Flesch Reading Ease

The Flesch reading ease score rates text readability from 0 to 100 using average sentence length and average syllables per word. Higher scores mean easier reading; 60-69 is standard, readable by high school students.

The numbers

Definition

The Flesch reading ease score rates how easy a text is to read on a scale that runs, in practice, from 0 to 100, using two measurable inputs: average sentence length in words (ASL) and average syllables per word (ASW). Higher scores mean easier text.

Because it relies only on counts, the score is fully deterministic: the same text always produces the same number, with no model, no training data, and no judgement involved. That is why it appears everywhere from word processors to SEO tools such as Yoast, which builds it into its readability analysis.

The formula

As given by Yoast, the score is 206.835 - (1.015 x ASL) - (84.6 x ASW), where ASL is total words divided by total sentences and ASW is total syllables divided by total words.

The weights matter. Adding one word to the average sentence costs 1.015 points, while adding a tenth of a syllable to the average word costs 8.46 points: word complexity moves the score roughly eight times faster than sentence length does. Both levers are worth checking before you rewrite anything.

Score bands

Yoast publishes the standard interpretation table. 90-100: very easy, understandable by an average 11-year-old student. 80-89: easy, middle school students. 70-79: fairly easy, teenagers aged 13 to 15. 60-69: standard, high school students.

50-59: fairly difficult, college students. 30-49: difficult, university graduates. 0-29: very confusing, best understood by specialists, academics, or experts. Yoast declines to set one universal target: "You should care about your score, but do not chase perfection. Balance readability with your audience's needs."

Limitations

The score "only looks at sentence and word length, ignoring context, tone, and audience", as Yoast puts it. A string of short, familiar words scores well even when the argument is incoherent; a long, well-signposted sentence scores badly even when it reads smoothly.

It also folds two causes into a single number, so a low score does not say whether sentences or vocabulary are the problem; Yoast now reports sentence length and word complexity separately for exactly that reason. The underlying correlation is real but coarse: ProWritingAid notes that as average sentence length increases, the grade level required to read the text also increases.

In the ScriptGrain analyser

Readability is one of the attributes ScriptGrain's free analyser at scriptgrain.com/tools/writing-style-analysis measures client-side: it counts your sentences, words, and syllables, then applies the Flesch arithmetic in the browser, so the score is inspectable and reproducible.

Read the result against your audience rather than a universal ideal. ProWritingAid notes that "most published fiction has an average grade level of 4 to 6", accessible to ages 9 to 12, while specialist material can sit far lower on the Flesch scale and still be exactly right for its readers.

Worked example

A two-sentence passage of 15 words each: "The analyser counts every syllable in the passage and then divides by the word total. Longer words and longer sentences push the score down, while shorter ones lift it higher." Hand counts: 30 words, 2 sentences, 45 syllables (24 in the first sentence, 21 in the second).

ASL = 30 / 2 = 15 words per sentence. ASW = 45 / 30 = 1.5 syllables per word. Score = 206.835 - (1.015 x 15) - (84.6 x 1.5) = 206.835 - 15.225 - 126.9 = 64.71.

A score of 64.71 lands in Yoast's 60-69 standard band, readable by high school students. Note the leverage: cutting ASW from 1.5 to 1.4 would add 8.46 points, over eight times the effect of trimming one word from the average sentence length.

Sources

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