List the numbers whose GCF you're trying to find vertically on a piece of paper. For example, suppose you want to find the greatest common factor of 72 and 96.
Write the factors for your first number. Remember that factors are numbers that can go into the larger number evenly. It is sometimes easier to list factors by pairs. Then you can easily rearrange them in numerical order. For example, the multiples of 72 are 1 x 72, 2 x 36, 3 x 24, 4 x 18, 6 x 12 and 8 x 9. You can list these factors as follows:
1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72
Write out the multiples for the second number. In the example, you would write, "1 x 96, 2 x 48, 3 x 32, 4 x 24, 6 x 16 and 8 x 12." So in order, the factors would be:
1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 96
Compare the lists of factors. Find the largest factor that is in all of the lists. This is your greatest common factor. Concluding the example, the GCF would be 24 because that is the largest number in both lists.