Plumbing Math, Sizing Calculations, and Isometric Interpretation
Math and sizing
Plumbing math covers unit conversion, slope, volume, pressure, fixture units, and isometric interpretation. Key conversions include 1 cubic foot = 7.48 gallons, 1 psi = 2.31 feet of water, and 1 inch = 25.4 millimeters. Slope in inches per foot is multiplied by run length in feet. Volume is length x width x height in consistent units, then converted to gallons. Fixture units are added from the tables, and pipe size is selected from the appropriate DFU or WSFU table.
Isometric interpretation
An isometric shows horizontal and vertical runs on a single drawing. Read the vertical risers, horizontal branches, and vent connections together; do not treat a diagonal line as a pipe at 45 degrees unless the drawing uses that convention. Developed length includes the run plus fittings, so a 10-foot horizontal run with several ells is longer than 10 feet for sizing.
Worked example
A rectangular tank measures 24 by 36 by 48 inches. Volume = 24 x 36 x 48 = 41,472 cubic inches. Divide by 1,728 to get 24 cubic feet; multiply by 7.48 to get about 179.5 gallons. If the tank is a receiver, the usable volume may be less because of the pump cutoff and inlet elevation.
Traps
Do not mix inches and feet in the same formula. Do not use the straight-line isometric length as developed length. Do not add fixture units without checking the table's fixture type and occupancy. Do not convert pressure to head with the wrong factor; 1 psi is 2.31 feet, not 1 foot.