What the scores measure
Every formula uses the same two things: how long your sentences are and how long your words are. Flesch Reading Ease (1948) and the Flesch-Kincaid grade level (developed for the US Navy in 1975) count syllables. Gunning Fog (1952) and SMOG (1969) count words of three or more syllables, and SMOG needs a sample of 30 sentences. Coleman-Liau (1975) counts letters instead (formulas and history). None of them can tell whether a text is clear, accurate or useful.
What to aim for on a web page
Nielsen Norman Group usually recommends a 6th to 8th grade level for a general audience, and says 10th to 12th grade suits experts (NN/g). On Flesch Reading Ease, 60 to 70 is plain English. Don’t chase a number: a Portent study of 756,297 ranking pages found top-30 content averaging around the 11th grade, with no link between reading level and position (Portent).
Does Google use readability scores?
Google hasn’t said it does. In a January 2018 hangout, John Mueller said that as far as he knew Google has no basic algorithms that count words to work out a reading level (Search Engine Roundtable). Google’s SEO starter guide asks for text that is easy to read, well organized and free of spelling and grammar mistakes, and mentions no scores. That was one answer eight years ago, so it’s a data point, not a guarantee. Use the numbers to find hard sentences, not as a ranking target.
How we count
- Words and sentences are split the same way as in the word counter. Lines wrapped in the middle of a sentence (text from a PDF or email) are joined. Headings and list items count as sentences, so paste body text for the cleanest score.
- Links and email addresses aren’t words, and numbers count as one syllable. Acronyms in capitals, like SEO and API, are read letter by letter and left out of Fog’s complex words, which can put Flesch a point or two away from other tools.
- Fog counts three-syllable words once -ing, -ed and -es are taken off, and skips names that don’t start a sentence. SMOG counts every word of three syllables or more.
- We flag sentences over 25 words. That’s our line, not a rule: Oregon’s plain-language guide aims for 14 to 20 (Oregon DAS). Passive voice and -ly adverbs are heuristics: use them as prompts to look, not as errors.
Using it to edit
Split the longest sentences first, then swap long words for short ones where the meaning holds. For length and reading time use the word counter; to see which phrases a draft repeats, the keyword density checker; for titles, the headline analyzer.