In July 2008, I wrote an article titled ‘The Average Sentence Length’. It has been the most popular of all my articles in Readability Monitor; and so I was tempted to revisit it. Hence this attempt titled ‘Syllable, Word And Sentence Length’.
A text is composed of letters (L), syllables (S) and words (W); and text difficulty is directly proportional to these independent variables. Though there is no law that governs the exact distribution of L and S and W, a sample passage of 100 syllables has on the average 300 letters and 60 words. The average syllable length is 3 letters and 0.6 words.
The average distribution of L and S and W in a sample passage resolves into this beautiful, though not exact, equation: 1 x L = 3 x S = 5 x W. In specific terms, L1 = S3 = W5 (L1, S3, and W5 being the number of letters in one sentence, syllables in three sentences and words in five sentences). Dividing by 10, we get L1/10 = S3/10 = W5/10. Three of my readability formulae the Character-count Index, the Strain Index and the Higher Grade Level are an outcome of this equation.
The average word length derived from the above equation is 5 letters per word; it is also 5/3 or 1.66… syllables per word.
Martin Cutts recommends an average sentence length (ASL) of 15 to 20 words. If W = 15, then ASL = 25 syllables and 75 letters. If W= 20, then ASL = 33.33… syllables and 100 letters. So, ASL = 15 to 20 words; 25 to 33.33… syllables; and 75 to 100 letters. If we take the midpoints and ignore the decimals, then ASL = 17 words; 29 syllables; and 87 letters.
Now for a quick summary. The average syllable length = 3 letters. The average word length = 1.66 syllables and 5 letters. The average sentence length = 17 words; 29 syllables; and 87 letters.