If I'd googled it with the right spelling I might have found something, doh!
Reading Ease
Scores of 90.0–100.0 are considered easily understandable by an average 5th grader. 8th and 9th grade students could easily understand passages with a score of 60–70, and passages with results of 0–30 are best understood by college graduates.
Reader's Digest magazine has a readability index of about 65,
Time magazine scores about 52, and the
Harvard Law Review has a general readability score in the low 30s. The highest (easiest) readability score possible is 121 (every sentence consisting of a one-syllable word); theoretically there is no lower bound on the score.
Grade Level
The
"Flesch–Kincaid Grade Level Formula" translates the 0–100 score to a
U.S. grade level.
The result is a number that corresponds with a grade level. For example, a score of 8.2 would indicate that the text is expected to be understandable by an average student in 8th grade.
The lowest grade level score in theory is -3.4, but, since there are no real passages that have every sentence consisting of a one-syllable word, this never occurs in practice.
Green Eggs and Ham by Dr. Seuss comes close, averaging 5.7 words per sentence and 1.02 syllables per word, with a grade level of -1.3. (Most of the 812 words are monosyllabic: only one word ("anywhere"), which occurs eight times, is not.)
So how does everyone fare on this?