Flesch Reading Ease and Flesch-Kincaid Grade Level: how the two readability formulas differ

Two classic readability formulas, Flesch Reading Ease and Flesch-Kincaid Grade Level, use the same inputs of words, sentences and syllables but weight them differently, so one scores higher for easier text while the other scores higher for harder text. The Reading Ease score is roughly 50 to 80 for everyday prose, while the Grade Level maps to a US school grade. The article works through both formulas by hand on two sample texts, one plain and one dense, and shows that the simpler text scores about 88 on Reading Ease and grade 2.5, while the dense text goes negative and above grade 20. It also provides a short JavaScript implementation. Tools often disagree because the formulas are fixed but syllable counting and sentence detection rely on heuristics that vary between implementations.