VA ClassesRounding Decimals
Rounding is picking the nearer of two signposts. One digit decides it, and it is never the digit you are keeping, which is where most of the lost marks come from.
The idea
Rounding means replacing a number with the nearest tidy one.
Put the number on a line between its two neighbours and ask which one it sits closer to. That is all rounding has ever been, and a child who has drawn it once can rebuild the rule from scratch when they have forgotten it, which is not something you can say for the rule on its own.
Which digit decides
The rule
Look at the digit in the place you are rounding to. Then look at the ONE digit immediately to its right.
If that next digit is 5 or more, round up. If it is 4 or less, leave the digit alone.
Everything after the rounding place then disappears.
Exactly one digit decides the outcome. Not the whole tail of the number, and not the digit you are keeping either, which is a surprisingly common misreading of the rule.
Why five rounds up
Five sits exactly halfway, so neither neighbour is closer. Somebody had to choose, and the agreed choice is up. It is a convention, not a fact about distance, and telling a child that honestly saves them wondering.
Rounding to decimal places
"To one decimal place" means keep one digit after the point, and "to two decimal places" means keep two. The method itself never changes, only the place where you stop and look.
Take 3.267:
- To the nearest whole number: keep 3, look at 2. Two is under five, so it stays 3.
- To one decimal place: keep 3.2, look at 6. Six is five or more, so round up to 3.3.
- To two decimal places: keep 3.26, look at 7. Round up to 3.27.
The same number rounded up in two of those cases and stayed exactly where it was in the third, which is worth pointing out, because children often assume a number is either a rounding-up sort of number or a rounding-down one. It is neither. The answer depends entirely on where you were told to stop.
Worked examples
To the nearest whole number
- The whole number part is 12, so the two neighbours are 12 and 13.
- Look at the very next digit, the tenths: it is 6.
- Six is 5 or more, so round up.
- Everything after the point goes, leaving 13. The 2 in the hundredths never had a vote.
To one decimal place
- One decimal place means keeping 5.8, so the neighbours are 5.8 and 5.9.
- Look at the next digit only, the hundredths: it is 4.
- Four is under five, so 5.8 stays as it is.
- The 9 at the end is a trap. It is two places away and does not get a vote.
When rounding up carries
- Keeping one decimal place means starting from 9.9.
- The next digit is 7, so round up. But 9 tenths cannot become 10 tenths.
- Ten tenths is one whole, so it carries: 9.9 plus one tenth is 10.0.
- Write 10.0, not 10. The question asked for one decimal place, so the zero has to stay.
Common mistakes
1. Chain rounding
4.46 gets rounded to 4.5, and then that 4.5 gets rounded again to 5. Each step looks correct on its own, which is exactly why this is so hard to spot in a child's working. The original number was 4.46, comfortably under halfway, and it should have gone to 4. Rounding a number that has already been rounded quietly nudges it across the line, so always go back to the original and do it in one move.
2. Looking at the wrong digit
5.849 comes out as 5.9, because the 9 on the end looks big and important. Only the digit immediately after the rounding place gets a say, and everything beyond it is decoration. Have the child cover the rest with a finger before deciding, and the temptation vanishes along with the digits.
3. Dropping a zero the question asked for
The question asks for one decimal place and the answer comes back as 10, or asks for two and gets 3.4. When a question specifies decimal places it is telling you how precise the answer has to look, so it must show exactly that many even when the final digit is a zero. That zero is carrying information about precision, not taking up space.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
Check yourself
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For parents
Make them draw the line every time, at least for a week. Two neighbours, a halfway mark, a dot for the number. It takes about fifteen seconds and it converts a half-remembered rule into a picture the child can rebuild from nothing, which is what you actually want under exam pressure. Children who draw it stop chain rounding almost immediately, because the second rounding has nowhere to hide once the first one is on the page.
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