Ross MackellUK-qualified primary teacher · 10 min read

Your child is working out 7.2 − 3.65. They carefully line the numbers up, work through the columns, and proudly write 4.45. It looks sensible. It's wrong. And neither of you can see why.

Adding and subtracting decimals trips up more children than almost anything else in upper primary. Not because it's hard, but because there are two or three small rules that nobody quite spells out. Get those right and it's just column addition and subtraction with a dot in the middle.

I'm Ross, a UK-qualified primary teacher with over ten years in the classroom. In this guide I'll show you exactly how UK schools teach adding and subtracting decimals, step by step, with worked examples, the zero trick that fixes most mistakes, and a 10-minute practice routine with answers.

What your child needs first

Adding and subtracting decimals uses skills your child has already learned. If they're struggling, check these first:

• Place value, including tenths and hundredths. They need to know that in 3.46, the 4 is worth four tenths and the 6 is worth six hundredths. My guide on place value covers this, and partitioning shows how to split a decimal into its parts.
• Column addition and subtraction with whole numbers, including carrying and exchanging. If that's wobbly, start with column addition and subtraction. Decimals won't stick until this does.

Children meet decimals in Year 4. The national curriculum suggests Year 5 pupils "practise adding and subtracting decimals", and calculations like these turn up regularly in the Year 6 SATs arithmetic paper.

The 3 golden rules

Everything in this guide comes back to three rules. If your child remembers nothing else, make it these:

1. Line up the decimal points. Not the right-hand edge of the numbers. The dots.
2. Fill the gaps with zeros. 2.8 becomes 2.80. This doesn't change its value, it just makes the columns tidy.
3. Drop the decimal point straight down. It goes in the answer in exactly the same column.

Rule 2 is the one I see skipped most, and it causes the majority of wrong answers, especially in subtraction.

Adding decimals step by step: 3.46 + 2.8

First, line up the decimal points and add a zero so both numbers have two decimal places. 2.8 becomes 2.80.

1. Hundredths: 6 + 0 = 6. Write 6.
2. Tenths: 4 + 8 = 12 tenths. That's 1 whole and 2 tenths. Write 2 and carry the 1 into the ones column.
3. Decimal point: drop it straight down.
4. Ones: 3 + 2 + the carried 1 = 6. Write 6.

3.46 + 2.8 = 6.26. A quick sense check: 3 and a bit plus 3 is about 6. Yes, 6.26 sounds right. Teaching your child to estimate first is one of the best habits you can build.

A bigger one: 14.7 + 5.385

Different numbers of decimal places? Same rules. 14.7 has one decimal place, 5.385 has three. So 14.7 becomes 14.700.

• Thousandths: 0 + 5 = 5.
• Hundredths: 0 + 8 = 8.
• Tenths: 7 + 3 = 10. Write 0, carry 1.
• Ones: 4 + 5 + 1 = 10. Write 0, carry 1.
• Tens: 1 + 1 = 2.

14.7 + 5.385 = 20.085. Notice the method didn't change at all. Decimals never get harder, they just get longer.

Subtracting decimals step by step: 7.2 − 3.65

Here's the question from the start, done properly. Line up the decimal points and turn 7.2 into 7.20.

1. Hundredths: 0 − 5? You can't. Exchange 1 tenth for 10 hundredths. The 2 tenths become 1, and the hundredths become 10. 10 − 5 = 5.
2. Tenths: now 1 − 6? You can't. Exchange 1 one for 10 tenths. The 7 becomes 6, and the tenths become 11. 11 − 6 = 5.
3. Decimal point: straight down.
4. Ones: 6 − 3 = 3.

7.2 − 3.65 = 3.55. Check it the other way: 3.55 + 3.65 = 7.20. It works.

So where did 4.45 come from? Without the zero, the hundredths column looked empty, so the child just brought the 5 down. Then in the tenths they did 6 − 2 instead of 2 − 6. Two tiny slips, both fixed by writing that zero.

Subtracting a decimal from a whole number

Questions like 10 − 4.38 look strange because 10 doesn't seem to have a decimal point. It does. It's just hiding. 10 is the same as 10.00.

This one needs exchanging across several zeros, which is where lots of children get muddled. The 1 ten becomes 9 ones, 9 tenths and 10 hundredths. Then it's 10 − 8 = 2, 9 − 3 = 6 and 9 − 4 = 5.

There's also a lovely mental way to do it, called counting up. Start at 4.38 and count up to 10 in friendly jumps:

• 4.38 to 4.40 is 0.02
• 4.40 to 5 is 0.6
• 5 to 10 is 5
• 0.02 + 0.6 + 5 = 5.62

This is exactly how shopkeepers used to count out change. If your child finds the column method fiddly with zeros, counting up is a perfectly good alternative for questions like this.

Watch me teach it

Here's a lesson from my Arithmetic Mastery course where I walk through adding decimals, step by step:

And because so many decimal mistakes are really subtraction mistakes, this one is worth five minutes too:

Quick mental methods

Not every decimal question needs a column. For small numbers, thinking in tenths is quicker:

• 0.7 + 0.5: 7 tenths + 5 tenths = 12 tenths = 1.2.
• 0.9 − 0.4: 9 tenths − 4 tenths = 5 tenths = 0.5.
• Number bonds to 1: 0.35 + 0.65 = 1, because 35 + 65 = 100. Practising these makes counting up and giving change much faster.

Money is the best way in. Most children who freeze at 0.35 + 0.65 can tell you instantly that 35p and 65p make £1. Use that. It's also a natural stepping stone to percentages and fractions, which are really just other ways of writing the same parts of a whole.

4 decimal mistakes and how to fix them

Mistake 1: Lining up the right-hand digits

For 3.46 + 2.8, your child lines up the 6 and the 8 and gets 3.74. They've added 8 tenths to 6 hundredths.

The fix: "Dots line up first, always." Squared paper with one digit per square helps hugely.

Mistake 2: Leaving gaps instead of zeros

In subtraction, an empty space gets treated as "nothing to do", so the bottom digit is just copied down. That's how 7.2 − 3.65 becomes 4.45.

The fix: make writing the zeros a habit before they calculate anything. Both numbers must have the same number of digits after the point.

Mistake 3: Forgetting the decimal point in the answer

The working is perfect, but the answer is written as 626 instead of 6.26.

The fix: drop the decimal point into the answer line before starting the calculation. Then estimate: "Is the answer about 6 or about 600?"

Mistake 4: Thinking longer means bigger

Children often think 0.125 is bigger than 0.5 because it has more digits. That causes trouble with ordering and estimating.

The fix: add zeros so they're the same length: 0.125 and 0.500. Now it's obvious which is bigger. Money helps here too: would you rather have 50p or 12 and a half pence?

A 10-minute practice routine

Same three steps I use for every skill: watch, copy, practise.

1. Watch: do one example yourself, out loud. "Line up the dots. Fill the gaps with zeros. Start on the right."
2. Copy: your child does a very similar one, saying the steps out loud too.
3. Practise: three or four on their own, then check each answer with the inverse (add back to check a subtraction).

Only move on when they're getting three in a row right. If one step keeps going wrong, it's usually the whole-number column method, so spend a few days on that instead. Ten minutes, a few times a week, is plenty.

Frequently asked questions

How do you add decimals?

Write the numbers in columns with the decimal points lined up. Fill any empty places with zeros so both numbers have the same number of decimal places. Add from right to left, carrying as normal, and put the decimal point in the answer directly below the others.

How do you subtract decimals?

Line up the decimal points, fill empty places with zeros, then subtract from right to left using the column method, exchanging where you need to. For example, 7.2 − 3.65 is written as 7.20 − 3.65, which equals 3.55.

Why do you add a zero when adding or subtracting decimals?

Adding a zero at the end of a decimal doesn't change its value, so 2.8 and 2.80 are the same. It gives both numbers the same number of decimal places, which keeps the columns lined up and stops children skipping a column in subtraction.

What year do children learn to add and subtract decimals?

Children are introduced to decimals in Year 4 and practise adding and subtracting them in Year 5. In Year 6 they use decimals with up to three decimal places, and these calculations appear in the SATs arithmetic paper.

How do you subtract a decimal from a whole number?

Give the whole number a decimal point and zeros, so 10 becomes 10.00. Then subtract with the column method, exchanging across the zeros. Alternatively, count up from the smaller number to the whole number in friendly jumps and add the jumps together.

Sources and further reading

• Department for Education, National curriculum in England: mathematics programmes of study (decimals in Years 4 to 6)
• Oak National Academy, Adding decimals (free pupil lesson)
• Standards and Testing Agency, 2025 key stage 2 mathematics mark schemes
• Ross Mackell, Teacher's Perspective on YouTube