Plumbing Pipe Size Calculator
Calculate required nominal pipe diameters for potable water supply lines (WSFU/GPM), sanitary drainage sewer lines (DFU/Slope), and natural gas fuel systems according to International Plumbing Code (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 both residential and commercial buildings. Undersized water supply lines cause sudden pressure drops and temperature fluctuations when toilets flush, while undersized drain lines cause chronic siphonage, poor venting, and frequent 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.