You can use Microsoft Word to do this.

a. Go to File>Options (make sure you use scroll bar all the way to right to scroll to bottom)>Proofing.

b. Under “When correcting spelling…”, check the box labeled “Show readability statistics.”

c. After that, go to “Review” on top menu bar and click “Spelling & Grammar” on far right.

d. Some box (or a few) might come up about conciseness or something else, click “Ignore once.”

e. Then another box will appear called “Readability Statistics.” Look at “Averages” and you will find your sentence average, as well as average sentence per paragraph and average character length for words.

If you don’t have Microsoft Word you can use analyzemywriting.com and paste your writing there and click “Analyze Text!”. Toward bottom, the last chart “Sentence Length” gives you the mean and median for your sentences (as well as a distribution of all of your sentence according to number of words).