One of the more useful techniques for simplifying expressions and numbers is finding the greatest common factor. The largest number (factor) that divides two or more numbers is called the **greatest common factor** or **GCF**. Two methods are used to find the greatest common factor. The first method includes writing down all the factors of two or more numbers. After that, we need to write all the common factors for each number. The greatest common factor of the numbers is the largest number in that list.

**First GCF method**

Let’s try this method on the next example. We need to find the __largest common factor for numbers 36 and 24__.

First, we need to list all the factors for each number. In order to find all the factors of the number 36, we need to see if the number can be divided by a number between 1 and 36. If 36 can be divided by that number without leaving a remainder, then that number is one of the factors of the number 36. So, let’s start:

-> 36 can be divided by 1 so one of the factors is **1**.

-> 36 can be divided by 2 so one of the factors is **2**.

-> 36 can be divided by 3 so one of the factors is **3**.

-> 36 can be divided by 4 so one of the factors is **4**.

-> 36 can’t be divided by 5.

-> 36 can be divided by 6 so one of the factors is **6**.

-> 36 can’t be divided by 7 and 8.

-> 36 can be divided by 9 so one of the factors is **9**.

-> 36 can’t be divided by 10 and 11.

-> 36 can be divided by 12 so one of the factors is **12**.

-> 36 can’t be divided by any number between 13 and 17.

-> 36 can be divided by 18 so one of the factors is **18**.

-> 36 can’t be divided by any number between 19 and 35.

-> 36 can be divided by **36** and this is the last factor.

We need to list the factors. They are 1 2 3 4 6 9 18 and 36.

We need to repeat the process for number 24:

-> 24 can be divided by 1 so one of the factors is **1**.

-> 24 can be divided by 2 so one of the factors is **2**.

-> 24 can be divided by 3 so one of the factors is **3**.

-> 24 can be divided by 4 so one of the factors is **4**.

-> 24 can’t be divided by 5.

-> 24 can be divided by 6 so one of the factors is **6**.

-> 24 can’t be divided by 7.

-> 24 can be divided by 8 so one of the factors is **8**.

-> 24 can’t be divided by 9, 10 or 11.

-> 24 can be divided by 12 so one of the factors is **12**.

-> 24 can’t be divided by any number between 13 and 23.

-> 24 can be divided by **24** and this is the last factor.

Now we need to list these factors. They are: 1 2 3 4 6 8 12 and 24.

Let’s compare them to the factors of 36: 1 2 3 4 6 9 12 18 and 36.

Now, we need to list the common factors for these numbers and pick the highest one: 1 2 3 4 6 12.

We can see that the *greatest common factor* is of 24 and 36 is 12.

**Second GCF method**

The second method for finding the greatest common factor is to list __all the prime factors for the two numbers__. After that, we need to multiply the common prime numbers and we will get the greatest common factor.

Let’s try this method out with the same numbers from the previous example.

First we need to list the prime factors of 36.

To find the prime factors, we need to start dividing 36 with the lowest possible number that can divide 36.

-> We first start with number 2. 2 can divide 36 and the result is 18. That means that 2 is the first prime factor.

-> 2 can divide 18 and the result is 9. That means that the second prime factor is 2.

-> 2 can’t divide 9. The next number that we should try is 3. 3 can divide 9. The result is 3, so 3 is the third prime factor.

-> 3 can divide 3. The result is 1. The fourth prime factor is 3.

-> Since the last result is 1, that means that the calculation of the prime factors is done.

-> The prime factors of number 36 are:

Using the same method, we can find the prime factors for 24.

The prime factors of 24 are:

The prime factors of number 36 are:

Now we need to pick the common prime factors. We can see that the common prime factors are 2,2 and 3. When we multiply we get 12 which is the greatest common factor.

## Greatest common factor worksheets

**GCF of two numbers up to 30** (92.3 KiB, 434 hits)

**GCF of two numbers up to 50** (97.8 KiB, 605 hits)

**GCF of two numbers up to 100** (112.0 KiB, 407 hits)

**GCF of two numbers up to 500** (146.7 KiB, 658 hits)

**GCF of two numbers up to 1000** (165.3 KiB, 1,416 hits)

**GCF of three numbers up to 30** (79.4 KiB, 495 hits)

**GCF of three numbers up to 50** (101.6 KiB, 330 hits)

**GCF of three numbers up to 100** (109.0 KiB, 1,436 hits)

**GCF of three numbers up to 500** (135.7 KiB, 1,274 hits)

**GCF of three numbers up to 1000** (189.3 KiB, 1,305 hits)