A decrease of one grade level in readability thus comes from ~10 additional words per sentence or ~0.11 additional syllables per word.
This misstates the Flesch-Kincaid math. In the standard formula, one grade-level change corresponds to about 2.6 words per sentence or 0.085 syllables per word, and adding either of those makes text harder, not easier.
Full reasoning
Microsoft’s documentation gives the Flesch-Kincaid Grade Level formula as:
(.39 × ASL) + (11.8 × ASW) - 15.59
where ASL is average sentence length and ASW is average syllables per word.
From that formula:
- changing the grade level by 1.0 via sentence length alone requires
1 / 0.39 ≈ 2.56additional words per sentence, not ~10; - changing it by 1.0 via syllables per word alone requires
1 / 11.8 ≈ 0.085additional syllables per word, not ~0.11.
The direction is also backwards: because both coefficients are positive, adding words per sentence or syllables per word raises the Flesch-Kincaid grade level (i.e. makes the text harder), rather than producing a decrease in grade level.
So the post’s sentence is numerically incorrect and reverses the sign of the relationship.
1 source
- Get your document's readability and level statistics - Microsoft Support
The formula for the Flesch-Kincaid Grade Level score is: (.39 x ASL) + (11.8 x ASW) - 15.59, where ASL = average sentence length ... ASW = average number of syllables per word.