Measure reading levels with readability indexes (2024)

How to calculate readability with the Flesch Reading Ease, Flesch-Kincaid and Gunning Fog tests

Since 1847, scholars and others have been measuring how hard copy is to read ¹. Over the years, these folks have created some 200 readability indexes — from the Flesch to the Fry, from the Fog to the SMOG, from the Spache to the LIX.

All of these indexes boil readability down to a mathematical formula that shows how well your readers can comprehend the text. Although the formulas are different, they usually rely heavily on two factors:

• Sentence length — syntactic, or structural, difficulty. Most formulas measure the average number of words per sentence.
• Word length — semantic, or meaning, difficulty. Most formulas measure the average number of syllables or number of characters per word.

There are a million indexes that measure readability of English text out there, from the SMOG Index to the FOG test, from the Automated Readability Index to the Coleman Liau Index. Here are three of the most popular formulas for determining how well your readers can understand the text:

1. Flesch Reading Ease

In 1946, lawyer, author and writing consultant Rudolph Flesch published a readability formula in his dissertation, “Marks of a Readable Style.” That formula, the Flesch Reading Ease index, was the original Flesch test. The formula is:

206.835 – (1.015 x words per sentence)
– (84.6 x syllables per word)
= reading ease

Flesch’s work with the Associated Press helped bring the reading level of front-page newspaper stories down by five grade levels. Publishers increased readership by 40% to 60% with the formula. Today, the Flesch test is one of the most widely used, most tested and most reliable readability formulas. U.S. Department of Defense, government agencies and Florida use this Flesch test.

Scores range from 0 to 100. The higher the score, the easier your message is to read.

Aim for 60 or higher. To increase your score, reduce the length of your sentences and words.

2. Flesch Kincaid Grade Level

In 1976, the U.S. Navy commissioned J. Peter Kincaid and his team to recalculate the Flesch Reading Ease to help sailors read Navy training manuals faster and understand them better.

The resulting formula ³ — The Flesch-Kinkaid Grade Level — is:

(.39 x average number of words per sentence)
+ (11.8 x average number of syllables per word)
– 15
= reading grade level

The Flesch-Kincaid Test is now a standard for the U.S. Department of Defense, the Internal Revenue Service and the Social Services Administration. Plus, many states now require insurance policies and other legal documents to weigh in at no higher than a 9th grade reading level on the Flesch-Kincaid formula.

Theoretically, the score bottoms out at -3.40. Dr. Seuss’s Green Eggs and Ham comes close: With 5.7 words per sentence and 1.02 syllables per word, it achieves a grade level of -1.3.

The lower the score, the easier your message is to read.

Aim for 8th grade or lower. To improve your score, reduce your average sentence length and word length.

3. Gunning Fog Index

In the mid-1930s, textbook publisher Robert Gunning realized that much of America’s reading problem was actually a writing problem. He found that news and business writing was full of “fog,” or unnecessary complexity. ⁴

In 1944, he founded the first readability consulting firm, consulting with more than 60 newspapers and magazines. He also correlated magazine reading levels with total circulation. (The lower the Fog, the higher the circulation.)

He developed the Fog Index in 1952. That formula is:

Words per sentence
+ (100 x percentage of words with three or more syllables)
x .4= reading grade level

Gunning worked with the United Press, helping bring the reading level of front-page newspaper stories by five grade levels. He also helped The Wall Street Journal reduce its level from 14th to 11th grade. In the process, the Journal’s circulation rocketed from less than 50,000 to more than 1 million in a decade.

Keep your score in the single digits. To improve your score, make your sentences and words shorter.

^[1] DuBay, Smart Language: Readers, Readability, and the Grading of Text (PDF), Impact Information, Jan. 25, 2000

^[2] Rudolph Flesch, The Art of Readable Writing, Harper (New York), 1949

^[3] Dubay, Dubay, The Principles of Readability (PDF), Impact Information, Aug. 25, 2000

^[4] William H. Dubay, The Principles of Readability (PDF), Impact Information, Aug. 25, 2004

Related