Plumbing Math, Sizing, and Fixture Units
Plumbing Math, Sizing, and Fixture Units. This domain tests calculations used in layout, sizing, and estimating. Expect questions on fixture units, slope, pipe volume, pressure, conversions, and developed length. Show units and convert inches to feet before multiplying.
Core rules
- Drainage fixture units (DFU) and water supply fixture units (WSFU) are separate load scales. Use the correct table for the system being sized.
- Slope fall = slope rate × run length. Keep units consistent: ¼ in per ft over 20 ft is 5 in, not 5 ft.
- Pipe volume: convert diameter to feet, find radius, use area = πr², multiply by length, then convert cubic feet to gallons using 7.48 gal per ft³.
- Pressure from elevation: about 0.433 psi per foot of water column. Developed length includes fittings and valves as allowed by the sizing method.
Worked example
Find the volume of 100 ft of 2 in pipe. Diameter = 2/12 = 0.1667 ft; radius = 0.0833 ft. Area = π × 0.0833² = 0.0218 ft². Volume = 0.0218 × 100 = 2.18 ft³. Gallons = 2.18 × 7.48 = 16.3 gal. The reasoning: convert diameter to feet first; using inches directly gives an area in square inches and a wrong volume.
Common traps
Do not add DFU and WSFU. Do not forget to convert cubic feet to gallons. Do not confuse diameter with radius. Do not use run length when the code requires developed length.