In more detail
The formula is 206.835 − 1.015 × (words / sentences) − 84.6 × (syllables / words). 90 to 100 is very easy, 60 to 70 plain English and 30 to 50 difficult. The related Flesch-Kincaid grade gives a US school level: 0.39 × (words / sentences) + 11.8 × (syllables / words) − 15.59. Rudolf Flesch published the score in 1948. It looks only at sentence length and word length, so it says nothing about grammar, structure or whether the text is correct, and tools that count syllables automatically may disagree by a few points.
Why it matters
Shorter sentences and shorter words make instructions, contracts and web copy easier to follow, and many style guides aim for 60 or above for the general public. The score is calibrated for English: Spanish and French texts score differently because of their longer words, so adapted formulas such as Fernández Huerta (Spanish) or Kandel-Moles (French) are used instead.
Example
| Text | Words / sentences / syllables | Score |
|---|---|---|
| "The cat sat on the mat." | 6 / 1 / 6 | 116.1 |
| "The cat sat on the mat. It was happy there." | 10 / 2 / 11 | 108.7 |
| "Organisational transformation necessitates comprehensive stakeholder communication." | 6 / 1 / 26 | −165.9 |
The score is not limited to 0-100: very simple text goes above 100 and very dense text below 0. Check your own copy with the Readability Score Checker.