How to Find Common Denominators

By Andrea Hermitt

In math, when dealing with fractions, it is often necessary to find the common denominator before doing any other math function. Common denominators are also used in other daily functions. Therefore, it is important to know how to find the common denominators. Here are the steps you should use. In this example, let us find the common denominators of 1/2, 1/3 and 1/5.

Write your first fraction down on paper, and then looking at the denominator, write down all multiples of that denominator. For example, if your first fraction is ½, then your multiples would be 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30… and so on.

Write your second fraction on the page below the first. Looking at the denominator, write down all multiples of that second denominator. So for your second fraction, of 1/3, your multiples would be 3, 6, 9, 12, 15, 18, 21, 24, 27, 30… and so on.

If you have a third fraction, which in this case you do, write it down below the other fractions and looking at your denominator of 1/5, your multiples would be 5, 10, 15, 20, 25, 30, 35, 40… and so forth.

Circle each number that the denominators of all three fractions have in common. For all three fractions, you would have the numbers 30, 60, 90, 120, and so forth in common. These would be common denominators of the set of numbers. If you were only looking to find the common denominators of ½ and 1/3, your common denominators would be 6, 12, 18, 24, 30 and so forth.

About the Author

Andrea Hermitt is an artist and writer who loves to research and write about new things. She's been a content writer since 2000, contributing to, the blog Notes From A Homeschooling Mom and other online publications. Hermitt has a Bachelor of Arts in fine art and English from the State University of New York at Albany.