Finding the volume of cylindrical tanks requires the use of the formula: Volume = πr2h This is read: “volume equals pi times the radius squared times the height.” Figure 4-23 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 conver- sion factors and examples of their use are given in Figure 4-24. English-metric conversion factors are provided in the Useful Information section. Computing Slope of Pipe Horizontal runs of sewer and drain pipe must slope to permit flow and prevent blockage. Computing the total amount of slope for a hori- zontal run of pipe permits the builder’s level to be used to accurately position the run of pipe, Figure 4-25. Generally, horizontal pipe runs in sewers and drains are required to slope ⅛ ″ to ¼″ per foot of run. 8′ Area = πr2 πr2 = 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 4-21. Formula for computing the area of a circle. 6′ 12′ 4′ Volume = Length (L) × Width (W) × Height (H) This formula is frequently written V = LWH Substituting the dimensions from the above drawing V = 12′ × 4′ × 6′ V = 288 cubic feet Goodheart-Willcox Publisher Figure 4-22. Determining the volume of a rectangular tank. 6′ 2′ Volume = Pi × Radius squared × Height This formula may be written: V = πr2h Substituting from the above drawing: V = 3.14 × (2′)2 × 6′ V = 75.36 cubic feet Goodheart-Willcox Publisher Figure 4-23. Determining the volume of a cylindrical tank. 80 Section 1 Introduction to Plumbing Copyright Goodheart-Willcox Co., Inc.