Adding and Subtracting Fractions with Unlike Denominators

By the Practice Buds team · October 16, 2026

Short answer

To add or subtract fractions with different denominators, first rewrite both fractions so they share the same denominator. Then add or subtract the tops and keep the bottom the same. Short sessions with printable adding fractions worksheets give your child the repetition that makes the routine automatic.

Why kids find this hard

With the same denominator, fractions are easy. Your child just adds the tops, so 2/7 + 3/7 = 5/7 and nothing else changes. Unlike denominators break that habit, and many kids try to use it anyway.

The deeper trouble is that halves and thirds are different sized pieces. You cannot count them together any more than you can add 2 apples and 3 oranges and call the answer "5 apples." The fix is to cut both into the same size piece first. That step feels like extra work, so children skip it, and then the answer is wrong.

This usually shows up in 4th and 5th grade, after your child is comfortable with the basics in fractions explained for parents.

How to teach it

Have paper, a pencil and two same-size paper strips ready. Ten minutes is plenty.

  1. Check the starting point. Ask your child to add 1/5 + 2/5 and say why the bottom stays 5. If that is shaky, review before moving on.
  2. Show why unlike denominators are a problem. Fold one strip into 2 equal parts and another into 3 equal parts. Ask, "Can we count these pieces together?" They are different sizes, so not yet.
  3. Find a common size. Fold both strips into 6 equal parts. Now 1/2 is 3 of the small pieces and 1/3 is 2 of them. Practice this idea with our equivalent fractions worksheets.
  4. Find the common denominator. List the multiples of each bottom number until one matches. For 2 and 3, the first match is 6. This is the least common multiple, and the least common multiple worksheets build that skill.
  5. Rewrite each fraction. Multiply the top and the bottom by the same number so the fraction keeps its value.
  6. Add or subtract the tops. Keep the bottom the same.
  7. Check the answer. Ask, "Is it close to what I expected?" Then simplify if you can.

Subtraction works the same way. Once your child has the routine for addition, move to subtracting fractions and point out that only the last step changes.

Worked example

Let's add 1/2 + 1/3.

Step 1: The denominators are 2 and 3. They are different, so we need a common denominator.

Step 2: Multiples of 2 are 2, 4, 6, 8. Multiples of 3 are 3, 6, 9. The first match is 6.

Step 3: Rewrite 1/2 as sixths. Since 2 × 3 = 6, multiply the top and bottom by 3, and 1/2 = 3/6 is the result.

Step 4: Rewrite 1/3 as sixths. Since 3 × 2 = 6, multiply the top and bottom by 2, and 1/3 = 2/6 is the result.

Step 5: Add the tops and keep the bottom, so 3/6 + 2/6 = 5/6 is the sum.

So 1/2 + 1/3 = 5/6 in total. It makes sense: half a strip and a third of a strip is a bit less than one whole strip.

Now a subtraction. For 3/4 − 1/6, the multiples of 4 are 4, 8, 12 and the multiples of 6 are 6, 12. The match is 12. Rewrite 3/4 = 9/12 and also 1/6 = 2/12 for the next step. Subtract the tops: 9/12 − 2/12 = 7/12 left over. So 3/4 − 1/6 = 7/12 in all.

One more, where the answer needs no simplifying: 2/3 + 1/4. The common denominator is 12. Then 2/3 = 8/12 and 1/4 = 3/12, so 2/3 + 1/4 = 11/12 in total.

Common mistakes

Adding the tops and the bottoms. Your child writes 1/2 + 1/3 as 2/5. Spot it when the answer is smaller than one of the fractions you started with. Fix it by going back to the folded strips: half a strip plus a third can never be less than half.

Changing only the bottom. Your child turns 1/2 into 1/6 by changing the 2 to a 6 and leaving the top alone. That makes the piece much smaller. Remind your child that what you do to the bottom, you also do to the top, like a fair trade.

Subtracting the smaller digit from the larger. For 3/4 − 1/6 your child gets confused after finding twelfths and subtracts in the wrong order. Have your child write the bigger fraction first and check that the answer is smaller than it.

Forgetting to simplify. Your child ends with 4/8 and stops. Ask, "Can both numbers be divided by the same number?" Then 4/8 becomes 1/2.

Practice ladder

  • First: Same-denominator addition and subtraction, and a quick review of equivalent fractions.
  • Next: Unlike denominators where one is a multiple of the other, such as 1/2 and 1/4. Only one fraction needs rewriting.
  • Then: Unlike denominators where neither divides the other, such as 1/2 and 1/3.
  • Later: Mixed numbers, word problems and simplifying answers. A good pace is three or four short sessions a week for two weeks on each rung.

The full set of skills lives on the fractions worksheets page.

FAQ

What grade do kids learn to add fractions with unlike denominators? Most children meet this in 4th and 5th grade, after they understand equivalent fractions.

Do we have to find the least common denominator? No. Any common denominator works. For 1/2 + 1/3, using 12 gives 6/12 + 4/12 = 10/12 and that simplifies to 5/6. The least one just means smaller numbers and less simplifying.

My child keeps adding the bottoms. What now? Go back to the paper strips and ask whether the answer is bigger or smaller than each fraction you started with. Seeing it makes the rule stick.

How many problems should we do in a session? Four to six, done carefully, beat twenty rushed ones.

Practice at home

Pair a quick fold-the-strips warm-up with a printed page so the steps become habit. Browse our printable adding fractions worksheets and pick one that fits where your child is today. Printing needs a free account.