence length is the number of words divided by the number of sentences. The percentage of complex words is their number divided by the number of words, times 100. Words and sentences follow the same rules as in our word and character counter, so both tools give the same totals.

Take this passage: “Our new library opens on Monday. It offers comfortable reading rooms and a large collection of international magazines.” It has 18 words in 2 sentences, so the average sentence is 9 words long. Five of the words have three or more syllables: library, comfortable, collection, international and magazines. That makes 27.8% complex words. The two numbers add up to 36.8, and 0.4 times 36.8 gives a fog index of 14.7.

A complex word has three or more syllables[1]. Gunning left out three kinds of long words that readers handle easily: proper nouns, compounds made of short words such as housekeeper, and verbs whose third syllable is the -es or -ed ending, such as donated and trespasses[3].

The calculator applies these rules wherever the spelling shows them. A word that starts with a capital letter in the middle of a sentence counts as a proper noun. The first word of a sentence is judged by its syllables, as Gunning's rule says[3]. A hyphenated compound counts only when one of its parts has three syllables, so well-known stays easy and self-explanatory is complex. Compounds written as one word, such as housekeeper, are counted by their syllables like any other word.

Sentences end at a period, a question mark, an exclamation mark or an ellipsis, and an abbreviation such as Mr. or Dr. doesn't end one. Gunning's 1952 method also counted each clause as a sentence. That step was dropped in the 1980s, and later guides generally recommend the sentence count alone[1]. To see the older result, put a period between the independent clauses before you paste the text.

When sentences are long, splitting one of them lowers the score more than replacing one long word. Both parts of the formula enter the sum with the same weight, and the difference comes from how far one edit moves each of them.

Take a passage of 100 words. Each complex word in it equals one percentage point, so replacing one with a shorter word lowers the index by 0.4. If the same passage has five sentences, the average sentence is 20 words long. Split one sentence in two, and the average drops to 16.7 words, which lowers the index by 1.3.

The calculator shows which edit your text needs. A high Share of complex words calls for shorter words from the Which words raise the index card. A long Average sentence length calls for more periods.

Texts for a wide audience generally need a fog index below 12, and texts that almost everyone has to understand need one below 8[1]. Gunning's magazine counts show the same pattern. Reader's Digest, the slicks and the pulps, each read by millions, scored between 6 and 9, while class magazines with fewer than a million readers reached 12[2].

We measured this page with the same calculator. Its running text, without tables and captions, scores 10.5 with an average sentence of 17.6 words. For a second view of the same text, check it with the Flesch Reading Ease score, which works with syllables per word instead of counting complex words.

Gunning built his formula on reading tests where the reader had to answer 90 percent of the questions correctly. Only the SMOG formula uses a stricter criterion, and the grades predicted by these two formulas tend to be higher than those of other formulas[2]. We saw this on the introductions of 312 tools on this site: in English they score 9.1 on the fog index and 6.7 on the Flesch-Kincaid grade level.

Programs also count syllables and sentences in different ways, which moves the result even when they use the same formula[2]. Microsoft Word shows the Flesch Reading Ease score and the Flesch-Kincaid grade level in its readability statistics, so a fog score for a Word document comes from pasting the text into a calculator like this one[6].

The formula sees only how long the sentences and words are. Some long words are familiar to almost every reader, and some short words are rare, yet the index judges both by their syllables[1]. The formula also has no way to judge meaning, so a text of short words can still be hard to follow. On a screen, how easily the letters can be read also depends on color, and the color contrast checker tests text and background against the WCAG levels.

The table of passages shows the effect. Three correct, short sentences scored 36.8, because nearly every word in them was long. The score tells you how much effort the wording asks of a reader, and the reading time calculator tells you how many minutes the text takes.

The index was also built for English. The same tool introductions on this site score 9.1 in English, 11.6 in French, 16.5 in Spanish and 16.6 in Italian, because Romance languages use more long words for the same content. The other language versions of this page explain what that means for each language.