How to size a water supply pipe by fixture units
Every fixture in a building never runs at once, so supply piping is not sized by adding fixture flow rates. Instead each fixture is assigned a water supply fixture unit (WSFU) value, the units are totaled, and Hunter's probability curve converts that total into a peak demand in gpm. This calculator uses the private-use WSFU values from IPC Appendix E Table E103.3(2), including 2.2 for a flush tank water closet and 0.7 for a lavatory.
Demand is converted with an approximation that follows the flush tank branch of Hunter's curve, Q(gpm) = 4.35 x WSFU^0.526. Pipe size then comes from continuity, d = sqrt(4Q / pi v), rounded up to the next nominal size. At 100 WSFU and 8 ft/s the peak demand is about 49 gpm and the required inside diameter is roughly 1.58 in, which rounds to a 2 in main.
Systems dominated by flushometer valves draw more at low fixture counts, so raise the adjustment factor or read the flushometer column of the original table directly. Nominal size is not inside diameter, and it varies with material and wall thickness, so confirm the real ID for your piping. Pressure loss and the residual pressure at the highest fixture, commonly 15 to 20 psi, still need a separate check by a plumbing engineer before permit drawings.
Frequently asked questions
Fixtures almost never run at the same moment, so demand per fixture unit drops as the total load grows. Hunter's curve builds that probability into the conversion, and adding raw flow rates would oversize the main by a wide margin.
Mains are commonly sized at 6 to 8 ft/s, and quieter residential work often drops to 5 ft/s or less. Above about 8 ft/s in copper, flow noise, water hammer, and erosion of the pipe wall all become concerns.