Copyright Goodheart-Willcox Co., Inc. 108 Section 1 Introduction to Plumbing purposes, pi is assumed to equal 3.14. Figure 6-21 illus- trates how to use area formulas for circles. To find the volume of a rectangular tank, multiply length, width, and height as shown in Figure 6-22. To find the volume of cylindrical tanks, use the fol- lowing formula: Volume = πr2h where π = pi (3.14) r2 = radius squared h = height Figure 6-23 illustrates the use of this formula. After calculating volume in some convenient units of cubic measure, it may be necessary to convert the cubic units of measure to a volume measure, such as gallons. The most common conversion factors and examples of 8′ Area = ˜r 2 ˜r 2 = 3.14 r = Radius = 1⁄2 diameter Since the diameter is 8′, the radius is 4′. Therefore, Area = 3.14 × (4)2 = 3.14 × 16 square feet = 50.24 square feet or Area = .7854d2 Area = .7854 × (8)2 = .7854 × 64 square feet = 50.24 square feet Goodheart-Willcox Publisher Figure 6-21. Formula for computing the area of a circle. as you would when measuring the run, except use the other side of the rule. Remember that in all cases, travel (hypotenuse) is greater than the actual length of the pipe that must be cut. Fitting allowances must be subtracted from the travel to obtain the actual pipe length. Fitting allowances vary by size of pipe and are discussed in Chapters 21 and 22. 6.4 Computing Area and Volume The surface area of a square or rectangular surface can be computed by multiplying the length by the width, Figure 6-20. However, computing the area of circles requires the use of one of the two special formulas shown here: Area = πr2, where π = pi (3.14) r2 = radius squared, or Area = .7854d2 where d2= diameter squared. Pi (π) is a mathematical ratio frequently used when making calculations for circles. For most practical Travel Run 45° Offset scale US Customary rule 7.07″ Travel 5″ Run Goodheart-Willcox Publisher Figure 6-19. To determine the travel length, measure the run distance with the 45° offset scale. 8′ 4′ Area = Length × Width Area = 8′ × 4′ Area = 32 square feet Goodheart-Willcox Publisher Figure 6-20. This formula can be used to compute the area of a rectangular surface.