The weight of a word is determined by assigning A=1, B=2 through Z=26 and adding the letter values together. The density, or average letter value, of a word is found by dividing its weight by the number of letters in the word. Words with a low density are 'lightweight' words; those with a high density are 'heavyweight' words.
"Lighter and Heavier,"
Word Ways: Vol. 36
, Article 9.
Available at: http://digitalcommons.butler.edu/wordways/vol36/iss3/9