Number & Operations in Base Ten
Rounding Whole Numbers
Rounding to the nearest 10 or 100 using place value.
Finding a Nearby Round Number
Sometimes an exact number isn't needed — just a close, easy-to-work-with estimate. Rounding replaces a number with the nearest round number, like the nearest ten or nearest hundred, using place value to decide which round number is closer.
Definition — Rounding
To round to the nearest ten, look at the ones digit. If it's 5 or more, round up to the next ten; if it's 4 or less, round down to the current ten. Rounding to the nearest hundred works the same way, but looks at the tens digit instead.
Worked Example — Rounding to the Nearest Ten
Worked Example — Rounding to the Nearest Hundred
Tip
Common Mistakes
Checking the wrong digit, such as looking at the ones digit when rounding to the nearest hundred instead of the tens digit.
Always look at the digit immediately to the right of the place being rounded to — the tens digit decides rounding to the nearest hundred, and the ones digit decides rounding to the nearest ten.
Rounding down a number whose deciding digit is exactly 5, such as rounding 45 down to 40.
A deciding digit of exactly 5 always rounds up, not down — 45 rounds up to 50.
Key Takeaways
- Rounding replaces a number with the nearest round number at a chosen place value.
- A deciding digit of 5 or more rounds up; a deciding digit of 4 or less rounds down.
- Rounding to the nearest hundred checks the tens digit; rounding to the nearest ten checks the ones digit.
Summary
Rounding gives a fast, useful estimate using place value. That same estimating skill is useful for checking the next lesson's work — adding and subtracting larger, multi-digit numbers.
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