Plumbing Math and Measurement
Plumbing Math and Measurement tests practical calculations: slope, volume, pressure, fixture units, and unit conversion. The exam expects clean setup and unit discipline more than complex formulas.
Core rules and concepts
- Slope fall = run length × slope per foot; keep the units consistent.
- Pipe volume = cross-sectional area × length; convert cubic feet to gallons by multiplying by 7.48.
- Area uses radius, not diameter: radius = diameter ÷ 2.
- Pressure from elevation = feet × 0.433 psi per foot.
- Fixture-unit calculations require adding values correctly and applying the correct table for the pipe or system being sized.
Worked example
Find the water volume in 100 feet of 1-inch-inside-diameter pipe. Radius = 0.5 inch = 0.04167 foot. Area = π × (0.04167)² = 0.005454 square feet. Volume = 0.005454 × 100 = 0.5454 cubic feet. Gallons = 0.5454 × 7.48 = 4.08 gallons. The common error is using 1 inch as the radius, which quadruples the area and gives about 16.3 gallons.
Traps that cost marks
Do not mix inches and feet in the same formula. Do not forget to convert diameter to radius. Do not add fixture units from different fixture types without checking the table. Do not round too early; carry decimals through the calculation.
Exam focus
Be able to compute slope, elevation pressure, and pipe volume, and to explain each step. Show units so a wrong conversion is obvious.