Are People Reading What You Write?

A Quick Formula to Burnish Your Prose

September 2006 By Denny Hatch

multipage

Until that time, readability measurement had been the exclusive province of educators. But their formulas were complicated and required the laborious counting of certain not easily discerned factors. Many systems—even the highly touted yardsticks of readability guru Rudolph Flesch—employed four-decimal-place multipliers that were difficult to remember and gave a false sense of scientific accuracy.

Gunning discovered that only two qualities were critical to determining readability: the average number of words in sentences plus the percentage of “hard” words that might cause a reader to stumble. In his 1952 book, “The Technique of Clear Writing,” he described how to apply his innovative “Fog Index,” which is ideal for any kind of copy. It’s simple to calculate and easy to carry in your head.

1. Count the number of words in successive sentences. In a long piece, take several 100-word samples distributed evenly throughout. (Stop the sentence count with the sentence that ends nearest each 100-word total.) Divide the total number of words in each sample by the number of sentences. This yields the average sentence length of the copy.

2. Count the number of words of three syllables or more per 100 words. Don’t count proper names, word combinations of short words (i.e., bookkeeper, manpower, etc.) or three-syllable plural or past-tense verb forms as a result of adding -ed or -es (i.e., created or trespasses). This figure is the percentage of “hard” words in the copy.

3. Add these two numbers and multiply the result by 0.4.

This yields the Fog Index, which correlates exactly with school grade reading levels as determined by the McCall-Crabbs Standard Test Lessons in Reading. Thus, if your copy is addressed to a hypothetical audience with a fifth-grade educational level, the Fog Index of the tested copy should be no higher than 5.

The following table compares the Fog Index with reading levels by grade.

Fog Index Reading Level By Grade
17 College graduate
16 College senior
15 College junior
14 College sophomore
13 College freshman
- - - - -D A N G E R L I N E- - - - - - - - -
12 High school senior
11 High school junior
- - - -EASY READING BELOW THIS LINE - - -
10 High school sophomore
9 High school freshman
8 8th grade
7 7th grade
6 6th grade
5 5th grade

Two words of caution: A “sentence” in Gunning’s terms is not always the distance between a capital letter and a period, which makes it ideal for the punchy writing of direct mail copy. Units of thought (marked off by semicolons, colons, dashes or ellipsis points) are treated as sentences and so are independent clauses. An independent clause (sometimes called a main clause) is a group of words that make a complete statement, no matter what punctuation comes before or after. Such a clause can stand as a regular sentence, beginning with a capital letter and ending with a period. Or several independent clauses may be separated by semicolons or commas. (For example, J.N. Hook’s parody of Julius Caesar in his Guide to Good Writing contains three independent clauses: “She came, I saw, she conquered.”) What is a “word” sometimes causes trouble, too. In applying the Fog Index, we may define a word as anything with white space on either side of it.

If your copy tests 13 or more, you are beyond Gunning’s danger line of reading difficulty, and your readers are likely to find what you have written to be heavy going.

After measuring the readability of magazines, Gunning discovered a correlation between a magazine’s circulation and his Fog Index. Magazines with the highest mass appeal had the lowest average sentence length and the lowest percentage of difficult words.

Gunning also applied his formula to books and found a direct relationship between a low Fog Index and popularity. Still in print, Margaret Mitchell’s best-selling “Gone with the Wind” has a 6th grade reading level. “Peyton Place” also scores a 6. Mickey Spillane’s hard-boiled novels come in at 5, as do the works of Harold Robbins. J.D. Salinger, John Cheever and Truman Capote all average a Fog Index of 7, not markedly lower than Somerset Maugham, Sinclair Lewis or Ernest Hemingway. Harper Lee’s prize-winning novel “To Kill a Mockingbird” has a Fog Index of 5, which must account for its astonishing continuing sales in the millions and its presence on nearly every school reading list.

Applying his formula, Gunning gave English poet John Milton a Fog Index of 26. This was 11 points higher than Lincoln’s “Gettysburg Address,” which came in at 15. Gunning believed that 16 to 20 words was a good average word count for sentences in newspaper articles.”

A low Fog Index for copy you have written is merely an assurance that the piece can be easily read by the widest audience—that readers of it will be induced to move along from one sentence to the next without stumbling and without excessive demands on their attention. The Fog Index cannot make you or anyone a great writer. Rather it is a measure of complexity in writing—for determining whether a piece of copy is geared to its intended audience. The Fog Index is a measuring tool, but only for use after you have written, not as a pattern before you write. “Good writing must be alive,” Gunning advised. “Don’t kill it with a system.”

About Robert Scott
Bob Scott and I met when we were book club directors at Macmillan in the late 1960s. After leaving Macmillan, Bob became publisher at Stoeger Publishing Co. At that time, it was the publishing arm of a gun company whose list had fewer than 10 titles on guns and shooting. In the short space of a couple of years, Bob boosted Stoeger’s list to a hundred titles and broadened the areas of interest to include fishing, camping, pets, the outdoors and the environment. He’s the author of “Office at Home” (Scribner’s, 1985), one of the first books to anticipate the working-from-home revolution. For 18 years, Bob was a pro bono instructor in the Writer’s Digest School, teaching correspondence courses in basic and advanced nonfiction writing and conducting a workshop in book-proposal writing and book marketing. For a number of years Bob was consulting editor to my wife Peggy’s and my newsletter, WHO’S MAILING WHAT! where this essay on Gunning’s Fog Index first appeared.

A jack of all trades and master of most of them, he’s still going strong at the age of 88 as an occasional ghost writer, publishing consultant and literary agent. He’s also a working journalist, writing feature articles on local history and a column on political and military affairs for a Westchester weekly newspaper, The Gazette. His blog, Postscripts, is described as “an online large print magazine offering a pastiche of articles on current affairs, history, technics, opinion, advice, humor and trivia” and includes a regular department, “The Writing Clinic.” The blog’s address is www.notorc.com.