Top 10 Mistakes Students Make in GCSE Maths
After marking thousands of GCSE maths papers, we've seen the same mistakes appear again and again. The frustrating thing? Most of them are completely avoidable.
These aren't about not knowing the content, although that should always come first. They're about communication. This can be understanding what is being communicated to you in the question, or how you are communicating your understanding of the situation given. Lack of mathematical communication skills can cause silly errors and habits to cost you marks without you even realizing. Here are the ten mistakes we see most often, and more importantly, how to fix them.
1. Not showing your working
This is the big one. Students think if they get the right answer, that's all that matters. Wrong.
GCSE maths papers award method marks. This means even if your final answer is incorrect, you can still pick up two, three, or even four marks for showing the right approach. But if you just write down an answer with no working? One arithmetic mistake and you get zero marks, even if you knew exactly what to do.
How to avoid it: Write down every step, even the ones that feel obvious. Show what you're doing and why. Think of your working as a conversation with the examiner. They can only give you marks for what they can see on the page. This is most common with percentages. How often have you written ‘10%=24’ instead of ‘240÷10=24’. The 1st method is not showing your working however the 2nd will retain all method marks going forward in the situation where you make an arithmetic error (or even miscopy your value as 290 for example).
2. Rushing through questions without reading them properly
You see "area" and immediately start multiplying given values. Except the question actually gave you the area and asked for the perimeter. Or it gave you measurements in different units. Or it wanted your answer in a specific form.
This happens constantly under exam pressure. You skim the question, spot familiar words, and jump straight in without actually reading what's being asked.
How to avoid it: Pay attention to each line of the question. Use a ruler or a piece of paper to stop yourself from reading further until you have fully understood each word in that line. If the question mentions ‘perimeter’ then pause and remind yourself ‘perimeter is the distance around the outside. If the question tells you that Andy has 20% of the 300 coins, then work out what 20% of 300 is as that is a more useful piece of information, you do not need to finish reading the question to know that you need to calculate that. If Andy is making flapjacks, then picture yourself as Andy, real life context should help with the calculations.
3. Not checking your answer makes sense
You calculate someone's age as 247 years old. Or work out that a car travelled at 400mph on a residential street. Or find that a probability is 1.7.
When you're focused on the calculation, you stop thinking about whether your answer actually makes sense in real life. Examiners see this all the time, and it's a clear sign you're not engaging your brain beyond just following steps.
How to avoid it: After every answer, pause and ask yourself: "Is this reasonable?" Does this person's height make sense? Could this actually be the cost of a pizza? Is this probability possible? If your answer seems weird, check your working. You've probably made a mistake somewhere.
4. Forgetting to include units
You calculate the area as 24. Not 24cm². Just 24.
Or you work out the speed as 60. Not 60mph or 60km/h. Just 60.
It seems picky, but units matter. In some questions, missing units can cost you the final mark even if your number is correct. Similarly you can sometimes gain a mark for the correct units of the answer with no correct calculations. In measurement questions especially, examiners want to see that you understand what your answer represents.
Keeping units within your calculations can also prevent mistakes and keep the flow of the question, for example if you need to calculate the distance but your speed is in km/h and your time is in minutes. Or you calculate a value in pence but think the final answer is in £.
How to avoid it: Write the units down as you go, not just at the end. If you're calculating area, write "Area = length × width = 6cm × 4cm = 24cm²" rather than doing all the calculation and trying to remember the units at the end. This will help remind you what each value is representing, as well as ensuring correct units are given. Remember that values cannot have more than one unit, for example £5.25p is 2 units and not an acceptable answer, it is either in £ or pence, not both.
5. Calculator errors (and not spotting them)
Your calculator is not your friend if you don't know how to use it properly.
Common calculator mistakes: entering fractions wrong, forgetting brackets in complex calculations, having it in the wrong angle mode for trigonometry (degrees vs radians), not clearing the previous answer before starting a new question.
The worst part? These errors are often wildly wrong (like getting 0.017 instead of 17), but students don't notice because they're just trusting whatever the calculator says.
How to avoid it: First, learn your calculator properly. Know how to enter fractions, use brackets, and switch angle modes. Second, always do a sense check on calculator answers. If the question asks for 20% of £50 and your calculator gives you 0.1, something's wrong. Third, consider a quick estimate in your head first. If you know 20% of 50 is roughly 10, you'll immediately spot when your calculator gives you something ridiculous.
6. Mixing up basic operations and formulas
Sin and cos get swapped. Area and perimeter get confused. Expanding and factorizing get muddled.
These aren't knowledge gaps. You know what sin and cos are. But under pressure, these fundamental things get mixed up, and suddenly you're using the wrong formula for the entire question.
How to avoid it: Create a mistakes log. Every time you mix something up, write it down: "I always confuse sin and cos - SOH CAH TOA." Review this list weekly. Your brain will start to catch these errors before you make them. Also, when you see a question involving formulas, take three seconds to write down the formula with each value substituted -with negative numbers in brackets- before calculating anything, this will ensure you get the marks for substitution.
7. Actually answering the question asked
The question says "solve x² = 25" and you write "x = 5" and move on.
But there are two solutions: x = 5 and x = -5. You've done half the work and lost half the marks.
This happens everywhere. Quadratic equations with two solutions. "Give two examples" when you only give one. "Show your working" when you just write the answer. Questions with multiple parts where you answer (a) but forget (b) exists.
Commonly this happens in the written questions when asked ‘do you agree with Charlie’ or ‘Can Dani afford the rug?’.
How to avoid it: Always read the last line of a question when you think you have finished. If it says "solutions" (plural), you need more than one. If it says "show," you need to show steps, not just state an answer. If it says ‘simplify your answer’ then simplify. If it says ‘give your answer in the form…..’ then make sure that’s what you have given. If it askes where it is cheaper, make sure to tell them with your values for evidence. If there's a part (b), make sure you actually answer it. Before moving to the next question, glance back and check you've done everything asked.
If it is a written answer, then the first thing to do is answer the question before giving your reason. ‘Do you agree?’ Yes/no because…… ‘Are they correct?’ yes/no because….. ‘Which is more likely?’ yellow because…. These answers can be supported with a correct calculation, a counter example to prove otherwise, or a quick explanation.
8. Not knowing when to round (or rounding too early)
Sometimes you round to 2 decimal places when they wanted 3 significant figures. A common way to lose marks is when rounding in the middle of a calculation and then use that rounded number for the next step, making your final answer inaccurate.
Rounding rules lose students marks constantly, and most don't even realize they're making mistakes.
How to avoid it: Read what the question wants. If it says "give your answer to 3 significant figures” then your answer needs to be accurate to 3 significant figures. So any rounding in the middle of a question needs to be to more than that to avoid rounding errors and losing accuracy marks. I usually use at least 5 significant figures, but you can never be too accurate. It’s a good idea to use the ‘ans’ key in your calculator to get the most accurate values, but make sure you still write your values (and calculations) down to ensure marks and that you don’t lose your full values if you need to go back a step.
9. Panicking on unfamiliar questions instead of breaking them down
You turn the page and see a question that looks nothing like anything you've practiced. Panic mode activates. Your mind goes blank. You skip it or write random numbers hoping for the best.
Here's the thing: unfamiliar questions are deliberately designed to test whether you can apply what you know to new situations. They're not testing secret content you've never seen. They're testing if you can think.
How to avoid it: When you see an unfamiliar question, stop and breathe. Read it carefully. What is it actually asking? What maths topic does this relate to? Read one line at a time and don’t continue reading until you have understood every word and context in that line. Can you write any of that in a different way? Maybe using algebra, or probability, there might be a formula for that. Sometimes questions on topics we understand can be given to us backwards, so we have been given what we would usually recognise as the answer, so use what you know and work it backwards (or forwards but with ‘x’ in place of the missing value to create an equation). Start with what you can do. Show your thinking. Even partial work gets partial marks.
10. Leaving questions blank
This is the ultimate mistake. A blank answer scores zero. Always.
But here's what students forget: you can get method marks even if you're not sure. You can get marks for showing you understand part of the problem, even if you can't finish it. You can get marks for drawing a diagram, writing correct units, substituting into a correct formula, rewriting the question algebraically, finding a probability, saying what percentage the given value is representing.
How to avoid it: Don’t rush into convincing yourself that you do not understand, take your time reading and understanding the question. Write some things down or calculate whilst reading the question. You may be surprised at how much you can write down before you have finished reading the question. Write the formula you think might be relevant. Draw a diagram. Show the first step even if you don't know what comes next. Set up an equation even if you can't solve it. These aren't desperate moves - they're strategic mark-grabbing in an exam where every mark counts. Sometimes your initial thought is the right one.
The pattern here?
Notice something? Most of these mistakes aren't about not knowing maths. They're about exam technique, reading carefully, showing working, checking answers, and not panicking.
This is actually good news. It means you can improve your grade significantly by ensuring that you are fully communicating your understanding. You just need to be more strategic about how you approach exam questions.
Start practicing these habits now, not the night before your exam. Build them into your revision. When you practice past papers, don't just focus on getting answers right. Focus on showing working clearly, checking units, reading questions twice, and never leaving blanks.
These small changes add up to big grade improvements.
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