Math Help: How to Round Decimals and Whole Numbers

How to Round Numbers

You can round numbers when you don't need a precise answer. We often use rounding when dealing with money. For example, if you owe someone $1.99, you might just give them $2.00 if neither of you has any change. Or, if you owed someone $1.03, they might just accept a dollar if you didn't have three cents.

Whole Numbers

To round a whole number like 43 or 127, start by looking at the digit in the ones place (the last digit to the right). If the number is five or higher, you'll round the number up. If it's four or lower, you'll round the number down.

Rounding Up

To round up, change the digit in the ones place to zero. Then, find the number in the tens place (it's one digit to the left of the ones place) and add one. For instance, since the number in the tens place of 127 is two, you'll change it to three and replace the seven with zero. The result is 130.

Rounding Down

If you're rounding down, simply change the number in the ones place to zero, and leave the rest of the number alone. For example, 43 would become 40, and 111 would become 110.

Using a Number Line

You can also think of rounding as finding the closest number on the number line. For example, 127 is in between 130 and 120 on the number line, but it's closer to 130. Likewise, 43 is closer to 40 than 50. Although numbers that end in five are technically at the midpoint between two base-ten numbers, they are traditionally rounded up.

Decimals

Closest Whole Number

Remember that decimals can also be found on the number line. If you're asked to round a decimal to the nearest one, you'll round it to the closest whole number. Numbers with decimals that are 0.5 or greater are rounded up, and those with decimals that are less than 0.5 are rounded down. Here are some examples:

3.7 rounds to 4.0

3.2 rounds to 3.0

3.5 rounds to 4.0

Nearest Tenth or Hundredth

You may also be asked to round decimals to the nearest tenth or hundredth. In this case, the same rules you followed for rounding whole numbers apply. If the last digit is five or greater, round up; if it's less than five, round down. The only difference is that you delete the last digit instead of replacing it with zero. Here are a few examples:

2.58 rounds to 2.6

2.54 rounds to 2.5

2.569 rounds to 2.57

2.563 rounds to 2.56