How to find GCF (Greatest Common Factor)

To find the GCF of a number. You need at least two numbers. List the prime factors of each number and multiply the factors they have in common.

For an example we will use 12 and 20 for our numbers.

Now to get the prime factors.

12

2 * 6 = 12

2 is prime

3 * 2 = 6

3 is prime

2 is prime

20

2 * 10 = 20

2 is prime

2 * 5 = 10

2 is prime

5 is prime

The prime numbers that they have in common is 2.

So we multiply 2 * 2.

One from each number we are factoring.

GCF = 4