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. They're about exam technique, silly errors, and habits that 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 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.
2. rushing through questions without reading them properly
You see "calculate the area" and immediately start multiplying length by width. Except the question actually 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: Force yourself to read each question twice before you start. Circle or underline key words like "perimeter," "volume," "simplify," "prove," or "give your answer in standard form." Check what units they want. Check if they want an exact answer or a decimal. These five seconds of careful reading can save you from losing easy marks.
3. not checking your answer makes sense
You calculate someone's age as 247 years old. Or work out that a car traveled 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. In measurement questions especially, examiners want to see that you understand what your answer represents.
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.
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 £0.10, something's wrong. Third, consider estimating 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
Sine and cosine get swapped. Area and perimeter get confused. Expanding and factorizing get muddled.
These aren't knowledge gaps. You know what sine and cosine 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 sine and cosine - 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 before plugging numbers in. This forces you to think about which formula you actually need.
7. stopping at the first answer when the question wants more
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.
How to avoid it: Circle the command words in the question. 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 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.
8. not knowing when to round (or rounding too early)
Sometimes you round to 2 decimal places when they wanted 3 significant figures. Sometimes you round in the middle of a calculation and then use that rounded number for the next step, making your final answer inaccurate. Sometimes you give an exact answer when they wanted a decimal, or vice versa.
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 2 decimal places," underline that. Do it. If the question doesn't specify, the rule is usually 3 significant figures or leave it exact (like π or √2). And never round in the middle of a calculation. Keep full accuracy until your final answer, then round once at the end.
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? Can you break it into smaller steps? Often these questions have easier parts at the start that unlock the harder parts. 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 or writing down a relevant formula.
There is literally no benefit to leaving it blank. Even an educated guess with some working shown gives you a chance at marks.
How to avoid it: Adopt a "no blanks" policy. If you're stuck, write down something. 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.
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 without learning loads of new content. 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.
*Want practice materials that help you build better exam technique? Our 15-mark GCSE worksheets use real exam questions and are designed to sharpen your problem-solving, timing, and approach.