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Common factors of two numbers are the whole numbers that divide both of them evenly. The largest one is the greatest common factor (GCF). This guide shows how to find every common factor and what they are good for.
What are common factors?
A factor of a number divides it with no remainder. A common factor of two numbers divides both. Every pair shares at least the factor 1.
How to find them
List the factors of each number with the Factors Calculator, then pick out the values that appear in both lists.
Worked example: 18 and 24
Factors of 18: 1, 2, 3, 6, 9, 18. Factors of 24: 1, 2, 3, 4, 6, 8, 12, 24. The common factors are 1, 2, 3 and 6.
Common factors vs the GCF
The biggest common factor — here 6 — is the GCF. In fact, every common factor of two numbers is itself a divisor of their GCF, which is a neat way to check your list.
A useful shortcut
Find the GCF first, then list its divisors: those are exactly the common factors. Since the GCF of 18 and 24 is 6, and the divisors of 6 are 1, 2, 3, 6, you instantly have every common factor.
Key takeaways
- Common factors divide both numbers evenly.
- The largest one is the GCF.
- Every common factor divides the GCF.
- List the GCF\u2019s divisors to get them all.
Factors Calculator
List every factor of a number so you can spot the ones two numbers share.
Open the Factors CalculatorFrequently asked questions
What are common factors of two numbers?
Whole numbers that divide both numbers evenly. Every pair shares at least the factor 1.
How are common factors related to the GCF?
The greatest common factor is the largest common factor, and every common factor is a divisor of the GCF.
What is the quickest way to list all common factors?
Find the GCF, then list its divisors — those are exactly the common factors of the two numbers.
The LCM Calculator Team
Math educators and engineers building free, accurate calculators with step-by-step solutions, visual diagrams and AI insights.