The idea, step by step
An estimate is a sensible nearby answer
We estimate when an exact count is not needed yet. If one jar holds about 100 counters, four similar full jars hold about 400 counters. An estimate should come from information, not a random guess.
For calculations, choose nearby tens or hundreds that are easy to work with. To estimate 198 + 304, use 200 + 300 = 500.
Check, then decide whether exact is needed
An estimate helps catch a surprising answer. If 402 − 197 is reported as 605, an estimate of 400 − 200 = 200 shows that something is wrong.
Different sensible estimates can be accepted. Say which easy numbers you used. For buying enough materials, an estimate slightly above the amount may be more useful than one below it.
A common trap
Treating an estimate as an exact counted answer.
Instead: Use words such as about or approximately. Give an exact answer only after calculating or counting exactly.
See the thinking
Estimate a total
Estimate 296 + 208 using nearby hundreds.
- 296 is close to 300.
- 208 is close to 200.
- 300 + 200 = 500.
Estimate from equal groups
One tray holds about 20 seedlings. Estimate how many seedlings are in 6 similar full trays.
- Use the estimate of 20 for each tray.
- Six groups of 20 make 120.
Check your understanding
Would 801 be a sensible answer to 398 + 205? Use an estimate.
Show answer
No. A sensible estimate is about 600.
400 + 200 = 600, so 801 is much too large.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Estimate 192 + 311 using nearby hundreds.
- Estimate 603 − 198 using nearby hundreds.
- A bag has about 50 beads. Estimate the number in 4 similar bags.
- A class needs 97 sheets of card. Is a pack of about 100 sheets likely to be close to the amount needed? Explain.
- Noah says 487 + 212 = 999. Show an estimate that checks this claim.
- Estimate 72 + 39 with nearby tens. State the numbers you used.
Read the worked solutions
- Estimate 192 + 311 using nearby hundreds.
200 + 300 = 500.
About 500. - Estimate 603 − 198 using nearby hundreds.
600 − 200 = 400.
About 400. - A bag has about 50 beads. Estimate the number in 4 similar bags.
4 × 50 = 200.
About 200 beads. - A class needs 97 sheets of card. Is a pack of about 100 sheets likely to be close to the amount needed? Explain.
100 − 97 = 3. An estimate does not guarantee the exact supply.
Yes; 97 is only 3 below 100. Check the pack contains at least 97 before relying on it. - Noah says 487 + 212 = 999. Show an estimate that checks this claim.
500 + 200 = 700.
About 700, so 999 is not sensible. - Estimate 72 + 39 with nearby tens. State the numbers you used.
72 is close to 70 and 39 is close to 40; their sum is 110.
About 110, using 70 + 40.
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics
Numbers beyond 10,000; place value; ordering; estimating quantities and answers.
Official curriculum source ↗