Water Supply Pipe Sizing Calculator

Minimum trunk/branch diameter from IPC 2021 Table E104.1 by WSFU, developed length, and pressure — plus UPC Table 610.4 comparison

Water-supply fixture units are not drainage fixture units. Too small a pipe and the second shower starves. This calculator applies the 2021 International Plumbing Code Table E104.1 fixture-unit method for a common trunk or branch: pick the pressure band, multiply developed length by 1.2, take the next length column, and read the smallest distribution diameter whose cell covers the load. This is standard prescriptive guidance for that table method only — not a hydraulic calculation, not a wet vent, and not a substitute for the plumbing code your city adopted.

Not sure your WSFU total? fixture unit calculator. How to total WSFU: supply guide. Low pressure when two fixtures run: troubleshooting. Why this is not the vent worksheet: WSFU vs DFU. Vent diameter: DWV vent sizing calculator. Drain fall: drain pipe slope calculator. Plumbing index: plumbing calculators hub.

Rural wells that feed the house still need trunk/branch pipe after the pressure tank: size supply with this calculator, then size the pump with the Well Pump Sizing Calculator.

Basement groundwater is a different worksheet: Sump Pump Sizing Calculator (basin, inflow GPM, total head — starting point, not a named-model pump curve).

Prescriptive starting point only — trunk/branch fixture-unit method, not a substitute for the plumbing code your city adopted. This calculator applies 2021 International Plumbing Code Table E104.1 (Appendix E): minimum meter, service, and distribution pipe from water-supply fixture units, developed length (actual length × 1.2 for fittings), and available static pressure. Uniform Plumbing Code 2021 Table 610.4 is shown for comparison (Hawaii Plumbing Code 2021 / UpCodes). Table E103.3(2)/(3) assigns WSFU and converts to gpm for the full hydraulic method — this tool is the table method, not that calc. Building supply is never less than 3/4 in. Long runs, elevation, PRVs, and flushometers may need Appendix E / Appendix A. Not a permit.
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How to Use This Water Supply Pipe Sizing Calculator

Type the WSFU on this segment, the developed length to the remote fixture, the pressure band after elevation and any PRV, and copper / PEX / CPVC. Say whether this is a branch or the building supply. The calculator looks up IPC Table E104.1 (length × 1.2, next column, smallest distribution diameter whose cell covers the load) and shows UPC Table 610.4 for the same inputs. PEX and CPVC use the same table; if the cell is 80% or more used, you get a one-size friction recommendation from Table E105.1’s smaller ID, not a fake second WSFU table.

IPC pipe = smallest Table E104.1 distribution diameter whose WSFU cell ≥ load at (developed length × 1.2) and the pressure band  ·  building supply ≥ 3/4 in

What WSFU actually counts: supply guide. Low pressure when two showers run: troubleshooting. Why this is not the vent calculator: WSFU vs DFU. Drain fall: drain pipe slope calculator.

What This Number Is (and Is Not)

The diameter is the IPC Table E104.1 minimum for a trunk or branch serving the WSFU load you typed, at the developed length and pressure band you typed. It is not a gpm from Table E103.3(3). It is not permission to skip a PRV calc. It is not DFU. It is not a substitute for the code book on your permit card.

IPC Table E104.1 vs UPC Table 610.4

Both are fixture-unit lookup tables: pressure band, length column, WSFU cell, pipe size. The numbers are not the same. IPC multiplies developed length by 1.2, uses 40–500 ft columns, and four pressure bands. UPC uses the entered length, 40–1,000 ft columns, and three pressure bands. A 16.5 WSFU house on a 100-foot run at ~55 psi is 1 in in both books in this tool. A 6 WSFU branch at 40 ft and 35 psi is 1/2 in on UPC 610.4 and 3/4 in on IPC E104.1 (IPC 1/2 in only holds 2.5 WSFU in that cell). That is why both results print. Use the family your inspector adopted.

Developed length is not a laser shot. IPC E104.1 Step 2 measures along the pipe to the most remote fixture, hot through the heater or cold, then × 1.2 for fittings. A 40-foot joist bay with four 90s is not 40 feet. Count the fittings or you will undersize a snaky 1/2 in branch that looked short on the plan.

When this tool is the wrong tool

Flushometer-valve batteries, buildings taller than the table’s pressure math, a PRV plus a softener plus 30 feet of elevation, parallel manifold (home-run) PEX sized from a manufacturer’s chart, fire sprinklers (UPC 612 / IRC P2904), and anything past 2-1/2 in / 533 WSFU. Those are Appendix E hydraulic charts or a plumber with a pressure gauge. Drain slope is Table 704.1. Vents are 906.2. Getting this diameter right does not set the vent or the fall.

Shower dies when the washer starts?That can be pipe, a starved PRV, or the street. Sort it before you upsize the whole house.

Troubleshooting →

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Water Supply Layout Gear

A WSFU diameter is a number until you cut the pipe, crimp or sweat it, hang it, and keep the thermal expansion from lifting a T&P. These are the pieces that actually leave the truck.

iCrimp PEX crimp tool kit

PEX Crimp Tool Kit

1/2 in and 3/4 in copper-ring crimps (ASTM F1807), go/no-go gauge, cutter, and ring puller. The iCrimp IWS-1807CN is how a PEX branch actually becomes the diameter Table E104.1 just printed.

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RIDGID close-quarters tubing cutter

Close-Quarters Tubing Cutter

Copper Type L still shows up on water heaters and meter tails. A RIDGID 57008 C-style cutter is the tool that squares a 3/4 in stub in a joist bay the wheel cutter will not fit.

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Amtrol thermal expansion tank

Thermal Expansion Tank

A closed supply (PRV or check at the meter) needs somewhere for the heater to expand. Undersized pipe plus a dead-headed heater is how you get relief-valve drips that look like a pressure problem.

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RIDGID 14 inch pipe wrench

14" Pipe Wrench

Meter couplings, galvanized nipples, and the last iron fitting on a remodel still need a wrench that bites. Same 14 in RIDGID that turns DWV no-hubs turns supply thread too.

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Adjustable pipe hanger

Pipe Hangers

PEX sags. Copper rings. Hang the run so the developed length you typed is the developed length that exists after the drywall goes up.

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Water supply pipe sizing calculator

Run the Supply Size

WSFU, developed length, pressure band, and copper / PEX / CPVC. IPC Table E104.1 plus UPC Table 610.4.

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