Copyright Goodheart-Willcox Co., Inc. Chapter 6 Mathematics for Plumbers 109 their use are given in Figure 6-24. US customary to metric conversion factors are provided in the Reference Section. 6.5 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 horizontal run of pipe per- mits the builder’s level to be used to accurately position the run of pipe, Figure 6-25. 6.6 Metric Measurement Some industries have adopted the International Sys- tem of Units (SI) for measurement, commonly known as the metric system. Since many construction trades, such as plumbing, are not involved in international commerce, there is less pressure for them to use metric measurements. However, some fixtures, faucets, and valves are now available in metric sizes. The base units of length measurement in the metric system are shown in Figure 6-26, along with comparable US customary units. One advantage of using the metric system is the ease of changing from one metric unit to another simply by multiplying or dividing by multiples of 10. This is easier than using the various conversion factors required in the US customary system. Code Note IPC Note The requirement for the slope of horizontal drainage pipe per foot of run varies from 1/16″ to 1/4″, depending on the pipe diameter. 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 6-22. Determining the volume of a rectangular tank. 6′ 2′ Volume = Pi × Radius squared × Height This formula may be written: V = ˜r 2h Computing using given values: V = 3.14 × (2′)2 × 6′ V = 75.36 cubic feet Goodheart-Willcox Publisher Figure 6-23. Determining the volume of a cylindrical tank. Total slope = Slope per foot × Feet of run = 1⁄8″ × 50 = 50⁄8″ = 61⁄4″ 50′ Total slope Outlet Flow Inlet Line of sight Builder’s level or transit Goodheart-Willcox Publisher Figure 6-25. If a 50′ horizontal run of building sewer is to be installed, what is the total slope of the run if a slope of 1/8" inch per foot is to be established? To Convert To Procedure Example Cubic inches Cubic feet Gallons Gallons Divide by 231 Multiply by 7.48 376 cu. in. = 376 231 = 1.63 gal. 6 cu. ft. = 6 × 7.48 = 44.88 gal. Goodheart-Willcox Publisher Figure 6-24. Converting cubic inches and cubic feet to gallons.