What is rounding numbers?
What is rounding numbers?
Rounding numbers means adjusting the digits (up or down) to make rough calculations easier. The result will be an estimated answer rather than a precise one.
Rounding numbers to the nearest 10
A good way of explaining this is to use a number line.
If the unit of the number is less than five, the number needs to be rounded down.
If the unit of the number is 5 or above, the number needs to be rounded up.
So 32 would be rounded down to 30, 35 would be rounded up to 40 and 38 would also be rounded up to 40:
Rounding numbers to the nearest 100
If the tens digit is less than 50 the number is rounded down.
If the tens digit is 50 or more, the number is rounded up. (The units digit can be ignored when rounding a three-digit number to the nearest 100.)
So 834 would be rounded down to 800, 851 would be rounded up to 900 and 876 would be rounded up to 900:
Rounding numbers to the nearest 1000
In Year 4, children need to round four-digit numbers to the nearest ten, hundred or thousand. Example questions may be as follows:
What is 4231 rounded to the nearest ten? (Answer: 4230)
What is 8163 rounded to the nearest hundred? (Answer: 8200)
What is 2839 rounded to the nearest thousand? (Answer: 3000)
Children are also expected to use their knowledge of rounding to estimate answers to calculations when checking their work.
For example: imagine a child has worked out 38 x 42 and got the answer 466.
In order to check if this is correct, it would be a good idea for them to round both 38 and 42 to the nearest ten and then multiply them, so the calculation they would do would be 40 x 40 = 1600. Although, 1600 is an approximate answer, it is completely different to 466 which means they would know that they had done something wrong in their initial calculating.
Rounding decimal numbers
Children also need to round decimals in Years 4, 5 and 6. In Year 4, they will be introduced to rounding decimals with one decimal place to the nearest whole number. In Year 5 they will move onto rounding decimals with two decimal places to the nearest whole number and to one decimal place. They may come across questions such as these:
|What is 3.97 rounded the nearest whole number?|
(Look at the first number after the decimal point - 9. Because it is more than 5 you round the 3 to the left of it up to 4.)
|What is 8.42 rounded to one decimal place?|
(Look at the second digit after the decimal point - 2. Because it is less than 5, you leave the 4 to the left of it the same.)
|What is 3.928 rounded to two decimal places?|
(Look at the third digit after the decimal point - 8. Because it is more than 5, you round the 2 to the left of it up to 3.)