Compute Flesch Reading Ease and grade level.
Both Flesch measures run on exactly two ratios: average sentence length and average syllables per word. Reading Ease returns a 0-100 style score where higher is easier, with 60-70 corresponding to plain English; Flesch-Kincaid rearranges the same inputs to output a US school grade level instead. Because syllables per word carries a coefficient of 84.6 against 1.015 for sentence length, replacing long words moves the score far more than splitting sentences — though shorter sentences remain the easier edit to make.
Flesch Reading Ease
FRE = 206.835 - 1.015 x (words / sentences) - 84.6 x (syllables / words)
Flesch-Kincaid Grade Level
FKGL = 0.39 x (words / sentences) + 11.8 x (syllables / words) - 15.59
Most word processors and readability tools report syllable counts directly. Counting by hand, treat each vowel sound as one syllable and ignore silent final -e; 1.5 to 1.7 syllables per word is typical for general prose.
Aim for 60-70 Reading Ease for a general audience, which is roughly 8th to 9th grade. Technical documentation usually lands between 30 and 50, and pushing it much higher tends to require sacrificing precision.
They use the same two ratios with different coefficients and opposite directions, so a text heavy in long words but short sentences can look moderate on one scale and harder on the other. Report both rather than choosing whichever flatters the draft.