In performing all calculations care is required to see that the correct units are used. Oftentimes, through haste and confusion, inches instead of being first changed into feet are multiplied by feet to obtain area in square feet. This error is often overlooked because there are many formulas or rule-of-thumb methods that have been abbreviated to their lowest terms by cancellation so that in their final form it is possible to multiply inches by feet or pounds. Therefore in using a formula, care should be exercised to see that it is correct and that the proper units are employed.

To illustrate: the formula for determining the thickness of a lead pipe necessary for a given head of water is:

T = h X s / 750 where T is thickness of pipe in inches, s is size of pipe expressed s decimal of an inch, and h is the head of water in feet.

In this formula, feet are multiplied by a decimal of an inch. As an example, the thickness of a half-inch pipe carrying a 50-foot head of water would be:

T = 50 X .5 / 750 = 25 / 750 = 1 / 30 = .033 in.