Readability formulas turn a text into a number using a few countable features: how long the sentences are, how long the words are, and how many “hard” words appear. They are quick, objective and surprisingly useful (as long as you remember what they can’t see).
The formulas
Flesch Reading Ease (1948)
Rudolf Flesch’s formula gives a score from roughly 0 to 100, where higher is easier:
206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words)
| Score | Difficulty |
|---|---|
| 90–100 | very easy (about age 11) |
| 60–70 | plain English (ages 13–15) |
| 30–50 | difficult (college) |
| 0–30 | very difficult (graduate, legal) |
Several US states use Flesch scores in plain-language rules for insurance policies.
Flesch–Kincaid Grade Level (1975)
Developed for the US Navy, it uses the same two inputs but outputs a US school grade:
0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59
A score of 8 means an eighth-grader (about 13–14) should follow it.
Gunning Fog Index (1952)
Robert Gunning counted complex words, three or more syllables, excluding proper nouns, hyphenated compounds and words that reach three syllables only through -es, -ed or -ing:
0.4 × [(words ÷ sentences) + 100 × (complex words ÷ words)]
SMOG (1969)
G. Harry McLaughlin’s “Simple Measure of Gobbledygook” is popular in health communication because it was validated against full comprehension:
1.043 × √(polysyllables × 30 ÷ sentences) + 3.1291
It was designed for samples of 30 sentences; shorter texts give less stable results.
Coleman–Liau (1975) and ARI (1967)
Both count letters or characters instead of syllables, which made them easy to compute on early machines:
Coleman–Liau: 0.0588 × L − 0.296 × S − 15.8 (L = letters per 100 words, S = sentences per 100 words)
ARI: 4.71 × (characters ÷ words) + 0.5 × (words ÷ sentences) − 21.43
LIX (1968)
Carl-Hugo Björnsson’s Swedish formula uses words longer than six letters, which makes it usable across many European languages:
words ÷ sentences + 100 × (long words ÷ words)
What the formulas can’t see
- Meaning. “The cat sat on the mat” and “Is gnosis a ruse?” score similarly.
- Familiarity. “Television” is four syllables but easy; “ennui” is two but hard.
- Structure. Headings, lists and white space make text easier without changing the score.
- Context. Technical readers handle jargon fine; a score can’t know your audience.
How to use them well
- Pick a target by audience. Grade 6–8 for the general public; lower for health information; higher is acceptable for specialists.
- Watch the trend, not the decimal. Edit, re-score, compare.
- Fix causes, not numbers. Split long sentences, prefer common words, turn noun stacks into verbs.
- Test with readers when it matters.
Sources
- Flesch, R. (1948). A new readability yardstick. Journal of Applied Psychology, 32(3).
- Kincaid, J. P., et al. (1975). Derivation of new readability formulas for Navy enlisted personnel. Naval Technical Training Command.
- McLaughlin, G. H. (1969). SMOG grading: A new readability formula. Journal of Reading, 12(8).
- Coleman, M., & Liau, T. L. (1975). A computer readability formula. Journal of Applied Psychology, 60(2).
- Björnsson, C. H. (1968). Läsbarhet. Liber.