Plumbing Math, Pipe Sizing, and Fixture Unit Calculations
What the exam tests
This domain tests basic arithmetic, unit conversion, fixture-unit totals, and the use of pipe-sizing tables. You must distinguish drainage fixture units (DFU) from water-supply fixture units (WSFU), use developed length, and convert pressure to head correctly.
Core rules
- DFU describes drainage load; WSFU describes water-supply demand. Never add them together or use a drainage table for a supply pipe.
- Developed length includes fittings and elevation changes, not just plan-view distance.
- Slope is fall per run. A 1/4 in/ft slope equals 2.08%; 1/8 in/ft equals 1.04%.
- Pressure and head: 1 psi equals 2.31 ft of water. An elevation rise reduces static pressure.
- Sum fixture units first, then use the adopted sizing table. Do not guess a pipe size from fixture count alone.
Worked example
A meter gauge reads 62 psi. The highest fixture is 18 ft above the meter. Convert the elevation to pressure: 18 / 2.31 = 7.8 psi. Static pressure at the fixture is 62 - 7.8 = 54.2 psi. If the fixture needs 8 psi flowing and the line has 12 psi friction loss, the residual is 54.2 - 12 = 42.2 psi, which is adequate. The common error is subtracting 18 psi instead of converting feet of head.
Traps that cost marks
Adding DFU and WSFU; using plan length instead of developed length; subtracting feet directly from psi; rounding a pipe size down to save material; ignoring residual pressure at the farthest fixture.