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GCSE Maths: How to Check Your Answers in the Exam

Check GCSE Maths answers with a four-step routine: re-read, size, units, then a second method, with a worked example and a practice plan.

Flat editorial illustration of a ruler, a calculator and a magnifying glass over a graph-paper page with a tick and a small clock on a cream background, with open space on the left.

You finish a GCSE Maths question, write your answer, and move on. Later you realise you gave the area when the question asked for the perimeter, or that your answer says a bus ticket costs £450. Those are not knowledge gaps. They are checking gaps, and they are some of the easiest marks to protect. This guide gives you a short checking routine you can practise now and use in every paper.

Why checking is worth practising

Exam boards say this themselves. AQA's 10 tips for avoiding common pitfalls in GCSE Maths exams tells students to check whether an answer is realistic, to re-read the question after answering it, and to use any spare time at the end to go back through the paper. It gives the example of an answer in centimetres when the question wanted metres, and notes that students can do plenty of credit-worthy work and still lose the final mark this way.

Pearson Edexcel's June 2025 principal examiner report for Paper 2F makes the same point from the marker's side. Its summary advises learners to check that the size of an answer is sensible, that units are appropriate and that the required accuracy is shown. For one percentage question, it says learners should read back through the question to check they have not skipped a necessary stage of the work. That report covers one Foundation calculator paper, but the habit applies to every tier and board.

The four-check routine

Checking everything twice is not realistic in a timed paper. Instead, use four quick checks that each take 10 to 20 seconds. Run them in this order after you reach an answer.

  1. Re-read the question. Underline what is being asked for (area or perimeter, the amount or the percentage, the new price or the discount) and tick it off against your answer line.
  2. Check the size. Ask whether the answer is the right sort of number. A length cannot be negative, a probability cannot be above 1, and a person's height will not be 17 metres.
  3. Check the units and accuracy. Look for the unit the question wants, and for any instruction such as 'give your answer to 2 decimal places' or 'in standard form'.
  4. Check the method with a second route. Estimate by rounding, work backwards with the inverse operation, or substitute your answer into the original equation.

A worked example

Here is a question written for this guide, in the style of a calculator paper: a jacket costs £60 and is reduced by 15% in a sale. What is the sale price?

Suppose you type 60 × 0.15 and get 9. That is the amount of the reduction, not the sale price. Check 1 catches it: the question asks for the sale price, so you need one more stage. Subtract to get £60 − £9 = £51.

Now run the other checks. Check 2: £51 is a little under £60, which is sensible for a small reduction. Check 3: money needs a pound sign, so write £51 or £51.00. Check 4: a 15% reduction means you pay 85%, so 60 × 0.85 should also give 51. It does, so you can be confident in the answer.

Notice that the second route is a different calculation, not the same one repeated. Repeating the same keystrokes will often repeat the same mistake.

Quick checking tools for common topics

Different topics suit different second routes. These are worth learning because they take seconds.

  1. Arithmetic: round each number to one significant figure. For 4.9 × 21.2, think 5 × 20 = 100. If your calculator shows 1038.8, you know a decimal point has slipped.
  2. Equations: substitute your solution back in. If you solved 3x + 4 = 19 and got x = 5, check 3 × 5 + 4 = 19.
  3. Tables of data: add across and down. Edexcel's report points out that it is easy to check that the additions in a two-way table work vertically and horizontally.
  4. Geometry: compare your answer with the picture. If a side you found is longer than the hypotenuse, something has gone wrong.

How to practise checking

A routine only helps if it is automatic, so build it into your normal revision rather than saving it for exam day.

  1. Take a past paper question set and cover the answer line for each question. Solve it, then run the four checks before you look at the mark scheme.
  2. Keep a short list of your own slips, such as wrong units, forgotten stages or misread questions. Review it before each practice paper so your checks target your real habits. The post on learning from practice-test mistakes shows how to sort errors by cause.
  3. Practise the timing. Aim to finish with a few minutes spare so you can use them as AQA suggests, going back through the paper to check your work.
  4. Show your working clearly. AQA notes that organised working helps the examiner find marks and also makes it easier for you to check later.

When not to check

If you have spent more than five minutes on one question, AQA's advice is to leave it and come back. Checking should not become a reason to run out of time on questions you have not yet attempted. Do your quick checks as you go, and keep a longer review for the end if you have time.

Your next step

Tonight, pick five questions from a past paper you have already attempted and run the four checks on your original answers. Note any that would have changed. That list is the start of your personal checking plan, and it tells you which check to prioritise in the exam.