Plumbing Pipe Size Calculator
Calculate nominal pipe diameters for water supply lines (WSFU/GPM), sanitary drains (DFU/Slope), and natural gas systems per IPC standards.
Water Supply Fixture Unit (WSFU) Count
Add the number of active plumbing fixtures connected to this main trunk or branch line.
Minimum recommended main trunk line diameter for this fixture load.
Plumbing Pipe Sizing Engineering Principles
Correct pipe sizing is critical for residential and commercial buildings. Undersized supply lines cause pressure drops during toilet flushes, while undersized drains cause trap siphonage and clogs.
Hunter's Curve Probability
Because not all fixtures run at the exact same second, Dr. Roy B. Hunter developed probability curves (WSFU to GPM conversion). This prevents severe oversizing of supply trunks while guaranteeing robust dynamic flow during peak usage.
Water Velocity & Water Hammer
Water moving faster than 8 feet per second creates loud acoustic knocking and hydraulic shock waves when fast-acting solenoid valves (washing machines, dishwashers) shut. Sizing up reduces velocity and prolongs valve lifespan.
Gas Piping CFH & Pressure Drop
Natural gas systems operate at very low pressure (0.25 to 0.50 PSI / 7" w.c.). High-demand appliances like tankless units (199,000 BTU) or standby generators (250,000 BTU) require dedicated 1" to 1-1/4" lines to prevent starvation shutdowns.