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Skill guide · Grades 4–6
Adding and subtracting decimals
How to add and subtract decimals by lining up the decimal point, with a worked example, the common mistakes and ideas for practice at home.
What it is
Adding and subtracting decimals works exactly like whole numbers once the columns line up. The rule is to line up the decimal points, not the last digits. Tenths go under tenths, hundredths under hundredths.
Texas introduces adding and subtracting decimals to hundredths in fourth grade. In Common Core states it usually comes in fifth grade, after fourth graders learn what tenths and hundredths mean.
A worked example
12.5 + 3.75 = 16.25
1
1
2
5
0
+
3
7
5
1
6
2
5
Every step, the way the pad explains it
- Add the hundredths: 0 + 5 = 5. Write 5.
- Add the tenths: 5 + 7 = 12. Write 2, carry 1.
- Add the ones (with the carry): 1 + 2 + 3 = 6. Write 6.
- Add the tens: only 1 here. Write 1.
- 1Write the numbers with the decimal points lined up. Fill the empty place with a 0: 12.5 becomes 12.50.
- 2Hundredths: 0 + 5 = 5.
- 3Tenths: 5 + 7 = 12. Write 2, carry 1 to the ones.
- 4Ones: 1 + 2 + 3 = 6. Tens: 1.
- 5Bring the decimal point straight down: 12.5 + 3.75 = 16.25.
Common mistakes
Lining up the last digits instead of the decimal points.
Draw the decimal points first, in a column, then fill in digits around them.
Forgetting the decimal point in the answer.
Bring it straight down before adding anything.
Thinking 0.25 is bigger than 0.5.
Write 0.5 as 0.50 and compare: 50 hundredths against 25 hundredths.
Leaving a gap and treating it as nothing when subtracting.
Fill empty places with zeros, especially on the top number.
Grade range and practice
Usually taught in grades 4–6. Schools order topics a little differently, so your child’s class may meet it earlier or later.
- Use money: $12.50 plus $3.75 is a decimal problem kids already understand.
- Read decimals out loud as “twelve and fifty hundredths” to keep the place value in view.
- Estimate with whole numbers first: about 13 + 4.
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