5 Common Maths Mistakes (and How to Avoid Them)
From sign errors to misreading questions — these are the marks students lose most often and how to keep them.
Most dropped marks in GCSE Maths aren't caused by topics students can't do. They're caused by five repeatable slips. Fix these and your grade usually moves before you've learnt anything new.
1. Sign errors when expanding and rearranging
Expanding −3(x − 4) as −3x − 12 is the single most common error in algebra. Write the multiplication out in two steps rather than doing it in your head, and put brackets around anything negative you substitute.
2. Rounding in the middle of a calculation
Rounding an intermediate answer and then using it changes the final figure, and you lose accuracy marks. Keep full precision on your calculator (use the ANS key) and round only the final answer, to the accuracy the question asks for.
3. Missing or wrong units
- Area is cm², volume cm³ — check your answer's dimension matches.
- Convert before you calculate, not after: mixing cm and m loses the mark.
- Speed, density and pressure answers all need units to earn full marks.
4. No working shown
A wrong final answer with correct working still earns method marks. A right answer with nothing written earns full marks only if the question didn't ask for working — and on multi-step questions it usually does. Write every line.
5. Answering a question you weren't asked
Questions ask for the perimeter when you found the area, the smaller angle when you found the larger, or the value of x when they wanted the length. Underline what's being asked before you start, and re-read it before you move on.
How to stop repeating them
Keep a single page of your own errors. Every time you mark a paper, add the slip and what you'll do differently. Read that page before every Maths session — it's the fastest revision you'll do.