Every year, thousands of GCSE Maths students lose marks on exactly the same mistakes. Not because they don't understand the content, but because of small, avoidable errors that stack up across the paper.
I'm Aadam, and I've been tutoring GCSE students for over five years at SHLC. I mark hundreds of practice papers every year, and I can tell you that around 30% of students lose marks simply from repeating the same predictable mistakes.
The frustrating part? These aren't conceptual gaps. They're habits. And habits can be changed.
Here are the ten most common GCSE Maths mistakes I see repeatedly, why they happen, and exactly how to fix each one.
Mistake 1: Not Showing Working (The £23,000 Error)
What it looks like: Your child writes down the final answer with no working shown. The answer is actually correct, but they only get one mark instead of three because the examiner can't see their method.
Why it costs so many marks: Research shows that over 20% of students miss easy marks because they fail to show workings. In GCSE Maths, method marks often make up 40-50% of the total marks available. You can get a question completely wrong but still earn 2 out of 3 marks if your method was correct.
Given that each grade improvement is worth around £23,000 in lifetime earnings, this single mistake could literally cost your child thousands of pounds.
How to fix it: Make it a non negotiable rule: every answer must show every step, even for "easy" questions. Think of working as insurance. Even if your final answer is wrong, correct working earns marks.
Practice this with SHLC past papers. After completing each question, check the mark scheme to see which steps earned marks. You'll quickly see that showing working is essential, not optional.
My mock exam marking service specifically identifies where students lose method marks and teaches them exactly what working examiners want to see.
Mistake 2: Misreading the Question
What it looks like: The question asks for an answer in centimetres, your child gives it in metres. Or it says "estimate" but they give an exact answer. Or it asks them to "show that" something equals 5, but they just calculate that it equals 5 without showing the working.
Why it happens: Under exam pressure, students rush through the question text. Research from Cambridge Assessment shows around 30% of students lose marks simply because they didn't understand what was being asked.
The classic examples:
- Calculating a probability correctly but leaving it as 0.65 when the question asked for a fraction
- Finding the correct area but forgetting to include cm²
- Solving an equation correctly but not answering the actual question asked
- Missing key words like "estimate," "simplify," or "give your answer to 2 decimal places"
How to fix it: Develop the "underline and check" habit:
- Read the question twice before starting
- Underline key instruction words (estimate, simplify, show that, round to)
- Circle what form the answer should take (fraction, percentage, 2 d.p.)
- Before writing your final answer, check it against what you underlined
Practice this systematically. After completing a paper, go back through and check you've actually answered what each question asked for.
Mistake 3: Forgetting Units or Using Wrong Ones
What it looks like: Your child calculates the area as 24 but writes just "24" instead of "24 cm²". Or they work in centimetres throughout but give the final answer in metres without converting.
Why it costs marks: Examiners take accuracy seriously. A perfect answer without correct units can cost you the final mark. This is especially common in area, volume, and speed/distance/time questions.
Common unit errors:
- Writing cm instead of cm² for area
- Writing cm² instead of cm³ for volume
- Mixing units within one calculation (adding 2 metres to 50 centimetres without converting)
- Forgetting currency symbols (writing 45 instead of £45)
How to fix it: After every calculation, ask: "What units should this answer have?"
For area: units squared (cm², m²) For volume: units cubed (cm³, m³) For speed: distance units per time unit (km/h, m/s) For money: currency symbol (£, p)
Add a final check to your exam routine: before moving to the next question, glance at your answer and verify the units make sense.
Mistake 4: Not Using Reverse Checking
What it looks like: Your child finishes a question, writes the answer, and immediately moves on. No sense checking. No verification. They might have made a calculation error but they'll never spot it.
Why it matters: A quick sense check catches ridiculous answers before they cost marks. If you calculate someone's height as 0.3 metres or a shopping bill as £450 for three items, something's clearly wrong.
How to fix it: After every answer, ask: "Does this make sense?"
For geometry: Is this angle realistic? Can a triangle have sides this length? For percentages: Is 150% of something actually bigger than the original? For real world problems: Would a person really be this tall? Would this journey really take this long?
Students working towards grade 7–9 should also do reverse checking where possible. For example:
- Solved an equation? Substitute your answer back in to verify
- Found a percentage increase? Calculate backwards to check you get the original
- Worked out coordinates? Plot them to see if they make sense
Mistake 5: Silly Arithmetic Errors (Because Calculators Make Us Lazy)
What it looks like: Your child calculates 7 × 8 = 54. Or adds 23 + 19 = 41. Simple arithmetic that they'd get right in Year 6, but under exam pressure they rush and make careless errors.
Why it happens: Students become over reliant on calculators for even basic calculations. Then in the non calculator paper (which is one third of total marks), they panic because mental maths feels unfamiliar.
The most common arithmetic mistakes:
- Times table errors, especially 7×8, 6×7, 8×9
- Addition/subtraction errors when borrowing or carrying
- Negative number mistakes (especially subtracting a negative)
- Fraction arithmetic (adding fractions with different denominators)
- Division errors (especially long division)
How to fix it: Drill basic arithmetic daily. Use Corbett Maths 5-a-Day to maintain sharpness. The Foundation and Foundation Plus questions keep arithmetic skills active.
For times tables specifically, check out my guide on mastering times tables in hours.
Practice non calculator papers more than calculator papers. This forces mental arithmetic and reveals where gaps exist.
Mistake 6: Using Wrong Formulas (Or No Formula at All)
What it looks like: Your child confidently applies the wrong formula. They use the area of a triangle formula for a trapezium. Or they calculate volume of a cylinder but use the surface area formula instead.
Why it happens: Under pressure, similar looking formulas get mixed up. The volume of a pyramid (1/3 × base area × height) looks similar to the area of a triangle (1/2 × base × height). Students confuse them.
The most commonly confused formulas:
- Pythagoras' theorem vs trigonometry
- Circle area (πr²) vs circumference (2πr)
- Volume of cone vs pyramid
- Mean, median, mode (students mix up definitions)
- Compound interest vs simple interest
How to fix it: Create formula flashcards with three things on each card:
- The formula itself
- What each letter means
- A simple example using it
Test yourself daily. Write out all key formulas from memory. Use Maths Genie to practice applying each formula in context.
Crucially: understand WHY the formula works, don't just memorise it. Understanding prevents confusion between similar formulas.
Mistake 7: Rounding Too Early (The Cascade Error)
What it looks like: Your child rounds an answer to 2 decimal places halfway through a multi-step question, then uses that rounded number for the next calculation. By the final answer, the rounding error has compounded and they're miles from the correct answer.
Why it costs marks: Calculators can store full values, so there's no need to round until the very end. Rounding too early is just creating unnecessary errors.
How to fix it: One simple rule: Only round the final answer, never intermediate steps.
Keep full values in your calculator throughout multi-step questions. Use the ANS button or memory functions to carry exact values forward.
If you must write down intermediate answers (to show working), write them to at least 4 decimal places, but keep the full value in your calculator for the next calculation.
Mark schemes explicitly penalise premature rounding, so this is an easy fix that protects marks.
Mistake 8: Leaving Questions Blank (The Zero Mark Guarantee)
What it looks like: Your child sees a difficult question, panics, and writes nothing. Completely blank. Zero marks guaranteed.
Why it's so costly: Research shows around 30% of GCSE students admit to leaving questions blank due to panic or uncertainty. But even partial attempts earn marks.
Let me be clear: a blank answer gets zero marks. A partial attempt might get 2 or 3 marks out of 5.
How to fix it: Change the mindset from "all or nothing" to "something is better than nothing."
For geometry questions: Draw and label the diagram correctly, even if you can't solve it. That's often worth 1 mark.
For multi-step questions: Complete the first step. You might not know step 3, but step 1 earns marks.
For algebra: Set up the equation correctly, even if you can't solve it.
Practice this specifically. Take past papers and attempt questions you find difficult, focusing on gaining partial marks rather than perfect answers. My mock exam marking service shows you exactly where partial marks are available in every question.
Mistake 9: Not Managing Time Properly
What it looks like: Your child spends 20 minutes perfecting a 3 mark question at the start of the paper, then runs out of time and leaves the final 15 marks completely blank.
Why it happens: Students don't appreciate that not all marks are equal in terms of difficulty. A 3 mark question at the start might be easier than a 5 mark question at the end, but spending too long on it costs more marks overall.
The maths: Foundation and Higher papers both have 80 marks available across 1 hour 30 minutes. That's roughly 1 mark per minute, plus 10 minutes for checking.
If a question is worth 3 marks, you should spend roughly 3 minutes on it. If you've spent 8 minutes and you're stuck, move on and come back if time allows.
How to fix it: Practice papers under strict timed conditions. Set your phone timer and stick to it religiously.
Develop the habit of marking hard questions and returning to them. Don't let pride keep you stuck on one question whilst easier marks slip away elsewhere.
Use this timing strategy:
- Questions worth 1-2 marks: 1-2 minutes maximum
- Questions worth 3-4 marks: 3-5 minutes maximum
- Questions worth 5+ marks: 6-8 minutes maximum
- Final 10 minutes: checking
Track your timing when using SHLC past papers. Write down your start time for each question. This reveals where you're losing time.
Mistake 10: Not Converting Between Fractions, Decimals and Percentages Fluently
What it looks like: The question gives you 3/5 but you need to work with percentages. Your child wastes 3 minutes trying to remember how to convert, makes an error, and the entire question goes wrong.
Why this matters so much: Fractions, decimals and percentages appear in almost every paper, often hidden within other question types. An algebra question might require adding fractions. A geometry task might need converting decimals to percentages. These skills underpin everything.
The most common errors:
- Dividing the wrong way when converting fractions to decimals (doing denominator ÷ numerator)
- Forgetting to multiply by 100 when converting decimals to percentages
- Adding fractions without finding common denominators
- Calculating percentage increase incorrectly (especially using the wrong base)
How to fix it: These conversions must become automatic. Practice them daily until they're as natural as breathing.
Use Corbett Maths 5-a-Day Foundation Plus which regularly includes these skills.
Create a reference card with the key conversions:
- Fraction to decimal: numerator ÷ denominator
- Decimal to percentage: ×100
- Percentage to decimal: ÷100
- Common fractions: 1/2=0.5=50%, 1/4=0.25=25%, 1/5=0.2=20%, etc.
Memorise these common conversions so you're not calculating them under time pressure during exams.
Tracking These Mistakes (How to Actually Fix Them)
Knowing these mistakes exist means nothing if you don't track which ones YOUR child makes repeatedly.
Here's how to do this properly:
Step 1: Complete a past paper under timed conditions
Step 2: Mark it thoroughly using mark schemes
Step 3: For every lost mark, categorise the mistake Was it:
- Not showing working?
- Misreading the question?
- Wrong units?
- Arithmetic error?
- Wrong formula?
- Rounding error?
- Time management?
- Conversion error?
Step 4: Log the pattern Use my digital revision planner to track which mistakes appear repeatedly. After three or four papers, clear patterns emerge.
Step 5: Target the top three Don't try fixing everything at once. Identify the three mistakes costing the most marks and focus on eliminating those first.
Step 6: Retest after two weeks Complete another paper specifically watching for those three mistakes. Have they reduced?
This systematic approach is what I use with tutoring students at SHLC. Within 8-12 weeks, these repeated mistakes virtually disappear.
For Parents: How to Help Fix These Mistakes
Don't Just Mark Papers, Analyse Them Marking a paper and saying "you got 45/80" achieves nothing. Sit down together and categorise every lost mark. Which mistakes keep appearing?
Celebrate Improvement in Habits, Not Just Grades "You showed working on every question this time" deserves celebration, even if the grade hasn't jumped yet. Changed habits lead to changed grades, but it takes time.
Get Professional Feedback Periodically Even with the best intentions, parents can miss subtle patterns. My mock exam marking service provides expert analysis showing exactly where marks are being lost and why.
Make Checking Part of the Routine Build in time for checking. If your child completes practice papers in 85 minutes, they're not allowing time to catch mistakes. They should finish in 75-80 minutes with 10 minutes for checking.
Know When Expert Help Is Needed If these mistakes persist despite awareness and practice, your child might need teaching, not just reminders. Contact SHLC to discuss how targeted tutoring can eliminate these repeated errors.
The Bottom Line
These ten mistakes cost students thousands of marks every year. The frustrating part? None of them are about not understanding maths. They're about habits, exam technique and awareness.
The good news? Habits can change. With systematic practice, clear awareness and proper tracking, these mistakes reduce dramatically within weeks.
Remember: moving up just one GCSE grade in Maths is worth around £40,000 in lifetime earnings. Fixing these mistakes could literally earn your child tens of thousands of pounds.
Start tracking them today. The sooner you identify which mistakes are costing marks, the sooner you can eliminate them.
Want expert analysis of exactly which mistakes your child keeps making? My mock exam marking service provides detailed feedback on every lost mark. Get in touch with SHLC to discuss how I can help eliminate these repeated errors.