Readability Scores Explained: Flesch, Gunning Fog, and What They Can’t Tell You
How readability formulas actually compute their numbers, what a "grade 8" score means, why they are blind to meaning, and how to use word and sentence counts to write clearer prose.
Every formula measures the same two things
Flesch Reading Ease, Flesch–Kincaid Grade, Gunning Fog, SMOG, Coleman–Liau, ARI — they differ in constants and in whether they count syllables or characters, but under the hood each one combines sentence length and word length. Long sentences and long words push the score toward "hard"; short sentences and short words toward "easy." That is the entire model. Flesch Reading Ease runs 0–100 (higher is easier; 60–70 is plain English, most newspapers sit there); the grade-level formulas output the US school grade notionally needed to follow the text.
What the numbers are blind to
- Meaning. Rearrange a paragraph’s words at random and its score barely moves — the formula counts, it does not read.
- Jargon. "Cat" and "API" are both short; one is easy, one requires context.
- Structure. Headings, lists and whitespace drastically change how readable a page *feels* and count for nothing.
- Audience. A grade-12 score is a problem for a consumer app and normal for a legal filing.
Use scores as a smoke alarm, not a judge: a 30-word average sentence is a real signal that readers will struggle, regardless of what they are reading about.
The counts that actually help you edit
Three numbers do most of the practical work. Average sentence length: aim for 15–20 words for general audiences, and vary it — monotony is as tiring as length. Reading time: at 200–250 words per minute for prose, it sets expectations ("5 min read") and tells you when a section is overstaying its welcome. Keyword density: useful to catch a word you are leaning on unconsciously ("very", "really", "leverage"), not as an SEO dial — modern search engines penalize stuffing and reward natural language.
Plain-language habits that move every score
- One idea per sentence; if there is an "and which" in it, split it.
- Prefer the short Anglo-Saxon word to the Latin one: use, not utilize; help, not facilitate.
- Lead with the point, then the qualification — readers skim the first clause.
- Cut hedges and intensifiers; "very unique" is two mistakes.
- Read it aloud. Anywhere you run out of breath is a sentence to cut.