When the exact value is difficult to find or unnecessary for the use. Example When estimating paint, it may only be necessary to determine the next larger integer number of gallons. You do not need to know that 2.465 gallons are needed. You only need to know that more than 2 and less than 3 gallons are needed. It may not be possible to calculate exactly how much paint is needed, because the coverage afforded by one gallon is not known in advance with any accuracy.