How to teach rounding and estimation
Year 7 (ages 12 to 13)
Rounding simplifies a number to a chosen level of accuracy (a whole number, 1 decimal place, and so on), and estimation uses rounded numbers to quickly check whether an exact answer is reasonable.
How to teach it
- Teach the rounding rule explicitly: look at the next digit, 5 or more rounds up, less than 5 stays the same.
- Practise rounding the same number to several different levels of accuracy (whole number, 1dp, 2dp) side by side.
- Introduce estimation as 'round first, then calculate', using round numbers that are easy to add or multiply mentally.
- Always compare an estimate with the exact calculated answer, discussing whether the difference is reasonable.
- Discuss what level of accuracy suits different real contexts (money, measurements, populations).
Worked example
Estimate 297 + 486 by rounding each number to the nearest 10, then adding 297 rounds to 300, 486 rounds to 490 300 + 490 = 790
Common mistakes
- Rounding down when the next digit is exactly 5 (the standard convention is to round up).
- Rounding each digit in a number separately instead of rounding the whole number once, at the correct place.
- Using an estimate as if it were the exact answer, rather than just a reasonableness check.
- Choosing an inappropriate level of accuracy for the context (e.g. rounding money to the nearest whole dollar when cents matter).
Frequently asked questions
How do you round a decimal to a given number of decimal places?
Look at the digit just after the place you are rounding to. If it is 5 or more, round the previous digit up; if it is less than 5, leave it as is, then drop the remaining digits.
Why estimate an answer before calculating it exactly?
Estimating first, e.g. by rounding each number, gives you a quick check that your exact answer is reasonable. If the exact answer is wildly different from the estimate, you likely made an error.
What level of accuracy should you round to?
It depends on the situation: money is usually rounded to 2 decimal places (cents), a measurement might round to the nearest millimetre or whole unit, depending on what precision is meaningful for the context.
What year is rounding and estimation taught at this level?
In the Australian Curriculum this is a Year 7 skill (AC9M7N05): rounding decimals to a level of accuracy that suits the situation, and using rounding and estimation to check answers are reasonable.
Practise with free worksheets
Printable worksheets with answer keys that are never wrong.