The score in English
206.835 - 1.015 × (words ÷ sentences) - 84.6 × (syllables ÷ words)
That is the Flesch reading ease formula.
Short sentences and short words push the score up, and most writing lands between 30 and 80.
- 80 and up: very easy - children's books and good ads.
- 65 to 79: easy. Most blogs, captions and newsletters should aim here.
- 50 to 64: plain English, about the level of a newspaper.
- 30 to 49: a bit heavy. Fine for experts, tiring for everyone else.
- Under 30: hard going. Contracts live here.
The score in Spanish
206.835 - 62.3 × (syllables ÷ words) - (words ÷ sentences)
Spanish words carry more syllables, so Flesch's formula marks a plain Spanish text 30 to 50 points too low.
This is Szigriszt-Pazos's version of it, rebuilt for Spanish in 1993, and it is read on its own scale, called INFLESZ.
The checker picks the formula from the text itself, whatever language this page is in.
- 80 and up: very easy.
- 65 to 79: fairly easy. A blog, a caption or a newsletter should land here.
- 55 to 64: normal - the scale's own word for the middle.
- 40 to 54: a bit heavy.
- Under 40: very hard.
What it cannot do
It counts syllables by looking for written vowels, and each formula was set on one language's readers, so the score only works in English, Spanish, French, Portuguese and German.
Hebrew and Arabic leave most vowels unwritten, and the formula would hand any Hebrew text a perfect score.
So for every other language the checker gives no score at all, and still counts the words, the sentences and the long ones.
It cannot tell good writing from bad, either.
"The cat sat. It was fat." scores off the top of the scale and says nothing.
What we actually use
The long-sentence list, more than the score.
A sentence past 25 words usually holds two thoughts, and splitting it in two almost always reads better.
Reading time assumes 200 words a minute, the usual figure for reading on a screen.