What is rounding and why do we do it?
Rounding makes numbers easier to work with. Here are two examples:
- If a house is 4 meters and 3 centimeters tall we would say it’s 4 meters tall. Most times this is close enough to describe the height of the house.
- If you had a bucket with 598 marbles in it you could say you have about 600 marbles.
Remember, rounded numbers:
- are easier to work with in your head
- are only approximate – they do not tell you the exact answer
- are used when you just have to be close to the exact answer
Good rounding numbers resource from the web
How to round numbers
- Highlight the digit that may change.
- Underline the digit on the right.
- Round the highlighted digit
- If the underlined digit is 5 or more the highlighted digit is increased by 1.
- If the underlined digit is 4 or less the highlighted digit stays the same.
- All the digits to the right of the highlighted digit are changed to zeros.
Follow the menu options or use the following links to learn more and to see examples of how to round numbers:
There are also 3 rounding worksheet generators here that will provide an unlimited number of questions on rounding.
You can generate and make rounding matching cards here. Make as many pairs of cards as you wish. One way of using them when tutoring is to lay out the cards face-up and then challenge your children to pick up as many matching pairs as they can.
When you hear the following words you should know that they might mean you have to round numbers:
- close enough
Rules For Rounding Numbers
- Numbers can be rounded to the nearest ten, the nearest hundred, the nearest thousand, the nearest million, and so on.
- If the number you are rounding is followed by 5, 6, 7, 8, or 9, round the number up. Example: 79 rounded to the nearest ten is 80
- If the number you are rounding is followed by 0, 1, 2, 3, or 4, round the number down. Example: 74 rounded to the nearest ten is 70
- All the numbers to the right of the place you are rounding to become zeros.
- Rounded numbers are easier to remember and easier to use.
- Rounding decimals to the nearest whole number
- Rounding decimals to the nearest tenth
- Rounding decimals to the nearest hundredth
Estimating using rounding
Rounding helps when we are looking for an estimates amount. An estimate gives us a rough idea and it might be a little more or a little less than the exact amount. Here’s an example:
Let’s say we wanted to buy 6 pizzas and we knew they were $19 each. To get a rough idea of the total cost we can round 19 to the nearest 10 to get 20. We can then calculate 6 x $20 to give us an estimate of $120 for the total cost. Estimating is also a good way to check if our answers to a problem are reasonable.
By carrying out an estimate we can check that the answers to problems are sensible.
Try the matching games below for more practice with rounding.