Adding fractions with unlike denominators: a parent’s guide
Why you cannot just add the tops and bottoms, how to find a common denominator, and worked examples of adding and subtracting fractions with your child.
Skill guide · Grades 3–6
Adding and subtracting fractions
How to add and subtract fractions with unlike denominators, step by step, with a worked example and the mistakes children make most often.
What it is
A fraction’s denominator tells you the size of the pieces. You can only add or subtract pieces of the same size, so fractions with different denominators are first rewritten with a common denominator.
Children learn what fractions are in third grade, add fractions with the same denominator in fourth grade, and add and subtract fractions with unlike denominators in fifth grade.
A worked example
5/6 − 3/8 = 11/24
- 1The bottoms are different (6 and 8). Find a common denominator: the least common multiple of 6 and 8 is 24.
- 2Rewrite 5/6 as 20/24 (multiply the top and bottom by 4).
- 3Rewrite 3/8 as 9/24 (multiply the top and bottom by 3).
- 4Subtract the numerators: 20 − 9 = 11. Keep the denominator: 11/24.
- 511/24 is already in simplest form — 11 and 24 share no common factor except 1.
- 1Find a common denominator for 6 and 8. Multiples of 8 are 8, 16, 24. The first one 6 also divides into is 24.
- 2Rewrite 5/6 as 20/24 (multiply top and bottom by 4).
- 3Rewrite 3/8 as 9/24 (multiply top and bottom by 3).
- 4Subtract the numerators and keep the denominator: 20/24 − 9/24 = 11/24.
- 511 and 24 share no common factor, so 5/6 − 3/8 = 11/24 is already in simplest form.
Common mistakes
Adding or subtracting the denominators too.
The denominator is the piece size. Taking pieces away does not change their size.
Changing the denominator but not the numerator.
Whatever you multiply the bottom by, multiply the top by too.
Not simplifying the answer.
Check whether the top and bottom share a factor before calling it done.
Picking a number that is not really a common multiple.
List the multiples of both denominators and find the first match.
Grade range and practice
Usually taught in grades 3–6. Schools order topics a little differently, so your child’s class may meet it earlier or later.
- Fold paper strips into halves, thirds and sixths to see equivalent fractions.
- Practice rewriting fractions with a given denominator before adding them.
- Ask whether the answer should be more or less than one whole before working it out.
Read more on the blog
3rd and 4th grade math skills: a checklist for parents
The main math skills most 3rd and 4th graders work on, in plain language, with examples and simple ways to check how your child is doing at home.
Why kids struggle with place value, and how column alignment helps
Place value is behind most carrying, borrowing and decimal mistakes. Here is what it is, how to spot gaps, and simple ways to build it at home.