Estimate first, then be exact: addition
The Short Answer
Addition questions at this level are rarely lost on the adding. They are lost on the setting out, particularly when decimals of different lengths are lined up by their ends rather than by their points. Make the decimal places match first, then add. Estimate before you start, so a wrong answer announces itself.
What do addition questions test at this level?
Not adding. Your child can add.
What the 11+ tests is whether they can set a calculation out correctly when the numbers are long, when they arrive with different numbers of decimal places, and when there is a clock running.
Our promise for this topic is short: estimate first, then be exact.
Line up by the point, not by the end
Here is where the marks disappear.
18.6 + 9.45
18.6 has one decimal place. 9.45 has two. Line those up by their ends and the 6 sits underneath the 5 — tenths above hundredths — and the calculation is wrong before a single digit has been added.
So the first thing to do is make them match. 18.6 is exactly the same number as 18.60. Now both have two decimal places, the points line up, and every digit is in the column it belongs to.
Can you show me a worked example?
Why the estimate earns its two seconds
Children resist estimating because it feels like doing the question twice. It is not — it is doing a different, much smaller question, and it buys two things.
It places the decimal point. On a decimal calculation, nothing else will. A child who computes the digits correctly and then has to choose between 28.05, 280.5 and 2.805 is guessing unless they estimated.
It catches the big errors. A missed carry or a dropped digit almost always lands nowhere near the estimate.
Where do pupils lose marks?
Lining up by the last digit, which is the whole reason this post exists.
Skipping the estimate, and then having no way to check.
Losing a carry in a long column, which an estimate catches and a re-read often does not.
Try it yourself
Estimate first, then set out carefully. Make the decimal places match before you write anything in columns.
- 18.6 + 9.45
- Estimate 18.6 + 9.45 before calculating. What should the answer be near?
- 7.4 + 12.65
- Why does lining these up by their last digit go wrong?
- A shopper spends £12.65, £7.40 and £3.99. Estimate the total, then find it exactly.
- 28.05. Write 18.6 as 18.60 so both have two decimal places, then add the columns as usual.
- About 28. Round to 19 and 9, which gives 28 — so an answer of 2.805 or 280.5 is visibly wrong, and so is 27.05.
- 20.05. Again, pad the shorter one: 7.40 + 12.65.
- Because the 6 in 18.6 is six tenths and the 5 in 9.45 is five hundredths. Lining them up by their ends puts tenths under hundredths, and the calculation is wrong before a single digit is added.
- Estimate: 13 + 7 + 4 is about 24. Exact: £24.04. The estimate confirms the answer is sensible, which is the whole reason for making it.
How much practice does this type need at home?
Not much of the adding, and quite a lot of the estimating — which is the part that is rarely practised because it feels like it is not the real work.
Give your child a calculation and ask only for the estimate. Do not let them work it out. Ten of those takes two minutes, and it builds the instinct that makes every other maths question safer, because a child who knows roughly what the answer should be cannot be far wrong for long.
Free: a Maths workbook
Questions across the Maths question types, with the method printed at the top and full answers at the back. One workbook for the whole strand, not a sheet per question type.
Frequently Asked Questions
Why does the estimate matter if my child can just do the calculation?
Because on a decimal question the estimate is the only thing that tells you where the point goes. A child who calculates 1860 + 945 correctly and then has to decide whether the answer is 28.05, 280.5 or 2.805 has nothing to go on but the estimate. It also catches the larger errors — a missed carry or a dropped digit usually produces an answer nowhere near the estimate, and a child who has one will notice.
How do I explain why the decimals have to match?
Point at the columns. In 18.6 the 6 is six tenths. In 9.45 the 5 is five hundredths. If you line them up by the last digit, tenths sit above hundredths and you are adding things that are not the same size. Writing 18.6 as 18.60 does not change its value — it just fills the empty hundredths column so everything sits where it belongs.
Should my child still use column addition at this level?
For anything with more than two digits or any decimals, yes. Mental methods are quicker on easy numbers and become unreliable exactly where the paper gets harder. What we do encourage is a mental estimate first and then the written method, so speed comes from confidence rather than from skipping the setting out.
What does a good estimate look like?
Rough and fast. Round each number to something easy — 18.6 becomes 19, 9.45 becomes 9 — and add those. It should take about two seconds and it does not need to be close, only close enough to spot an answer that is ten times too big or has lost a digit. An estimate that takes as long as the calculation is not doing its job.
Sources
- GL Assessment — 11+ familiarisation materials and question type descriptions Published 2025.
- Clever Clogs Academy — our own Year 4 and Year 5 lesson records, Winslow Published September 2025 to July 2026.
We are not affiliated with, endorsed by or connected to GL Assessment or Buckinghamshire Council. Test arrangements change; always check the council's own pages for the current year's dates.
We Teach This Method Every Week in Winslow
Groups of no more than six pupils with two qualified teachers, in person or online. Come and see a session before you decide.