Rounding and Estimation

Grade 3 · Math · Study Guide

Learn how to round numbers to the nearest ten or hundred and use estimation to check answers.

Rounding means replacing a number with a nearby number that is easier to work with. To round to the nearest ten, look at the ones digit. If it is 5 or more, round up. If it is 4 or less, round down. For example, 47 rounds up to 50, and 43 rounds down to 40.

To round to the nearest hundred, look at the tens digit. If it is 5 or more, round up to the next hundred. If it is 4 or less, round down. For example, 360 rounds up to 400, and 330 rounds down to 300.

Estimation means using rounded numbers to get an approximate answer. This is useful when you need a quick answer or want to check if your exact answer is reasonable. For example, to estimate 48 + 31, round to 50 + 30 = 80. The exact answer of 79 is close to 80, so it is reasonable.

A number line can help you round. Find your number on the line and see which ten or hundred it is closest to. If it is exactly in the middle (like 25 or 250), round up by convention.

Key terms

Rounding: Replacing a number with a close number that ends in zero or another convenient value.
Estimation: Finding an approximate answer using rounded numbers.
Round up: Increase a number to the next ten or hundred when the deciding digit is 5 or more.
Round down: Keep the number at the current ten or hundred when the deciding digit is 4 or less.
Nearest ten: The multiple of 10 that is closest to a given number (e.g., 47 rounds to 50).
Nearest hundred: The multiple of 100 that is closest to a given number (e.g., 360 rounds to 400).

Flashcards

Rounding rule: ones digit is 5–9: Round up to the next ten.
Rounding rule: ones digit is 0–4: Round down — keep the same tens digit.
Round 83 to the nearest ten: 80 (ones digit is 3, so round down)
Round 85 to the nearest ten: 90 (ones digit is 5, so round up)
Round 650 to the nearest hundred: 700 (tens digit is 5, so round up)
Round 249 to the nearest hundred: 200 (tens digit is 4, so round down)
Estimate 61 + 38: 60 + 40 = 100 (exact answer is 99, which is close)
When is estimation useful?: When you need a quick answer, want to check work, or need to know 'about how many'.

Questions

What is 76 rounded to the nearest ten?
1. 70
2. 80
3. 75
4. 100
Show answer
Answer: 80
76 has a ones digit of 6, which is 5 or more, so round up to 80.
The number 45 rounded to the nearest ten is 40.
Show answer
Answer: False
45 has a ones digit of 5. When the digit is exactly 5, we round up, so 45 rounds to 50.
What is 342 rounded to the nearest hundred?
1. 300
2. 340
3. 350
4. 400
Show answer
Answer: 300
342 has a tens digit of 4, which is less than 5, so round down to 300.
To estimate 53 + 29, round each number to the nearest ten: 50 + 30 = ___.
Show answer
Answer: 80
53 rounds to 50 and 29 rounds to 30. 50 + 30 = 80.
A store has 478 items. About how many items is that, rounded to the nearest hundred?
1. 400
2. 470
3. 500
4. 480
Show answer
Answer: 500
478 has a tens digit of 7, which is 5 or more, so round up to 500.

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