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Use the calculator for your own numbers and compare another example instantly.
Round before adding or subtracting
Round each number to a place value that makes the mental arithmetic simple. For larger numbers, rounding to the nearest ten or hundred is often enough.
Estimated sum or difference ≈ rounded value ± rounded value
Worked example
For 862 − 421, round to 860 − 420 = 440. The exact difference is 441, so 440 is an excellent estimate.
Choose the right place value
Use finer rounding when the numbers are close together or when accuracy matters more. Use broader rounding when you only need a quick magnitude check.
Common mistakes
- Rounding one value to tens and the other to hundreds without a reason.
- Forgetting whether the estimate should be larger or smaller than the exact result.
- Using estimation when an exact value is required.
Frequently asked questions
What is a good estimate for 862 − 421?
860 − 420 = 440 is a convenient estimate.
Should I round both numbers the same way?
Usually yes, because consistent place-value rounding makes the estimate easier to interpret.
Can I estimate addition the same way?
Yes. Round both addends and add the rounded values.