There are about two million words in the English language and exactly twenty-six letters in the English alphabet. Therefore, the average English word should be less than five letters long, since there are over eleven million words that can be spelled with five letters. However, in fact the average length of an English word is around twelve letters.
Word Ways: Vol. 33
, Article 6.
Available at: http://digitalcommons.butler.edu/wordways/vol33/iss4/6