Plumbing Math, Sizing, and Calculations
Plumbing Math, Sizing, and Calculations tests your ability to convert code tables and field measurements into correct pipe sizes, slopes, and pressures. Most exam errors come from using the wrong length-measured versus developed-or from mixing inches and feet.
Core relationships
- Slope drop = run length × slope. A 1/4-inch-per-foot slope is 1 inch in 4 feet.
- Static pressure changes by 1 psi for every 2.31 feet of elevation.
- Roof runoff, in gallons per minute, is approximately roof area (ft²) × rainfall intensity (in/hr) × 0.0104.
- Drainage fixture units and water-supply fixture units are converted with code tables, not one universal formula.
- Developed length includes fittings and offsets; measured length alone underestimates pressure loss.
Worked example
A 3-inch building drain runs 60 feet at 1/8 inch per foot. What is the invert drop? 60 × 0.125 = 7.5 inches. Convert to feet: 7.5 ÷ 12 = 0.625 foot. If the upstream invert is 100.00 feet, the downstream invert is 100.00 − 0.625 = 99.375 feet. Reasoning: keep the slope in inches per foot, multiply by feet, then convert the total drop to feet before subtracting from an elevation.
Traps that cost marks
- Using 1/4 inch per foot for a 3-inch drain where the table permits 1/8.
- Adding fitting lengths to run length but then forgetting them in the pressure calculation.
- Mixing decimal feet with inches in the same subtraction.
- Treating fixture-unit values as gallons per minute without the code conversion table.