To ensure your ideas are expressed clearly, aim for an average sentence length of 18 to 24 words -- as a very rough rule of thumb -- and very few sentences longer than four lines of text. Keep in mind that we are suggesting averages; some sentences will necessarily be longer and some should be significantly shorter. Be particularly careful, however, to avoid runs of short sentences. The following diagram illustrates the pattern for short sentences.
Some readers unconsciously make assumptions about the competence or intelligence of writers based on the length of their sentences (among other things). Note how easily you can deduce the approximate age of the writer of the following letter:
I watched your show called Discovery. It was about spiders. Don't kill the spiders. I know you have to catch them for medicine. But only catch some spiders. I just don't think it's fair. Also, you have to be very, very careful around tarantulas and black widows. Another reason why is because if you hurt them they'll hurt you. I like spiders. But to tell you the truth, not very much.
The writer's age is probably quite close to her average sentence length -- about 7. If your sentences average less than 15 words, you may persuade readers that you are young and inexperienced. Or your readers could assume that you are oversimplifying things because you think their ability to understand is limited. Either way, too many short sentences can alienate readers.
On the other hand, if your sentences are too long, you risk confusing and frustrating readers. Writing is like a balancing act: you do not want to lean too far in either direction. However, what counts as too long or too short depends on a number of things, including the age, education, and prior knowledge of your readers.
The range of your sentence lengths is at least as important as their average length. If all your sentences are of roughly the same length, you risk putting your reader to sleep. You take the same risk if all your sentences are similarly structured. Variety helps maintain reader interest.