Estimate reading level using Flesch scores.
Both classic Flesch measures use the same two inputs — average sentence length and average syllables per word — because those are the strongest simple proxies for reading difficulty. Reading Ease runs from 0 to 100 with higher meaning easier, where 60 to 70 is plain English suitable for a general audience. Grade Level converts the same information into US school years, so 9.0 means a typical ninth-grader can follow it. Syllable counting is estimated by vowel groups, which is accurate to within a few percent on ordinary prose but under-counts unusual words, so an override is offered.
Flesch Reading Ease
Ease = 206.835 - 1.015 x words per sentence - 84.6 x syllables per word
Flesch-Kincaid Grade Level
Grade = 0.39 x words per sentence + 11.8 x syllables per word - 15.59
Grade level for communicating with a specific audience — public health material commonly targets grade 6 to 8, general journalism grade 9 to 11. Reading Ease is more useful as a single comparative score across drafts, since higher always means easier without needing a schooling reference.
Because working memory holds a clause while waiting for its resolution. A 40-word sentence forces the reader to keep several dependencies in mind at once, which is tiring regardless of how simple the individual words are. Splitting long sentences is usually the fastest way to lower a grade level.