GCSE Maths: The Most Common Exam Mistakes

By Russell Greenhill

Founder & CEO @ Greenhill Academics

Oxford Master’s Graduate • 8+ Years Tutoring Experience

GCSE Maths examiners publish a report after every exam series. Year after year, the same mistakes appear. Students who know what those mistakes are and fix them before the exam gain marks that other students consistently lose. This post covers the most common errors that cost GCSE Maths students marks. Specifically, it draws from AQA, Edexcel, and OCR examiner reports. It also reflects the patterns that specialist Maths tutors see repeatedly across students at every grade level.

If the grade is not reflecting your child’s effort, there is a strong chance one or more of these mistakes is involved.

Mistake 1: Not showing working

This is the single most consistent source of lost marks in GCSE Maths, cited in virtually every examiner report across all three major boards. Method marks often make up 40 to 50 percent of the available marks on multi-step questions. A student who writes only the final answer scores one mark regardless of whether it is correct. However, a student who shows full working can earn the majority of marks even when an arithmetic slip occurs.

The fix is straightforward. Every step of every calculation must be written down, even for questions that feel easy. This is especially important on non-calculator papers, where showing method is the only way to earn partial credit when an error occurs. Practise this until it becomes automatic. Your child should not be relying on remembering it under pressure.

Mistake 2: Misreading the question

Examiner reports note this consistently as one of the most frustrating sources of avoidable mark loss. A question might ask for the area of a shape, but a student calculates the perimeter. Another asks for an answer in centimetres, but the student gives metres. Another says “estimate”, but the student works out an exact answer. In each case, the student has done the maths correctly and scored zero.

The fix is to read every question twice before writing anything. Your child should identify specifically what is being asked, in what form, and in what units. Underlining the key instruction in the question is a simple habit that takes seconds and saves marks. On longer questions, checking the answer against what was actually asked, rather than what was assumed, catches this error before it costs marks.

Mistake 3: Rounding too early

On calculator papers, rounding intermediate values before the final step is a consistent source of error. If your child rounds a value mid-calculation, that rounding error compounds through the remaining steps. The final answer then differs from the mark scheme, even when every step of the method was correct. Examiners see this frequently and note it specifically in reports as an avoidable error.

The fix is simple: keep full values in the calculator until the very end. Round only the final answer. When a question asks for a specific number of decimal places or significant figures, that instruction applies to the answer, not to intermediate working. Building this habit in practice papers before the exam eliminates a consistent source of mark loss. Importantly, it has nothing to do with understanding the maths.

Mistake 4: Mishandling negative numbers and brackets

Algebra questions consistently generate mark loss through sign errors. The most common error, flagged repeatedly in AQA examiner reports, is incorrectly expanding a bracket with a negative coefficient. For example, expanding -(2x + 2) as -2x + 2 rather than -2x – 2 is a persistent error. It costs marks on questions students otherwise answer correctly. Similarly, misapplying the rules of indices or confusing the order of operations in complex expressions are regular sources of lost marks.

The fix is to slow down on any step involving a negative sign. Write each expansion explicitly rather than trying to do it mentally. For higher-tier students, regular timed practice on algebraic manipulation builds the accuracy the mark scheme rewards.

Mistake 5: Missing or wrong units

Forgetting to include units in answers costs marks on measurement, area, volume, and speed-distance-time questions. Examiners across all boards flag this regularly. A correct numerical answer without the correct unit loses the final mark on that question. Area questions require squared units (cm², m²). Volume questions require cubed units (cm³, m³). For speed, the correct unit is compound, such as km/h or m/s. Writing the unit is not optional. It is part of the complete answer.

The fix is to build a final-step check into every measurement question. Before moving on, your child should verify that the answer includes a unit. Specifically, the unit must match what the question asked for. This takes two seconds and is the difference between full marks and losing the final mark on an otherwise correct answer.

Mistake 6: Poor performance on multi-step problems

Multi-step problems require several linked calculations to reach a final answer. They are where the largest mark gaps open between students. Examiner reports note that many students make good progress on the first step. However, they then lose the thread, write haphazard working, or give up when they are unsure of the next move. Because these questions carry more marks, they have the biggest impact on the overall grade. Consequently, they deserve the most focused preparation.

The fix is to approach multi-step problems methodically. Your child should identify what information they have and what they are trying to find. In particular, they should plan the intermediate steps before writing anything down. Writing each step clearly and labelling intermediate values makes it far easier to catch errors. It also earns method marks even when the final answer is wrong. Practising these questions specifically, rather than avoiding simpler practice, is the most efficient way to close the gap between a grade 5 and a grade 7.

Which GCSE Maths tutors can help fix these habits?

Murray

Murray is reading Materials Science at Oxford (MEng, expected First), having achieved A* in Maths, Chemistry, and Physics at A Level. With over 150 hours of tutoring experience across the sciences and mathematics, he has a precise, methodical approach to identifying where students lose marks. Murray is particularly effective at building the systematic working habits that GCSE Maths rewards at the top grades. He helps students stop losing marks through the small, fixable errors that examiner reports flag every year.

Ejaz

Ejaz is reading MSci Mathematics at Imperial College London (First Class expected), having achieved a 7 in IB Higher Level Mathematics and 8 Grade 9s at GCSE. With over 100 hours of tutoring experience in Maths and Physics, Ejaz builds the systematic problem-solving approach that GCSE Maths rewards. Specifically, he focuses on the habit of showing full working on every question and writing answers in the precise form the mark scheme requires.

Martin

Martin holds an MSc in Mathematical Sciences from Oxford (Distinction) and a First Class BSc in Mathematical Sciences from Bath, having achieved A*A*A*A at A Level. He is currently completing a fully funded PhD in Applied Maths and Theoretical Physics at Cambridge. Martin tutors GCSE Maths with the depth of understanding that comes from working at the highest level of the subject. He helps students understand why methods work, rather than simply how to apply them.

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