Flesch Reading Ease

Flesch Reading Ease is a readability score for English text, developed by Rudolf Flesch in 1948. It's based on two things: average sentence length and average number of syllables per word.

$\text{FRE} = 206.835 - 1.015 \left(\frac{\text{total words}}{\text{total sentences}}\right) - 84.6 \left(\frac{\text{total syllables}}{\text{total words}}\right)$

Higher scores mean easier text. Scores around 60 to 70 are considered plain English, while scores below 30 are very difficult to read. The Flesch-Kincaid Grade Level is a related formula that maps the same inputs to a US school grade.

Because it only counts surface features, it can't tell whether the text actually makes sense. See Cloze procedure: A new tool for measuring readability for an approach that tests real readers instead.