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?
The worked example in place-value columns.
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 checks the decimal point. Lining the columns up by place value is what puts the point in the right place. The estimate confirms it: with about 28 in mind, 280.5 and 2.805 are plainly wrong.
It catches the big errors. A dropped digit or a misplaced point lands nowhere near the estimate. A small slip, such as a lost carry in the tenths giving 27.05, can still pass it — that is what setting out carefully and adding the columns a second time are for.
Where do pupils lose marks?
Lining up by the last digit, which is the whole reason this post exists.
Skipping the estimate, and then missing an answer that is ten times out.
Losing a carry in a long column. An estimate catches it only when the lost carry is a large one; adding the columns a second time catches the rest.
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. An estimate will not catch a small slip such as 27.05; adding the columns again, or subtracting to check, does that.
- 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 it is a two-second check on the size of the answer. Lining the numbers up by place value is what puts the decimal point in the right column, and the estimate confirms it: with a rough answer of 28 in mind, 280.5 or 2.805 is obviously wrong. It also catches the larger errors, such as a dropped digit. It will not catch a small slip like a lost carry in the tenths, so it sits beside careful setting out rather than replacing it.
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.