Greatest Common Factor Calculator


What is the Greatest Common Factor (GCF)?

The Greatest Common Factor (GCF), also known as the Greatest Common Divisor (GCD), of two or more numbers is the largest positive integer that divides all the numbers without leaving a remainder.

Prime Factorization Method

The Prime Factorization method for calculating the Greatest Common Factor (GCF) involves breaking down the numbers involved into their basic prime factors, then comparing these factors to find the largest ones they have in common. Here is a step-by-step guide on how to use this method:

Steps for the Prime Factorization Method:

  • List the Prime Factors: Break down each number into its prime factors. You can do this by dividing the number by prime numbers starting from the smallest (2, 3, 5, 7, 11, ...) until you are left with 1.
  • Create Prime Factor Trees (optional): It can be helpful, especially for visual learners, to create a factor tree for each number. This is a branching diagram where you write the number at the top and draw branches for each factorization step until you reach the prime numbers at the ends.
  • Identify Common Factors: Compare the lists of prime factors and circle or note the factors that appear in all lists. Be sure to include the smallest power of each common prime factor if they appear multiple times.
  • Multiply the Common Prime Factors: Multiply these common prime factors together to find the GCF.


Let's find the GCF of 84 and 120 using prime factorization. First, we break down each number into its prime factors:

  • 84 = 2×2×3×7 = 22×3×7
  • 120=2×2×2×3×5=23×3×5

Now, identify the common prime factors. In this case, the common prime factors of 84 and 120 are two 2's and one 3:

  • Common Prime Factors: 22×3

Next, multiply these together:

  • GCF: 2×2×3=4×3=12

So, the GCF of 84 and 120 is 12.

Listing the Factors Method

The "Listing the Factors" method to calculate the Greatest Common Factor (GCF) involves listing out all the factors of the numbers you're considering and then identifying the largest factor that they have in common. Here's how you do it step by step:

Steps for the Listing the Factors Method:

  1. List All Factors of Each Number: Write out all the numbers that divide into each of your given numbers with no remainder. These are the factors of each number.
  2. Identify Common Factors: Compare the lists and find the factors that are common to all the numbers.
  3. Select the Greatest Common Factor: From the list of common factors, choose the largest one. This is the GCF.


Let's find the GCF of 30 and 45 using the listing method.

  1. List All Factors:
    • Factors of 30: 1, 2, 3, 5, 6, 10, 15, 30
    • Factors of 45: 1, 3, 5, 9, 15, 45
  2. Identify Common Factors:
    • The common factors of 30 and 45 are: 1, 3, 5, 15
  3. Select the Greatest Common Factor:
    • The largest factor that 30 and 45 have in common is 15.

So, the GCF of 30 and 45 is 15.

Euclidean Algorithm Method

The Euclidean Algorithm is a time-honored technique for finding the greatest common factor (GCF) of two numbers. It's based on the principle that the GCF of two numbers is also the GCF of their difference. This process repeats until the remainder is 0, at which point the GCF is the last non-zero remainder.

Steps of the Euclidean Algorithm:

  1. Begin with Two Numbers: Identify the two numbers, a and b, such that a > b, for which you want to find the GCF.
  2. Divide a by b: Perform the division of a by b, and note the remainder r. Mathematically, this is a = bq + r, where q is the quotient.
  3. Update a and b: Replace a with b and b with r (the remainder).
  4. Repeat the Division: Continue the process of dividing the new a by the new b and updating them accordingly.
  5. Result When Remainder is Zero: When you reach a remainder of 0, the divisor (b) at that step is the GCF of the original two numbers.


Let's use the Euclidean Algorithm to find the GCF of 48 and 18.

  1. Divide 48 by 18: 48 ÷ 18 = 2 with a remainder of 12.
  2. Now, replace 48 with 18 (the previous divisor), and 18 with 12 (the remainder): 18 ÷ 12 = 1 with a remainder of 6.
  3. Next, replace 18 with 12, and 12 with 6 (the new remainder): 12 ÷ 6 = 2 with a remainder of 0.

Since the remainder is now 0, we stop. The divisor at this step, which is 6, is the GCF of 48 and 18.

So, the GCF of 48 and 18 is 6.

This algorithm is particularly powerful for large numbers because it significantly reduces the amount of computation compared to other methods like listing all factors. It's an excellent example of how a simple mathematical algorithm can be applied to solve problems that at first seem computationally intensive.

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