This article originally appeared in Science 80, a consumer magazine published by the journal Science. Over the years, it has circulated among thousands of editors, writers, and executives around the world.
By Terry Dunkle
Some years ago, as the story goes, a public information staffer at the National Bureau of Standards got an email from a New York plumber saying that he had found hydrochloric acid dandy for cleaning drains.
“The efficacy of hydrochloric acid is indisputable,” the bureaucrat wrote back, “but the ionic residues are incompatible with metallic permanence.”
“Thank you,” the plumber wrote back. “I thought it was a good idea, too.”
Reading this, the bureaucrat grabbed his phone. “Don't use hydrochloric acid!” he yelled at the plumber. “It eats hell out of the pipes!”
Foggy writing, according to Douglas Mueller, of the Gunning-Mueller Clear Writing Institute in Santa Barbara, California, costs American workers $1 billion a year in lost time, lost contracts, and lost customers. It also costs a good many people their promotions. In a recent survey, top executives of the Fortune 500 ranked communication skill as the most important quality for business leaders—ahead of technical, financial, and marketing abilities. “You can be a genius,” says Mueller, “but if you don't make yourself clear, you will fail.”
Mueller, formerly director of communications for the Borg-Warner Corporation, now travels around the country teaching managers, scientists, and engineers how to write—at $2,500 for a six-hour seminar. Clients say his most valuable tip is the Fog Index.
Devised 60 years ago by the late Robert Gunning, the Fog Index is a simple formula for quantifying the readability of English prose. Newspapers and magazines have used it to boost circulation. The federal government applies it to critical instruction manuals. Modern-day knockoffs are available in word-processing software, so that writers can check their readability with a mouse-click.
Even with pencil and paper, calculating a Fog Index takes only two or three minutes. Here's how to do it: Checking a sample of a least 200 words, find the percentage of Big Words—three syllables or more. (Don't count three-syllable words ending in ed or es, short-word compounds like butterfly and bookkeeper, or proper names.) Add the average sentence length in words, multiply by 0.4, and drop everything after the decimal point. The result is your Fog Index.
The paragraph above has a Fog Index of 9, meaning ninth graders could read it easily. The bureaucrat's email, with an index of 26, requires a Ph.D. and seven years of postdoctoral study.
What Fog Index should a writer aim for? That depends on the audience. Reader's Digest averages 10, the New Yorker 12, Time and Newsweek 11. Few magazines flourish above 13. Technical journals range a lot higher, but most are notoriously hard reading, even for specialists. Good technical memos, according to a study at Bell Laboratories, average only 14.
“The truth is,” says Mueller, “no matter what Fog Index your readers can tolerate, they prefer to get their information without strain. Any writer can drive his Fog Index under 15, with a little work. Einstein could. The trick is to keep your average sentence length under 20 and get rid of every useless word. And never use a Big Word unless you absolutely have to. Remember: the less energy your readers waste on decoding your language, the more they have left for your brilliant ideas.”
