Rounding Decimals
Rounding is choosing the nearest signpost. One digit decides it, and it is never the digit you are keeping.
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 is closer to. That is all rounding ever is, and drawing it once saves a lot of rule-following later.
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.
Only one digit decides. Not the whole tail of the number, and not the digit you are keeping.
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. "To two decimal places" means keep two. The method never changes, only where you stop.
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.
Notice that the same number rounds up in two cases and stays put in the third. The answer depends entirely on where you 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
Rounding 4.46 to one decimal place to get 4.5, then rounding that to 5. The original number is 4.46, which is under halfway, so it rounds to 4. Always round the ORIGINAL number in one step. Rounding a rounded number quietly moves it.
2. Looking at the wrong digit
Rounding 5.849 to one decimal place and getting 5.9 because of the 9 on the end. Only the digit immediately after the rounding place counts. Cover the rest with a finger and the temptation disappears.
3. Dropping a zero the question asked for
Writing 10 when the question said one decimal place, or 3.4 when it said two. If a question specifies decimal places, the answer must show exactly that many, even when the last digit is a zero. That zero is information, not decoration.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
Check yourself
No sign up. Nothing is sent anywhere, the score stays on this device.
For parents
Make them draw the line. Two neighbours, a halfway mark, and a dot for the number. It takes fifteen seconds and it turns a rule they half remember into a picture they can rebuild. Children who draw it stop chain rounding almost immediately.
What to learn next
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