Least Common Denominator

What is a ...
... Denominator?

fraction

The denominator is the bottom number in a fraction.

It shows how many equal parts the item is divided into

2/5 + 1/5 common denominator

... Common Denominator?

When the denominators of two or more fractions are the same, they have Common Denominators.

... Least Common Denominator?

it is the smallest of all the common denominators.

Why?

Why do we want common denominators?

Because we can't add fractions with different denominators:

13+16=?
1/3 1/6 pie huh

Before we can add them we must make the denominators the same.

Finding a Common Denominator

But what should the new denominator be?

One simple answer is to multiply the current denominators together:

3 × 6 = 18

So instead of having 3 or 6 slices, we will make both of them have 18 slices.

The pizzas now look like this:

618+318=918
1/3 1/6 pizza 9/18

They now have common denominators (but not the least common denominator)

(Read more about Common Denominators.)

Least Common Denominator

That is all fine, but 18 is a lot of slices ... can we do it with fewer slices?

Here is how to find out:

13List multiples of 3:  3, 6, 9, 12, 15, 18, 21, ...
16List multiples of 6:  6, 12, 18, 24, ...

Now find the smallest number that is the same:

multiples of 3:  3, 6, 9, 12, 15, 18, 21, ...
multiples of 6:  6, 12, 18, 24, ...

The answer is 6, and that is the Least Common Denominator.

So let us try using it!

We want both fractions to have 6 slices:

And our question now looks like:

26+16=36  
2/6 1/6 3/6 empty

One last step is to simplify the fraction (if possible). In this case 3/6 is simpler as 1/2:

26+16=36=12
2/6 1/6 3/6 3/6

 

And that is what the Least Common Denominator is all about.

It lets us add (or subtract) fractions using the least number of slices.

What Did We Do?

The trick was to list the multiples of each denominator, then find the Least Common Multiple

In the previous example the Least Common Multiple of 3 and 6 was 6.

In other words the Least Common Denominator of 13 and 16 is 6.

Here are the steps to follow:

dog looking ear down
  • Find the Least Common Multiple of the denominators (which is called the Least Common Denominator).
  • Change each fraction (using equivalent fractions) to make their denominators the same as the least common denominator
  • Then add (or subtract) the fractions, as we wish!

Example: What is 16 + 715 ?

The Denominators are 6 and 15:

multiples of 6:  6, 12, 18, 24, 30, 36, ...
multiples 15:  15, 30, 45, 60, ...

So the Least Common Multiple of 6 and 15 is 30.

 

Now let's try to make the denominators the same.

Note: what we do to the bottom of the fraction,
we must also do to the top

For the first fraction we can multiply top and bottom by 5 to get a denominator of 30:

 × 5 
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16=530
right under arrow
 × 5 

 

For the second fraction we can multiply top and bottom by 2 to get a denominator of 30:

 × 2 
right over arrow
715=1430
right under arrow
 × 2 

 

Now we can do the addition by adding the top numbers:

530 + 1430 = 1930

The fraction is already as simple as it can be, so that is the answer.

 

Least Common Multiple Tool

To find the least common denominator automatically use the Least Common Multiple Tool. Just put in the denominators, press the button, and the least common denominator is shown.

One More Example

Example: What is 38 + 512?

List the multiples of 8 and 12

multiples of 8:  8, 16, 24, 32, 40, ...
multiples 12:  12, 24, 36, 48, ...

The Least Common Multiple is 24

For the first fraction we can multiply top and bottom by 3 to get a denominator of 24:

 × 3 
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38=924
right under arrow
 × 3 

 

For the second fraction we can multiply top and bottom by 2 to get a denominator of 24:

 × 2 
right over arrow
512=1024
right under arrow
 × 2 

 

Now we can do the addition:

924 + 1024 = 1924

The fraction is already as simple as it can be, so that is the answer.