Fog = 0.4 x (A + L)
Where A is the average number of words (including short words like ‘a’ and ‘to’) in a sentence.
And L is the number of long words (of three or more syllables that are NOT (i) proper nouns, (ii) combinations of easy words or hyphenated words, or (iii) two-syllable verbs made into three with -es and -ed endings] per 100 words.
A Fog Index of 9 suggests the text should be readable by someone with 9 years of education. Twinkle, twinkle little star has a Fog Index of 3.