What is the GCF of 21 and 28?
GCF (Greatest Common Factor) of the numbers 21 and 28 is the largest possible number which divides 21 and 28 exactly without any remainder.
Answer: GCF of 21 and 28 is 7
We will explain the following two methods to find the greatest common factor of 21 and 28
Methods to find GCF of 21 and 28

Prime factorization Method

Listing the common factors Method
Method 1: GCF of 21 and 28 by Prime Factorization
Let us represent 21 and 28 as a product of their prime factors:
Prime factorization of 21 is 3 x 7
Prime factorization of 28 is 2 x 2 x 7
Common factors = 7
GCF is the product of the factors that are common to each of the given numbers.
GCF of 21 and 28 is 7
Method 2: GCF of 21 and 28 by Listing the Common Factors
Factors of 21: 1, 3, 7, and 21
Factors of 28: 1, 2, 4, 7, 14, and 28
Common Factors of 21 and 28: 1 and 7
The Greatest Common Factor of 21 and 28 from the list is 7
GCF of 21 and 28 is 7
By using any of the above methods, the answer to GCF of 21 and 28 is the same.