Rounding & Estimation
Round to any place, estimate answers, and judge whether a result is reasonable.
1 The rounding rule: look right
To round, look at the digit just right of your target place. 5 or more rounds up; 4 or less rounds down.
47 to the nearest ten: the ones digit is 7 (5 or more), so round up to 50.
47 → ones digit 7 means round up: 50.
145 → tens digit 4 means round down: 100.
2 Exactly halfway rounds up
When a number sits exactly halfway (like 150 between 100 and 200), the rule says round up. This is the one case to memorize.
150 is exactly halfway between 100 and 200 → round up: 200.
3 Estimation: round, then compute
An estimate is a quick, close-enough answer. Round each number first, then add or subtract — and round only once, from the original numbers.
Estimate 348 + 52: first round 348 to the nearest hundred → 300, then 300 + 50 = 350.
4 Is that answer reasonable?
Estimation is a lie detector for arithmetic. If Mia says 48 + 52 = 110, estimate (50 + 50 = 100) and you will smell the mistake.
5,612 to the nearest thousand is 6,000 — so an answer of 5,000 for 5,612 + 300 would be unreasonable.
5 Common mistakes
Two rounding traps.
Wrong: 150 → 100 (the tens digit 5 “feels” small). Right: halfway always rounds up: 150 → 200.
Wrong: 149 → 150 → 200. Right: round once, from the original number: 149 → 100.
6 Quick checks
Try these yourself, then reveal the answer.
7 Key points and vocabulary
| Word | What it means |
|---|---|
| Round | Replacing a number with a nearby “clean” number. |
| Estimate | A quick, close-enough answer found by rounding first. |
| Halfway | Exactly between two round numbers; always rounds up. |
| Reasonable | Close enough to the estimate to be believable. |
Remember
- Look right: 5 or more rounds up, 4 or less rounds down.
- Halfway rounds up — always.
- Round once, from the original number.
- Estimate before you trust a computed answer.