Readability: Why are long sentences over 20 words?

Siteimprove relates these word counts within a sentence to how difficult they are to read: 

• Fairly difficult (Medium long sentence length): 21-25 words
• Difficult (Long sentence length) : 25-30 words
• Very Difficult (Very long sentence length) : 30-40 words
• Extremely difficult (Extremely long sentence length) : 40+ words

Studies from the American Press Institute have shown that when the average sentence length is 14 words, readers understand more than 90% of what they are reading. At 43 words, comprehension drops to less than 10%. Long, complicated sentences force users to slow down and work harder to understand what they are reading. This is not something people want to do, even if they are familiar with the subject or language you are using. 

Similar research from the American Press Institute shows that readers rate the following difficulty levels based on the number of words per sentence (*Sources: “Readers’ Degree of Understanding,” American Press Institute):

Martin Cutts, the author of the book Oxford Guide To Plain English, offers the following guideline: “Over the whole document, make the average sentence length 15-20 words.” And what is the reason? He explains: "More people fear snakes than full stops, so they recoil when a long sentence comes hissing across the page."

Note: Some readability algorithms (LIX, SMOG, Flesch-Kincaid, Gunning Fog) ignore digits such as dates and phone numbers, while others include them (CLI, ARI). Therefore, the same sentence might not be considered "long" for all algorithms. Learn more about the different readability tests.

Additional resources for Readability

• Readability Tests 
• Why are polysyllabic words hard to read?
• What are complex words and how are they determined?