What is this skill?
Rounding means replacing a number with a nearby friendly number that is easier to work with. Third graders round whole numbers to the nearest ten or the nearest hundred. 347 rounded to the nearest ten is 350, because 347 is between 340 and 350 and closer to 350. Rounded to the nearest hundred it is 300, because it is between 300 and 400 and closer to 300.
Children first learn rounding on a number line, by finding the two tens or hundreds a number sits between and seeing which one it is nearer to. Then they learn the rule: look at the digit to the right of the place you are rounding to. If it is 5 or more, round up; if it is less than 5, round down. A number exactly halfway, like 345 to the nearest ten, rounds up to 350 by convention.
Why it matters
Rounding is the basis of estimation, one of the most useful everyday math skills. Estimating a total at the store, checking whether an answer is reasonable, or getting a rough sense of how long a trip will take all depend on rounding. A child who estimates 347 + 286 as about 350 + 290, or about 640, will notice if their exact answer comes out as 5,033.
Rounding also deepens place value. To round to the nearest hundred, a child has to know which digit is in the hundreds place and what the tens digit tells them. The same idea will be used to round larger numbers and decimals in fourth and fifth grade.
Common mistakes
- Looking at the wrong digit, such as using the ones digit when rounding to the nearest hundred. Underline the place being rounded to, then circle the digit right after it.
- Changing the digits after the rounding place incorrectly, and writing 347 rounded to the nearest ten as 357 or 35. Remind your child that the digits after the rounding place become zeros.
- Rounding down by lowering the digit, such as rounding 342 to 330. Rounding down means keeping the rounding digit the same.
- Struggling when rounding up crosses a hundred, as in 296 to the nearest ten, which is 300. A number line makes this one much clearer.
How to help at home
- Estimate at the grocery store: round prices to the nearest dollar or ten dollars and keep a rough running total.
- Draw a quick number line with two tens or hundreds at the ends, mark the number, and ask which end it is closer to.
- Round real numbers you see: the distance on a road sign, the number of pages in a book, the score in a game.
- Ask "Is it about 300 or about 400?" before working out an exact answer. Making estimating a habit is the real goal.
Worked examples
These are real questions from the Tutorsaurus rounding to 10 or 100 practice, with the answers worked out.
Example 1
Round 117 to the nearest ten.
Answer: 120. Look at the ones digit: 7. It is 5 or more, so round up to 120.
Example 2
Round 788 to the nearest hundred.
Answer: 800. Look at the tens digit: 8. It is 5 or more, so round up to 800.
Example 3
Round 248 to the nearest ten.
Answer: 250. Look at the ones digit: 8. It is 5 or more, so round up to 250.