Simple Rules for Rounding Numbers Correctly

Follow these steps to round any number quickly and accurately

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Rounding numbers is important to preserve significant figures in calculations and to record long numbers. In everyday life, rounding is useful in calculating a tip at a restaurant and dividing the bill among diners. or in estimating the amount of cash required for a trip to the grocery store.

When rounding whole numbers there are two rules to remember:

First, you must understand the term "rounding digit." When asked to round to the closest 10, yourrounding digit is the second number from the right, or 10's place, when working with whole numbers. When asked to round to the nearest hundred, the third place from the right is the rounding digit or 100's place.

Rules for Rounding Whole Numbers

Determine what your rounding digit is and look to the right side of it.

  • If the digit is 0, 1, 2, 3, or 4, do not change the rounding digit. All digits that are on the right-hand side of the requested rounding digit will become 0.
  • If the digit is 5, 6, 7, 8, or 9, your rounding digit rounds up by one number. All digits that are on the right-hand side of the requested rounding digit will become 0.

Rounding Rules for Decimal Numbers

Determine what your rounding digit is and look to the right side of it.

  • If that digit is 4, 3, 2, or 1, simply drop all digits to the right of it.
  • If that digit is 5, 6, 7, 8, or 9 add one to the rounding digit and drop all digits to the right of it.

Some teachers prefer another method, which provides more accuracy and is sometimes referred to as the "Banker's Rule." When the first digit dropped is 5 and there are no digits following or the digits following are zeros, make the preceding digit even (i.e., round off to the nearest even digit). Following this rule, 2.315 and 2.325 both round to 2.32—instead of 2.325 rounding up to 2.33—when rounded off to the nearest 100th. The rationale for the third rule is that approximately half of the time the number will be rounded up and the other half of the time it will be rounded down.

Examples of How to Round Numbers

765.3682 becomes:

  • 1,000 when asked to round to the nearest 1,000
  • 800 when asked to round to the nearest 100
  • 770 when asked to round to the nearest 10
  • 765 when asked to round to the nearest one (1)
  • 765.4 when asked to round to the nearest 10th
  • 765.37 when asked to round to the nearest 100th
  • 765.368 when asked to round to the nearest (1,000th)

Rounding comes in handy when you are about to leave a tip at a restaurant. Let's say your bill is $48.95. One rule of thumb is to round to $50 and leave a 15 percent tip. To quickly figure out the tip, say that $5 is 10 percent, and to reach 15 percent you need to add half of that, which is $2.50, bringing the tip to $7.50. If you want to round up again, leave $8—if the service was good, that is.