Why Rereading the Same Working Is Not Enough
If you miscopy a number, drop a sign or press the wrong button, rereading your working just retraces the same path with the same mistake on it. Checking works when it is independent of the working you already did. Every technique below has that in common: each one looks at the answer from a new angle, so an error has somewhere new to show up.
Ability does not protect you from this either, and strong Maths students still make easy mistakes for exactly this reason. Good checking is a habit, not a sign that you doubt yourself.
Start With a Sense Check
Before anything else, ask one question: is this answer reasonable? Rounding the numbers to something easy before you calculate gives you an estimate, and comparing your final answer against that estimate catches the wild ones. A phone contract costing £2,300 a month, a probability of 1.4, a rectangle with a negative area or a cyclist travelling at 60 mph all fail a basic sanity test, and each takes seconds to spot once you are in the habit of asking.
Substitute Your Answer Back In
If the question gave you an equation to solve, put your answer back into the original equation, not into your rearranged version. If x = 4 is right, both sides should match with 4 in place. The same idea works elsewhere: a mean should sit sensibly between your smallest and largest values, the angles of a triangle should sum to 180 degrees, and a hypotenuse should be longer than either other side but shorter than the two added together. Substitution checks the Maths itself, which is why it catches errors rereading never finds.
Reverse the Calculation
Every operation has an opposite, and reversing is one of the fastest checks available. Check a subtraction by adding, a division by multiplying, a square root by squaring. If you increased a price by a percentage, undo the operation on your final amount and see whether you land back on the starting number. Reverse checks work on percentages, rearranged formulae and multi-step problems alike, and they need almost no extra working.
Check Signs, Units and Copied Numbers
Three of the cheapest mark-losers are a dropped negative sign, the wrong unit and a number miscopied from the question. Circle the key numbers as you read, then compare them against your working at the end. Look at every sign, especially after moving terms across an equals sign, and check the units the question asked for: if the question wants metres and you worked in centimetres, the number can be arithmetically perfect and still wrong. Transcription slips are invisible to rereading because your working is internally consistent, it just started from the wrong number.
Check What You Actually Told the Calculator
Calculator errors are input errors far more often than machine errors. Before pressing equals, read the display and confirm the numbers, signs, brackets and mode are what you intended. After pressing equals, run the sense check from earlier on the result. Common slips include missing brackets around negatives and fractions, a degree mode left on from a trigonometry question, and rounding too early in a long calculation. Our guide to calculator mistakes that cost marks in GCSE Maths lists these with fixes for each.
Reread the Actual Question
There is a difference between rereading your working and rereading the question. The question tells you exactly what a correct answer looks like: the rounding required, the units, whether an exact value or a decimal is wanted, and whether the real context means a decimal answer such as 3.7 tins should become 4. Plenty of mathematically correct answers lose marks because they answered a nearby question rather than the one printed. The last read before you finish a question should be the question itself.
Use Your Remaining Time Well
A checking routine only helps if you have minutes to use, which is why it pays to manage your time across the whole paper and bank a few minutes for checking instead of spending everything on the hardest question. When the time comes, work in this order: sense check anything that felt rushed, substitute back in on the questions you flagged, then sweep the paper for signs and units. Our guide to what to do with the last 10 minutes of a GCSE Maths exam turns that into a minute-by-minute routine.
Build the Habit Before Exam Day
Checking is a skill, and skills need practice. When you do past papers, stop with time left, run the full routine above and note what it caught. After a few papers you will know your personal pattern: the two or three checks that reliably rescue your marks. Practise those until they are completely automatic. If slips keep surviving your checks, our guide on how to stop making silly mistakes in GCSE Maths works through the deeper habits behind them.
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