Flesch, Flesch–Kincaid, Gunning Fog and more, with the grade level they imply
Every index here rests on three counts: words, sentences and syllables. They come from first-language reading research, so they compare two passages well but say little in absolute terms.
Syllables are estimated from vowel groups, with a short list of common exceptions (people, science, idea…). Polysyllables have three syllables or more. The numbers are a guide, not a study plan.