Find the common denominator of a set of fractions by identifying the denominators of the fractions, creating a list of the multiples of those denominators, and then choosing the smallest multiple that both denominators share. The common denominator is used to add or subtract fractions that originally had different denominators.

**Find the denominators of the fractions**The denominator is the bottom number in a fraction. For example, if the fractions are 3/5 and 1/7, then 5 and 7 are the denominators.

**List the multiples of the denominators**Make a list of the first ten multiples of the denominators. In the example from Step 1, the first ten multiples of 5 are 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50. The first ten multiples of 7 are 7, 14, 21, 28, 35, 42, 49, 56, 63 and 70.

**Choose the lowest multiple that both denominators share**Look at the list of multiples for the denominators and choose the one that is the same in both lists. In the example, the number 35 is in both lists so that is the common denominator. If there is no shared number, continue listing multiples until there is a number that is the same in both lists. The easiest way to find this for just two fractions is to multiply the original denominators together.