Common GCSE Maths Mistakes
Many GCSE Maths marks are lost through avoidable mistakes. Students may understand the topic but lose marks because of unclear working, sign errors, calculator mistakes or rushing.
This page explains common GCSE Maths mistakes and how to avoid them.
1. Not showing enough working
GCSE Maths examiners often award method marks. If a student writes only an answer and it is wrong, they may lose marks that could have been earned from correct working.
Write down each important step, especially in algebra, ratio, geometry and multi-step problem solving.
2. Sign mistakes with negative numbers
Negative number mistakes are very common, especially in algebra and substitution.
A common mistake is thinking -3² means 9. Without brackets, -3² means -(3²), so the answer is -9. But (-3)² = 9.
Helpful topic: Negative Numbers
3. Forgetting BIDMAS
The order of operations matters. Multiplication and division should be done before addition and subtraction unless brackets change the order.
Helpful topic: BIDMAS
4. Rounding too early
Rounding too early can make the final answer inaccurate. Unless the question tells you to round during the calculation, keep full accuracy until the final step.
Helpful topic: Rounding and Estimation
5. Confusing decimal places and significant figures
Decimal places count digits after the decimal point. Significant figures start from the first non-zero digit.
For example, 0.0482 to 2 decimal places is 0.05, but to 2 significant figures it is 0.048.
6. Cancelling incorrectly in algebraic fractions
Students can cancel common factors, but not parts of terms joined by addition or subtraction.
Helpful topic: Algebraic Fractions
7. Not putting data in order before finding the median
To find the median, the data must be in order first.
Helpful topic: Mean, Median, Mode and Range
8. Mixing up area and perimeter
Perimeter is the distance around the outside. Area is the space inside. These are different and use different units.
Helpful topic: Area and Perimeter
9. Forgetting units
GCSE questions often require units such as cm², m³, degrees, kg or pounds. Missing or incorrect units can lose marks.
At the end of a question, check whether the answer needs units and whether the units are squared or cubed.
10. Misreading probability questions
Probability questions often include words such as “and”, “or”, “at least” and “not”. These words change the method.
Helpful topics: Probability Basics and Tree Diagrams
11. Drawing inaccurate graphs
In graph questions, students can lose marks through inaccurate scales, poorly plotted points or unclear lines.
Helpful topic: Straight Line Graphs
12. Calculator entry mistakes
Calculator errors are very common. Brackets, fractions, powers and standard form should be entered carefully.
Estimate before using a calculator. If your calculator answer is very different from your estimate, check the input.
How to reduce mistakes
- show working clearly
- circle or underline key information
- estimate answers before calculating
- check signs carefully
- repeat questions you got wrong
- practise timed exam questions
For a wider revision plan, read how to revise GCSE Maths effectively.
Video explanation
A short Worthing Maths Tutor video explanation for common GCSE maths mistakes can be embedded here later to improve student engagement and time on page.
Useful GCSE Maths Revision Pages
Need help with GCSE algebra?
If your child understands examples in lessons but struggles to apply them independently, structured GCSE maths tutoring can help rebuild confidence and close gaps step by step.