Fixture units → gpm → minimum pipe size.
Enter the fixtures on a job. Get the water supply demand from Hunter's method, the smallest copper size that meets velocity and pressure limits, and plain flags for anything that needs a second look — stamped with version, date and sources for your records.
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What goes in
- Fixture counts from Hunter's Table 6 (public and private, flush valve or flush tank)
- Continuous demand: hose bibbs, irrigation, equipment
- Optional pressure check: street psi, rise, developed length, meter, and the minimum pressure your fixtures need
- Type L or K copper, cold or hot service
What comes out
- Total fixture units and estimated peak demand in gpm
- Minimum pipe size with velocity and friction loss
- Flags: "pressure governs", "not enough pressure", and a list of fixtures that take no water
- A saved, printable result with version, time, sources and disclaimer
Example
One bathroom group plus a kitchen sink, flush tanks:
12 apartments, 55 psi street, 25 ft rise, 300 ft run, Flushmate 504, 2" meter → 2" Type L, flagged "Pressure governs: velocity alone allows 1-1/2"".
Drawings govern
This is a calculator and a minimum-requirements guide, not an engineering service: it works from the numbers you enter and doesn't know your project's conditions, so verify every result. If the drawings show a larger pipe, price the drawing. If they show a smaller one, raise an RFI. See method & limits.
For software and AI agents
The same calculator is available as a JSON API with an API key; each successful call uses one credit. See the API docs.