Plumbing sizing

Drainage — list the fixtures (a bathroom group counts a whole bathroom) and their quantities. The total drainage fixture units (DFU) give the smallest building drain at the slope you can get, a horizontal branch and a stack, with the capacity table to read across. Anything carrying a water closet is at least 3 in. Fixtures not in the table go in by trap size; pumps and condensate by gallons a minute (2 DFU per gpm). UPC values change with occupancy (private, public, assembly).

Vents — the vent size for the drain it serves, the DFU on it and its developed length. IPC stack vents and vent stacks come from Table 906.1; other IPC vents are half the drain, one size up past 40 ft. UPC vents come from Table 703.2.

Traps — the longest trap arm allowed from trap to vent (IPC Table 909.1, UPC Table 1002.2), and the minimum trap size for each fixture.

Slope — any two of run, slope and drop give the third, with the grade in per cent and degrees and the code’s minimum for the pipe size.

Water supply — fixtures in water supply fixture units (WSFU) → peak demand in gpm from Hunter’s curves (flush tanks or flushometer valves) → the smallest copper, PEX or CPVC pipe that stays under the velocity limit and whose Hazen–Williams loss fits the pressure left after the height of the highest fixture, the water meter, other losses and the pressure the fixture needs, over the developed length plus a fitting allowance. This is the available-pressure method of IPC Appendix E and UPC Appendix A.

Water heater — the busiest hour of hot-water use for a tank’s first-hour rating (the US DOE method), and for a tankless heater the flow you run at once and the temperature rise, giving the gas input or electric kW needed and what a given unit can deliver.

The code tables are data in the tool, cited where they are used: IPC Tables 704.1, 709.1, 709.2, 710.1(1), 710.1(2), 906.1 and 909.1 and Appendix E; UPC Tables 702.1, 703.2, 717.1, 1002.2 and 610.3 and Appendix A. Hazen–Williams loss is 4.52 Q^1.852 ÷ (C^1.852 d^4.87) psi per foot with C = 150 and inside diameters from ASTM B88 (copper), F876 (PEX) and D2846 (CPVC). The meter loss estimate scales the AWWA C700 limit (15 psi at the meter’s maximum flow) with flow squared; use the meter’s own curve if you have it. Hunter’s curves over-predict demand for modern low-flow houses; where your jurisdiction has adopted it, the Water Demand Calculator (UPC Appendix M, IPC Appendix N) is the newer method.

Not covered: wet venting, circuit venting, air admittance valves, island venting, combination waste-and-vent, storm drainage, and gas piping (see gas pipe sizing). Offsets and cut lengths are in pipe offsets and takeoffs; expansion and well tanks, pumps and recirculation in pumps, tanks and hot water.

Pipe flow for other fluids, Darcy–Weisbach and pump power are in fluid flow and pipe pressure drop. Heating loads are in heat loss and HVAC sizing; ducts in HVAC ducts and airflow; an electric water heater’s share of the service in the dwelling load calculator; a well pump on standby power in generator sizing.

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