Numbers · Lesson 28 of 36

Adding and Subtracting Decimals

Line up the decimal points so each place value sits under the same one, fill gaps with zeros, then add or subtract as with whole numbers.

Primary
Updated 14 November 2026

What you will learn

Add and subtract decimals by lining up place value.
Fill gaps with zeros so the numbers have the same length.
Check an answer with an estimate.

Before you start: you should know decimal place value and column addition and subtraction. See Decimals Explained and Column Addition.

The idea

Adding decimals is adding whole numbers, as long as the places match. Tenths go with tenths and hundredths with hundredths. The way to keep them matched is to line up the decimal points.

Three tenths plus four tenths is seven tenths: 0.3 + 0.4 = 0.7.

Line up the points

To work out $3.45 + 12.6$ , write the numbers one under the other with the decimal points in a line. Add a zero to $12.6$ so it has two decimal places.

	Tens	Ones	.	Tenths	Hundredths
		3	.	4	5
+	1	2	.	6	0
=	1	6	.	0	5

Add from the right. Hundredths: $5 + 0 = 5$ . Tenths: $4 + 6 = 10$ , write $0$ and carry $1$ . Ones: $3 + 2 + 1 = 6$ . Tens: $1$ . The answer is $16.05$ .

Using a number line

You can also jump. To add $0.6 + 0.7$ , jump $0.6$ and then $0.7$ more.

0.6 + 0.7: start at 0, jump 0.6, then jump 0.7 more to land on 1.3.

Subtracting

Line up the points and fill gaps with zeros. For $5 - 2.36$ , write $5$ as $5.00$ .

2.5 - 0.8: start at 2.5 and jump back 0.8 to land on 1.7.

Check with an estimate

Round each number first. $3.45 + 12.6$ is about $3 + 13 = 16$ . The exact answer, $16.05$ , is close, so it is sensible.

Worked examples

Example 1: Add

Work out $3.45 + 12.6$ .

Write $12.6$ as $12.60$ and line up the points.
$3.45 + 12.60 = 16.05$ .
Estimate: $3 + 13 = 16$ . Close.

Answer: $16.05$

Example 2: Subtract from a whole number

Work out $5 - 2.36$ .

Write $5$ as $5.00$ .
$5.00 - 2.36 = 2.64$ .
Check by adding: $2.64 + 2.36 = 5.00$ .

Answer: $2.64$

Example 3: Money

You pay with a £ $10$ note for items costing £ $2.35$ and £ $4.50$ . How much change is there?

Total: $2.35 + 4.50 = 6.85$ .
Change: $10.00 - 6.85 = 3.15$ .

Answer: £ $3.15$

Tip: if a number has no decimal point, it is a whole number. Put the point after the last digit and add zeros.

Why it matters

Shopping, measuring, timing and cooking all use decimals. A misplaced digit can change a price by a factor of ten, so lining up the points every time is worth the habit.

Practice

Your turn. Type or choose an answer, then press Check. Getting one wrong is fine, that is what practice is for. You can try as many times as you like.