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Common Mistakes

Common SATs Maths Mistakes (and How to Fix Them)

The errors children make most often in the KS2 maths papers — and exactly what you can do at the kitchen table tonight to fix each one.

Most lost marks come from a few fixable habits

When children lose marks in the maths papers, it is rarely because the work is too hard. Far more often it is the same handful of habits: adding fractions the wrong way, rushing past a step, or writing a bare answer with no working. Most of these go in a single short session at the kitchen table.

Below are the six mistakes children make most, each with a wrong-versus-right worked example, the fix, and one thing you can try tonight. If you want the marking system behind them — how marks are actually awarded — our guide to how SATs papers are marked covers that; this page is about the errors themselves and what you do about them.

For the bigger picture across all three subjects, see our Year 6 SATs revision hub, or jump straight into maths practice.

The six most common maths mistakes

Adding fractions by adding the denominators

What goes wrong: Your child adds the top numbers and the bottom numbers separately, so 1/4 + 1/3 becomes 2/7.

Why it happens: It feels like the natural way to add — but you can only add fractions once the pieces are the same size. That means rewriting both fractions over a common denominator first.

Wrong

1/4 + 1/3 = 2/7

Right

1/4 + 1/3 = 3/12 + 4/12 (rewrite both over a common denominator of 12) = 7/12

Sense-check: 2/7 is smaller than 1/3 on its own, and adding two positive fractions can never make the total smaller. So 2/7 has to be wrong.

The fix: Use the same three steps every time: find a common denominator, convert both fractions, then add only the numerators. The denominator stays the same. See more on our fractions topic page.

Try this at home tonight: Draw one bar split into quarters and another split into thirds, and ask your child why you cannot add them until both bars are cut into the same number of pieces (twelfths).

See this mistake in practice →

Throwing away the method mark in division

What goes wrong: On the 2-mark long-division question your child works it out in their head, writes a wrong answer with no method on the page, and scores zero — when the working alone was worth a mark.

Why it happens: According to the published mark schemes, the long-division and long-multiplication questions in the arithmetic paper are worth 2 marks each, and 1 mark is awarded for a correct formal written method even if there is a single arithmetic slip — but only when the working is there for the marker to see. (This applies to those specific multi-mark items, not to ordinary one-mark arithmetic questions.)

Wrong

4928 ÷ 16 = 38 (no written method, and the 0 in the answer is dropped)

Right

4928 ÷ 16 = 308 49 ÷ 16 = 3 remainder 1 → write 3 bring down 2 → 12; 12 ÷ 16 = 0 remainder 12 → write 0 bring down 8 → 128; 128 ÷ 16 = 8 remainder 0 → write 8 Check: 16 × 308 = 4928 ✓

The fix: Always write the full method, line by line, and keep a zero in the answer whenever the number you are dividing is smaller than the divisor. That zero is the digit children drop most often. See more on our long division topic page.

Try this at home tonight: Give one long-division question and ask your child to talk you through every line out loud. If they cannot show you the steps, that is the mark they are leaving behind.

See this mistake in practice →

Writing the answer with no working on reasoning questions

What goes wrong: On the reasoning papers your child writes a single final answer. If it is wrong, there is nothing for the marker to credit, so a 2- or 3-mark question scores zero.

Why it happens: Multi-mark reasoning questions carry method marks. The mark scheme can award a mark for a correct method even when the final answer is wrong — but a bare, incorrect answer gives the marker nothing to reward.

Wrong

A ticket costs £6. A group buys 7 tickets and pays with £50. How much change? Child writes: £2 (7 × 6 worked out wrongly in their head — 0 marks)

Right

7 × 6 = 42 50 − 42 = 8 Change = £8 (Even if the multiplication had slipped, the written "50 − ___" method earns the method mark.)

The fix: For every reasoning question, write the calculation, not just the answer. One line of working is the difference between zero and a method mark.

Try this at home tonight: Mark one of your child’s reasoning answers yourself using the "show me how you got there" rule — no working, no method mark.

See this mistake in practice →

Stopping halfway through a multi-step word problem

What goes wrong: Your child does the first calculation correctly, then answers with that number — missing the second step the question actually asked for. Or they pick the wrong operation under time pressure.

Why it happens: Reasoning questions often hide two or three steps in one short paragraph. Children who start calculating before reading to the end answer a different question from the one on the page.

Wrong

A baker makes 6 trays of buns with 8 buns on each tray. She sells 30 buns. How many are left? Child writes: 48 (stopped after 6 × 8 and forgot the buns sold)

Right

6 × 8 = 48 buns made 48 − 30 = 18 buns left Answer: 18

Sense-check: "How many are left?" must be fewer than the 48 made, so 48 cannot be the final answer.

The fix: Read the whole question first. Underline what it is actually asking, circle every number, and plan the steps before calculating. Then ask: does my answer make sense?

Try this at home tonight: Read one word problem together and have your child underline the question and circle every number before they pick up a pencil.

See this mistake in practice →

Working left to right instead of using order of operations

What goes wrong: Your child calculates 5 + 3 × 4 straight from left to right and gets 32, instead of doing the multiplication first.

Why it happens: Calculations follow a fixed order: brackets, then multiplication and division, then addition and subtraction. Reading straight from left to right gives the wrong answer, and the arithmetic paper tests this directly.

Wrong

5 + 3 × 4 = 8 × 4 = 32 (added first)

Right

5 + 3 × 4 = 5 + 12 (multiply first) = 17

The fix: Learn the order: brackets first, then × and ÷, then + and −. Scan the whole calculation for brackets, × and ÷ before writing anything down. See more on our order of operations topic page.

Try this at home tonight: Write 5 + 3 × 4 on a scrap of paper and ask for the answer. If they say 32, you have just found tonight’s ten-minute lesson.

See this mistake in practice →

Misaligning decimal columns and rounding the wrong digit

What goes wrong: Two related place-value slips: lining decimals up by their last digit instead of the decimal point, and rounding using the wrong column.

Why it happens: Place value is the foundation of decimal arithmetic. If the columns do not line up, every digit lands in the wrong place; if your child rounds the wrong digit, the answer is out by a whole unit.

Wrong

Adding 3.4 + 12.65 with the last digits lined up → wrong total Rounding 3.47 to 1 decimal place → 3.4 (the 7 was ignored)

Right

Line up the decimal points (fill the gap with a zero): 3.40 + 12.65 16.05 Rounding 3.47 to 1 d.p.: the next digit is 7, so round up → 3.5

The fix: Line up the decimal points, filling any gaps with zeros. When rounding, look only at the digit immediately to the right of the place you are rounding to.

Try this at home tonight: Ask your child to add 3.4 and 12.65 on paper and check they line up the decimal points, not the last digits.

See this mistake in practice →

Where this comes from

The maths SATs are set by the Standards and Testing Agency (opens in new tab), part of the Department for Education. The mark counts and method-mark rules above are the STA’s own, published in the mark schemes; the misconceptions come from the national curriculum. None of it is data of ours.

You can read the rules first-hand: the published KS2 past test papers and mark schemes (opens in new tab) show exactly how method marks are awarded, and the national curriculum mathematics programmes of study (opens in new tab) set out what children are expected to know by the end of Year 6.

The six mistakes at a glance

A quick checklist to screenshot and keep on the fridge.

The mistakeDo this instead
Adding the denominators (1/4 + 1/3 = 2/7)Find a common denominator first: 3/12 + 4/12 = 7/12.
No method shown in long divisionWrite every line of the method, and keep the zero in the answer.
A bare answer on a reasoning questionShow the calculation — a wrong answer with a correct method still earns a mark.
Stopping halfway through a word problemRead to the end, underline the question, do every step, then sense-check.
Working arithmetic strictly left to rightBrackets first, then × and ÷, then + and −.
Decimal columns misaligned / wrong digit roundedLine up the decimal points; round using the next digit only.

Related Guides

Frequently Asked Questions

What's the most common SATs maths mistake?

The most common avoidable one is losing method marks by not showing any working. On the multi-mark reasoning and long-division questions, a correct method earns a mark even when the final answer is wrong — but a bare, incorrect answer scores nothing. Close behind it is adding fractions by adding the denominators (writing 1/4 + 1/3 as 2/7 instead of 7/12).

What is a method mark?

A method mark rewards the correct approach rather than just the final answer. On specific multi-mark items — such as the 2-mark long-division and long-multiplication questions, and multi-step reasoning questions — the published mark schemes award a mark for a correct formal method even if there is one arithmetic slip. It only applies to those multi-mark questions, not to ordinary one-mark arithmetic, and only when the working is written down for the marker to see.

Why does my child keep getting fractions wrong?

Usually because they add the denominators as well as the numerators, which feels logical but ignores that the pieces must be the same size first. The fix is to rewrite both fractions over a common denominator before adding, then add only the numerators. A drawn bar model makes it click, and short, regular practice does the rest — you can try our fractions practice set to build the habit.

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