10 Common Mistakes Students Make in GCSE Maths - and how to overcome them

After marking thousands of GCSE Maths papers, we see the same mistakes again and again.

The frustrating thing is that most of them are avoidable.

These mistakes aren’t always about not knowing the maths. That should always come first.

They are often about communication.

You need to understand what the question is telling you. You also need to clearly communicate what you know.

Poor mathematical communication can lead to silly mistakes and lost marks.

Here are the 10 mistakes we see most often and, more importantly, how to avoid them.

1. Not showing your working

This is a big one.

Some students think that getting the right answer is all that matters.

It isn’t.

GCSE Maths papers award method marks. This means you can still get marks even if your final answer is wrong.

But if you only write down an answer, the examiner cannot see your method.

One small arithmetic mistake could then cost you all the marks.

How to avoid it:

Write down every step, even the ones that seem obvious.

Think of your working as a conversation with the examiner.

They can only give you marks for what they can see.

This is particularly important with percentages.

For example:

10% = 24

This does not show your method.

Instead, write:

240 ÷ 10 = 24

Now the examiner can see what you have done.

If you make an arithmetic mistake later, you can still keep your method marks.

2. Not reading the question properly

You see the word area and immediately start multiplying.

But perhaps the question gave you the area and asked for the perimeter.

Or maybe the measurements are in different units.

Or perhaps the question wants your answer in a particular form.

This happens a lot under exam pressure.

You spot a familiar word and start working before you have understood the question.

How to avoid it:

Read each line carefully.

Use a ruler or piece of paper to cover the lines below.

Don’t move on until you have understood what that line is telling you.

If the question says perimeter, pause.

Remind yourself:

Perimeter = the distance around the outside.

You can also start working before you have finished reading the whole question.

For example, if Andy has 20% of 300 coins, you already know you can work out 20% of 300.

Do it.

You don’t need to wait until you have read the entire question.

Try to picture the real-life situation too.

If Andy is making flapjacks, imagine yourself as Andy.

The context can help you understand what the maths means.

3. Not checking if your answer makes sense

You calculate someone's age as 247 years old.

Or a car travelling at 400 mph on a residential street.

Or a probability of 1.7.

When you focus on the calculation, it is easy to forget about the real-life context.

How to avoid it:

After every answer, stop and ask:

“Does this make sense?”

Could someone really be this height?

Could a pizza really cost this much?

Can a probability be greater than 1?

If your answer seems strange, check your working.

You may have made a mistake.

4. Forgetting units

You calculate an area as 24.

But the answer should be 24 cm².

Or you calculate a speed as 60 instead of 60 mph.

Units matter.

In some questions, forgetting the unit can lose you the final mark.

You can sometimes even gain a mark for the correct unit when your calculation is wrong.

Units can also help you avoid mistakes.

For example, you might have a speed in km/h but a time in minutes.

Writing your units down can help you spot this.

How to avoid it:

Write your units as you work.

For example:

Area = length × width

= 6 cm × 4 cm

= 24 cm²

This is much safer than trying to remember the units at the end.

Remember, a value can only have one unit.

For example, £5.25p is not correct.

It should be either £5.25 or 525p.

5. Making calculator mistakes

Your calculator is only useful if you know how to use it properly.

Common mistakes include:

• Entering fractions incorrectly

• Forgetting brackets

• Using the wrong angle mode for trigonometry

• Forgetting to clear an earlier calculation

The biggest problem is that students often trust whatever answer appears on the screen.

But calculators can’t tell you if you have entered something incorrectly.

How to avoid it:

Learn how to use your calculator properly.

Know how to enter fractions and brackets.

Know how to change between degrees and radians.

Then check whether your answer makes sense.

For example, if you need 20% of £50, you should know the answer is around £10.

If your calculator says £0.1, stop and check what you entered.

A quick estimate in your head can help you spot calculator errors.

6. Mixing up formulas and basic operations

Sin and cos get swapped.

Area and perimeter get confused.

Expanding and factorising get muddled.

You might know all of these topics. But under pressure, they can get mixed up.

How to avoid it:

Keep a mistakes log.

Every time you mix something up, write it down.

For example:

“I keep confusing sin and cos — remember SOH CAH TOA.”

Review your mistakes regularly.

When using a formula, take a few seconds to write it down first.

Then substitute your values.

Put negative numbers in brackets.

This helps you avoid mistakes and makes your method clear to the examiner.

7. Not answering the question that was asked

The question says:

Solve x² = 25

You write:

x = 5

But there are two solutions:

x = 5 and x = -5

You have only answered half the question.

This happens in lots of different ways.

The question might ask for two examples and you only give one.

It might say show your working and you only give an answer.

Or you might answer part (a) and forget about part (b).

This is particularly common in written questions.

For example:

“Do you agree with Charlie?”

You need to actually say whether you agree.

How to avoid it:

Always read the last line of the question when you think you have finished.

Look for key words.

• Solutions → there may be more than one.

• Show → show your working.

• Simplify → simplify your answer.

• Give your answer in the form... → use the required form.

• Which is cheaper? → say which one and give evidence.

• Give two examples → give two.

If there is a part (b), make sure you have answered it.

For written questions, answer the question first.

For example:

“Do you agree?”

Yes, because...

“Are they correct?”

No, because...

“Which is more likely?”

Yellow, because...

Then support your answer with calculations or reasoning.

8. Rounding too early

Sometimes the question asks for 3 significant figures, but you give 2 decimal places.

Another common mistake is rounding too early in a calculation.

You then use the rounded number for the next step.

This can make your final answer inaccurate.

How to avoid it:

Read the question carefully.

If it asks for 3 significant figures, make sure your final answer has 3 significant figures.

When working through a longer calculation, avoid rounding too early.

I usually recommend keeping at least 5 significant figures during your working.

Using the ANS button on your calculator can also help you keep the most accurate value.

But still write your calculations down.

You don’t want to lose your full values if you need to go back and check your work.

9. Panicking when a question looks unfamiliar

You turn the page and see a question that looks completely different from anything you have practised.

You panic.

Your mind goes blank.

You skip it or start writing random numbers.

Don’t.

Unfamiliar questions are designed to make you think.

They are testing whether you can apply what you know in a new situation.

How to avoid it:

Stop and take a breath.

Read the question carefully.

What is it actually asking?

What topic does it relate to?

Read one line at a time.

Make sure you understand each line before moving on.

Can you rewrite the information in a different way?

Could you use algebra?

Could you use a formula?

Could you draw a diagram?

Sometimes a question is simply giving you a familiar problem in an unfamiliar way.

You might normally be given the information and asked to find the answer.

This time, you might be given the answer and need to work backwards.

Use what you know.

Start with something you can do.

Show your thinking.

Even partial working can earn partial marks.

10. Leaving questions blank

A blank answer gets zero marks.

Always try something.

You can still get method marks even if you are unsure of the final answer.

You might get marks for:

• Drawing a diagram

• Writing down a relevant formula

• Substituting into a formula

• Rewriting the question using algebra

• Finding a probability

• Finding a percentage

• Writing the correct units

• Completing the first step of a calculation

How to avoid it:

Don’t immediately decide that you don’t understand.

Take your time.

Read the question carefully.

Start writing things down.

You might be surprised by how much you can work out before you have even finished reading the question.

Write down a formula.

Draw a diagram.

Set up an equation.

Complete the first step.

Every mark counts.

The pattern?

Notice something?

Most of these mistakes aren’t about not knowing maths.

They are about:

• Reading carefully

• Showing your working

• Checking your answers

• Communicating your thinking

• Managing exam pressure

That is actually good news.

These are skills you can practise.

You don’t need to wait until exam day.

Build these habits into your revision now.

When you practise past papers, don’t just focus on getting the answers right.

Focus on how you work.

Show your working.

Check your units.

Read questions carefully.

Check your answers make sense.

And never leave a question blank.

These small changes can make a big difference to your marks.

Be strategic. Communicate your maths. Get every mark you can.

*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.