Work through the examples below, then open Online Calculator to check your own numbers and practice the skill.
Why decimal alignment matters
Adding decimals follows the same place-value rules as adding whole numbers. Align ones with ones, tenths with tenths and hundredths with hundredths. The decimal points should appear in the same column in your written work.
If one number has fewer fractional digits, add zeros on the right without changing its value. For example, write 3.5 as 3.50 when adding it to 0.27.
Step-by-step decimal addition
Calculate 12.35 + 7.8. First write 7.8 as 7.80. Add hundredths: 5 + 0 = 5. Add tenths: 3 + 8 = 11 tenths, so write 1 tenth and carry 1 to the ones column. Then add whole-number digits.
12.35 + 7.80 = 20.15. Estimate first: 12 + 8 is about 20, so the answer is reasonable.
Step-by-step decimal subtraction
Calculate 20 − 3.47. Write 20 as 20.00. Borrow across the decimal positions as needed. The result is 16.53 because 16.53 + 3.47 = 20.00.
Another example is 5.2 − 0.85. Write 5.20 − 0.85 and subtract to obtain 4.35. Do not subtract 85 from 2; the last digits occupy different decimal places.
Money, measurements and negative results
Money is a natural application of decimal arithmetic. If an item costs $18.75 and delivery is $2.40, the total is $21.15. If you pay $25.00, the change is $3.85.
When subtracting produces a negative value, keep the sign: 2.5 − 4.1 = −1.6. Decimal alignment still works for positive and negative amounts; the sign communicates the direction of the result.
Practice questions and answers
Try these without a calculator: 0.46 + 0.7, 15.00 − 8.26, and 3.125 + 0.075.
Answers: 1.16, 6.74, and 3.200 (which is numerically 3.2). Use the online calculator to verify and then try a question with three addends.
Checking accuracy
Estimate to a nearby whole number, then check a subtraction by adding its answer to the smaller starting value. Watch for extra trailing zeros: a display of 3.200 and a display of 3.2 represent the same amount, even though they communicate different decimal precision.
Frequently asked questions
Why do we line up decimal points?
It aligns equal place values: ones with ones, tenths with tenths and hundredths with hundredths.
Can I add zeros at the end of a decimal?
Yes. Appending trailing zeros does not change its numerical value: 4.5 = 4.50.
What is 0.8 − 0.35?
Write 0.80 − 0.35. The difference is 0.45.
Use Online Calculator to verify a new example, then try the related practice activities.