01Why this matters
An estimate catches calculator mistakes and helps when an exact answer would imply more certainty than the measurements allow.
02See it
03Understand it
Rounding replaces a number with a nearby value at a chosen level of precision. Estimation finds an approximate answer useful for a purpose.
To round to a place, look at the next digit. If it is below 5, keep the chosen digit. If it is 5 or more, increase the chosen digit by one. This lesson uses the familiar school convention for positive numbers. Decide the place before rounding, and avoid repeatedly rounding intermediate calculations.
04Guided example
Round 3.476 m to two decimal places.
- Two decimal places means retain the hundredths digit, which is 7.
- Look at the next digit: 6 is at least 5, so round the 7 up to 8.
- The rounded result is 3.48 m. It lies within 0.005 m of the original.
05Try it
06Practise it
07Use it in the real world
To check a $19.80 item bought six times, estimate $20 × 6 = $120. The exact $118.80 total is plausible; $1,188 is not.
Go a little deeper
Rounding rules at exact ties can differ between software systems. Significant figures describe meaningful digits starting from the first non-zero digit, rather than a fixed number of decimal places.
08Quick check
09Summary
Rounding replaces a number with a nearby value at a chosen level of precision. Estimation finds an approximate answer useful for a purpose. The rounded result is 3.48 m. It lies within 0.005 m of the original.
Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.
10Key idea
19.8 × 6 ≈ 20 × 6 = 120
The ≈ symbol means approximately equal. Keep the original quantity’s units.
Round at the end when possible.
Find this in the formula library →