Rounding Numbers: Definition & Rules

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Rounding a number is useful when you want to estimate a value. To round a number, you need to follow three simple steps, which you will learn in this lesson.

Why Do We Need Rounded Numbers?

When we round a number, we make it simpler but keep its value close to the original number. Why is it ever necessary to round a number? There are several reasons. First, whole numbers are much easier to add, subtract, multiply and divide. If you just want an estimate of the answer and do not need to be exact, rounding is a great way to simplify such calculations. For example, suppose you are renting out a room in your home, and you need to determine the general square footage of the room for your online advertisement. You measure the room and find that it is 12.65 feet long and 10.43 feet wide. How would you determine the square footage?

area calculation

To figure this out, you could multiply these two numbers together to get an exact area for the room. But you don't really need to be that exact in this scenario, so you can simply round the dimensions of the room to 13 feet by 10 feet. Then, the area is about 130 square feet. That is exact enough to give potential renters an idea of the room's size, and it was a lot easier to calculate!

Also, sometimes you just don't need to have extra digits, and it's more sensible to round the number. For example, it wouldn't make sense to have more than two decimal places if you were calculating an amount of money, since we can't have a fraction of a penny. In that case, you would need to round to two decimal places.

Rules for Rounding Numbers

The rules for rounding a number are very simple. You can always do it correctly if you follow these three easy steps:

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