40 = 23 x 5.
40 has a representation as a sum of two squares: 40 = 22 + 62.
A regular 40-gon is constructible with straightedge and compass.
40 is 1111 in base 3.
40 can be partitioned in 31,185 ways.
40 is the number of equivalent hyperspheres in five dimensions that can touch an equivalent hypersphere without any intersection. Hence, 40 is the kissing number in five dimensions.
According to the Bible, rain fell for 40 days and 40 nights during Noah's flood.
40 is the only number in English whose letters are in alphabetical order.