WATER SUPPLY SIZE
WATER SUPPLY PIPE
size = f(Σ WSFU)
RESULT
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FILL IN ABOVE
Residential service entry, 40–60 psi static. PEX is bumped one nominal size vs copper because of thinner ID. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This water supply pipe sizing calculator gives the minimum supply pipe diameter for a residential service line, based on the fixtures it serves. Enter how many full baths, kitchens, washing machines, and hose bibs the line feeds; the calculator sums Water Supply Fixture Units (WSFU) per IPC Table E202.1, then picks the smallest pipe that can carry that demand at typical residential pressure (40–60 psi static). Copper Type L and CPVC use the nominal size returned. PEX has a smaller inside diameter for the same nominal size, so the PEX answer is bumped up one size for equivalent flow. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
What is a Water Supply Fixture Unit (WSFU)?
A WSFU is a dimensionless number assigned to each fixture in IPC Table E202.1 that captures both its peak flow rate and its likely simultaneous use with other fixtures. A toilet is 2.5, a tub is 4, a kitchen sink is 1.5. Summing WSFUs lets you size a single supply line that serves many fixtures without assuming all of them open at once — Hunter's probabilistic curve does the rest.
Why is the PEX size bigger than the copper size?
PEX has a thicker wall than copper for the same nominal size, so the inside diameter is smaller and friction loss is higher per foot. To deliver the same flow rate, PEX usually needs to be one nominal size up. A ¾" copper line is roughly equivalent to a 1" PEX line at typical residential pressure.
Does this work for well water systems?
Use this for the line downstream of the pressure tank, which sees the same 40–60 psi residential range. The line from the pump to the pressure tank sizes differently — driven by pump GPM and lift, not WSFU. For long underground service runs (over 100 ft), bump up one size to offset friction loss regardless of what this calculator returns.