The average sentence length of your text is found by counting the number of words in sentences and dividing by the number of sentences. But, just as for average word length, this is only a rough measure of how difficult or easy the text will be to read.
The ideal average sentence length depends on the audience. In general, the broader the audience, the shorter the sentences should be: an average of 15 words per sentence for text for a general audience and no more than 25 words per sentence for more technical text. Your content is more likely to be understood by those with limited English if it is expressed in shorter sentences.
This does not mean that shorter is always better. Even very short sentences can be less readable if they chop the material up too much. A paragraph of 4 very short sentences may actually be less readable and convey less meaning than a paragraph of 2 longer sentences:
It was hard to sleep. I had a job interview. I was nervous. I really wanted the job.
It was hard to sleep because I was nervous. I had a job interview in the morning, and I really wanted the job.
A mix of shorter and longer sentences that are connected is the best way to keep readers engaged. Too many short sentences can lose meaning; too many long ones lose the connections between sentences in a haze of words. See Clear and appropriate language for further discussion of sentence lengths for different publications.
