How to Find Common Denominators

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.

Instructions

1. 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.

2. 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.

3. 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.

4. 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.

Tips & Warnings

• If you wanted to find the least common denominator, you would look for the smallest number that all three fractions would fit into and that number would simply be 20.

• If you wanted to quickly the denominator in your head, and did not necessarily need the least common denominator, you could just multiply the denominators as the product of 2, 3, and 5 is 30, and also a denominator.