What is a Denominator?

Tricia Christensen
Tricia Christensen

The denominator is math terminology used when discussing fractions. Fractions have three parts: the numerator or top number, the vinculum or the line separating the numbers which means divide by, and the denominator or bottom number. The fraction actually suggests division. The denominator divides the numerator. In the fraction 3/4, for instance, this could be read as 3 divided by 4, .75, or 75%.

Woman holding a book
Woman holding a book

We often think of the fraction as a part of the whole. The top number represents the number of parts while the bottom is the actual whole amount. It could be said that the fraction represents what is used over what could be used. When children learn fractions, they often learn it based on slices of pie. If there are 8 slices, the potential whole is 8, and this is the denominator. If 2 slices are removed, there are now just 6/8 or six slices out of a possible eight slices.

Of course, there are instances where denominators are less than numerators. These are called improper fractions. They are actually a whole number and something left over and can be converted into a mixed number. For instance 5/2 can be changed to 2 1/2. Sometimes it’s easier to keep fractions in improper forms until all operations have been completed.

In learning about fractions, children begin to learn in third or fourth grade is that there are many fractions that represent the same thing. Any fraction multiplied by the same number on top and bottom will still work out to the same decimal or percentage. This information becomes useful when people must add or subtract fractions that do not have the same denominator.

When the denominators are the same, only the top numbers get added or subtracted. If they are different, other operations must be performed on the fractions first before addition or subtraction can take place. This is called finding the common denominator.

In the example 1/3 + 1/4, people must find the common denominator. They do this by looking at the denominators to see which numbers they might be factors of (go into). In this case, both 3 and 4 go into and are factors of the number 12. The operation is then to get each fraction converted into twelfths. This would be accomplished by multiplying 1/3 by 4/4 and multiplying 1/4 by 3/3, resulting in the new (but still the same) fractions 4/12 + 3/12. It’s now possible to add the fractions together (only the numerators!) and get the number 7/12.

Fraction operations can be more difficult and sometimes denominators may be written as a decimal or a fraction. These take a little more work. In simple understanding of the term though, it is very important that people realize one number can never be a denominator. Zero can never be placed in the bottom of the fraction since in math operations it cannot divide any number.

Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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Discussion Comments


I really learned fractions by learning to cook. I still use a lot of what I learned in school for halving or doubling a recipe. Of course, it's always nice to have that tool in an online recipe so I can specify the number of servings I want and have the converter do the fractions for the recipe.

Of course, a denominator is also the one something a number of different things have in common with each other. Police officers use this process all the time in solving murders. It's easier to solve a puzzle when you know the one common feature everything has.


My gosh, but does this take me back in learning fractions! I remember having to find the greatest common factor and the least common denominator to add and subtract them properly!

Multiplying and dividing factions was always easier, and a lot more fun. I am going to have to look up cross-canceling online to refresh my memory of how you do it. I know it's so you don't have to reduce the fraction when you finish the equation, but I'm not sure I remember how to do it. Reducing the fraction was never bothersome, though. I always hated finding the least common denominator, and then converting the fractions to common denominators so they could be added and subtracted, and then reduced again.

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