Lazy answer It depends on the size on the size of integer Lets suppose 16 bits integer create an array with 65536 positions populate as follow Position Number ofset bits [0] = 0 [1] = 1 [2] = 1 [3] = 2 .... [FF] = 16 To get the number of set bits of a int, just use the int as the index of the array lets say number is 65 number of set bits = [65] I told you, Lazy Answer Not so lazy answer. Use assembly. Assume int is 16 bits 1) start W = 0 2) start R = 0 3) start N = Number 4) shift right N and collect the shifted bit in c bit 5) if c=1 -> add W,1 6) add R,1 7) if R<16 jump to 4) 8) Answer is in W