Why round?
Exact answers are great, but sometimes you just need a quick estimate. Is this close to 500 or 1,000? Rounding helps you:
- Check if your calculator answer makes sense
- Make mental math easier
- Give a quick, “close enough” answer
The rounding rule
To round a number to a given place:
- Find the rounding digit (the place you’re rounding to)
- Look at the digit one place to the right
- If it’s 5 or more — round up
- If it’s 4 or less — round down (keep the rounding digit the same)
- Replace all digits to the right with zeros
Example: Round 3,847 to the nearest hundred
- Rounding digit: 8 (hundreds place)
- Digit to the right: 4 (tens place)
- 4 < 5, so round down: 3,800
Example: Round 3,847 to the nearest thousand
- Rounding digit: 3 (thousands place)
- Digit to the right: 8 (hundreds place)
- 8 >= 5, so round up: 4,000
Estimating with rounding
To estimate a calculation, round first, then compute:
Estimate 489 + 312:
- 489 rounds to 500
- 312 rounds to 300
- Estimate: 500 + 300 = 800
- Exact answer: 801 — our estimate was very close!
Find the digit
Look right
Decide
Result
Look at the digit to the RIGHT of your rounding place. 5 or more = round up. 4 or less = round down.
Practice
What is 6,473 rounded to the nearest hundred?
The hundreds digit is 4. Look right: 7 ≥ 5, so round up to 6,500.
What is 24,851 rounded to the nearest thousand?
The thousands digit is 4. Look right: 8 ≥ 5, so round up — 4 becomes 5, giving 25,000.
Round 7,345 to the nearest ten.
The tens digit is 4. Look right: 5 ≥ 5, so round up to 7,350.
Estimate 692 + 215 by rounding each number to the nearest hundred first.
692 rounds to 700, 215 rounds to 200. Estimate: 700 + 200 = 900.
A school has 1,247 students. The principal says 'about 1,200 students'. Which rounding did she use?
1,247 rounded to the nearest hundred: look at the 4 in tens place. 4 < 5, round down → 1,200.
Estimate 48 × 21 by rounding each factor to the nearest ten.
48 rounds to 50, 21 rounds to 20. Estimate: 50 × 20 = 1,000. (Exact answer is 1,008.)
Challenge
Quick-Fire Round