How Do You Find the Greatest Common Factor?

To find the greatest common factor of two or more numbers, find the factors of each number, then circle the shared factors. Choose the greatest factor that the numbers share.

  1. List the factors of each number

    Write the numbers so that one number is on top of the others. Make a vertical line to the right of the numbers. For each number, list its factors starting with the smallest factor. Factors are numbers that equal a whole number when they are divided into the original number. The numbers 3, 9 and 18 are factors of 18.

  2. Find the shared factors

    Determine which factors the numbers share, then circle the common factors.

  3. Choose the largest factor

    Determine which common factor is the largest. This is the greatest common factor. To find the greatest common factor of the numbers 12, 16 and 32, begin by listing the factors of 12, which are 1, 2, 3, 4, 6 and 12. The factors of 16 are 1, 2, 4, 8 and 16. The factors of 32 are 1, 2, 4, 8, 16 and 32. These numbers share the factors 1, 2 and 4. The greatest of these numbers if 4, so it is the greatest common factor of 12, 16 and 32.