Year 5 · Adding decimals
How do you add decimals with different lengths?
Your child is working away happily at the kitchen table, and then an answer appears that looks a little small. They are adding 6.35 and 1.4, and they tell you it comes to 6.49.
Something about that feels wrong, and it is worth naming. They have added more than one whole to 6.35, yet the total has barely moved. The reason is simple: the numbers were lined up by their right-hand ends instead of by the decimal point.
Why the mistake is a smart one
For years your child has lined up every column sum at the right-hand end, because with whole numbers that is exactly where the ones sit. If the question were 635 + 14, doing that would be completely correct.
They are applying a rule that has always worked, to a situation where it quietly stops working. Nobody has told them yet that the decimal point, not the end of the number, is what fixes the columns.
Line 1.4 up under 6.35 by the ends and the 4 slides into the hundredths column. One and four tenths becomes fourteen hundredths — 0.14 — which is why the total hardly changes.
The fix: three steps that untangle it
Step one — line up the points. Write the numbers one above the other so the decimal points form a single straight vertical line. They are like buttons on a shirt: every one has to be done up in the same column. That puts tenths under tenths and ones under ones. Squared paper helps a lot here.
Step two — fill the gap with a zero. One number is short a hundredths digit, so 1.4 becomes 1.40. Say out loud that 1.40 is worth exactly the same as 1.4 — the zero changes nothing about the value, it just fills the empty column so the sum is easier to read.
Step three — add, and bring the point down. Add the parts after the point, then the whole numbers, then bring the decimal point straight down into the answer line. 6.35 + 1.40 = 7.75. Worth pausing on: that answer is bigger than both numbers you started with, which is exactly what adding should do.
What to say tonight
- “Where does the decimal point go in each number?”
- “Does every column have a digit in it?”
- “Is your answer bigger than both the numbers you started with?”
How SmartMaths handles it
57 of our Year 5 questions carry this exact mix-up, and the wrong answer itself is labelled with it — so when a child taps 6.49, the app knows what they meant by it, not just that they were wrong.
It does not simply mark it. A calm panel opens over the question and walks the alignment three steps at a time, deliberately on a second example rather than the child’s own, so they still get to finish theirs. If it has not landed, “Explain it differently” re-teaches the same idea another way — a number line, a bar model — until one of them clicks. The rest of the year is on our Year 5 maths practice page.
SmartMaths is very affordable tutor-style maths at home — it explains mistakes the way a tutor would; free trial at smartmaths.ai.
Common questions
Do you have to line up the decimal points when adding?
Yes. Lining up the points is what guarantees you are adding tenths to tenths and ones to ones. Without it the digits shift into the wrong columns and the total is simply a different sum.
Can you add a zero to the end of a decimal?
Yes, a zero after the last decimal digit does not change the value — 1.4 and 1.40 are the same number. It is a tidy way to give both numbers the same number of columns before you add.
What decimals should a Year 5 child be able to add?
Year 5 covers 29 micro-skills in our curriculum model, and adding decimals of different lengths is one of them. Children often meet it through money, adding a price like £6.35 to a value with only one decimal place, which is a natural place to practise keeping the points aligned.