Round before checking
398 x 21 is about 400 x 20, so an answer near 8,000 is sensible.
Use friendly numbers to make the estimate quick.
Year 6
Use estimation to check answers to calculations and determine, in the context of a problem, an appropriate degree of accuracy
Estimate before or after calculating so you can check whether an answer is sensible and suitably accurate.
Lesson overview
Number - addition, subtraction, multiplication and division
An estimate is a quick, sensible value. It helps us spot impossible answers and decide whether a final answer needs to be exact or rounded for the context.
Accuracy depends on context
A population estimate might be rounded, but money in a bill may need exact pence.
nearest 1,000 people
total money
The question decides how exact the final answer should be.
Worked example
A calculation gives 8,358 for 398 x 21. Is that sensible?
| Estimate | Calculated answer |
|---|---|
| 8,000 | 8,358 |
- Round 398 to 400.
- Round 21 to 20.
- 400 x 20 = 8,000.
- 8,358 is close to 8,000, so it is sensible.
Try it
Estimate first or alongside the calculation, then compare the answer with your estimate.
Question 1
398 x 21 is about:
Question 2
Which answer is most sensible for 5,012 + 2,989?
Question 3
Which context usually needs the most exact answer?
exact money
estimate
rounded
Question 4
6,781 rounded to the nearest 100 is:
Question 5
A bus journey is measured to the nearest minute. Which answer is suitable?
journey time
nearest minute
Common mistakes
Watch for these when working through the lesson.
- Choosing an estimate that is not close enough to the original numbers.
- Thinking an estimate must match the exact answer.
- Giving an exact answer when the context only needs a rounded one.
Related topics
These ideas fit closely with this lesson.
- Round any whole number to a required degree of accuracy.
- Solve addition and subtraction multi-step problems in contexts.