There are two ways. The direct one is to test the whole numbers from 1 up to the square root of the number: each time the division is exact you gain two divisors, the tested one and the quotient. For 36, testing 1, 2, 3, 4, 5 and 6 gives the pairs (1, 36), (2, 18), (3, 12), (4, 9) and (6, 6). Factoring is faster for large numbers: break the number into primes and combine the powers, which is what the Step by step tab shows.