How to size water supply pipe
Fixture units total up, demand comes off Hunter's curve, and then the question that actually decides the job: after lift, meter, fittings and friction, is there pressure left at the top?
Supply sizing is usually taught as "pick a pipe size", which hides what is going on. It is a budget, and seeing it that way tells you something a pipe-size table cannot.
Step-by-step
- Total the fixture units from your code's fixture table, noting for each row whether it is a flush tank or a flushometer valve.
- Read the demand in GPM off Hunter's curve for that total — from the right curve for the fixture type.
- Check velocity against the pipe's actual bore.
- Work the pressure budget from the street to the highest fixture.
Why flushometers are kept separate
A flush tank refills gently over a minute or so. A flushometer valve takes its whole volume in a few seconds at a very high rate. The fixture unit values reflect that, and the demand curves for the two are genuinely different curves — not one curve read differently.
This matters most on small systems, where a single flushometer can dominate peak demand entirely. Total everything as though it were tank-fed and the main comes out too small; the symptom is a fixture that works fine until someone flushes.
Fixture units do not convert to GPM
There is no formula. The relationship is Hunter's probability curve, printed as a graph or table because it has no closed form, and it flattens hard — a linear guess overshoots badly on large systems. Any formula a tool offered you here would be a fabrication, and it would get quoted back as though it were the code.
Sizing is a subtraction
There is a fixed amount of pressure at the street and four things spend it: lifting the water to height, the meter, the fittings, and friction in the pipe. What survives has to satisfy the fixture.
Seeing it this way makes one thing obvious that a pipe-size table hides: only friction responds to a larger pipe. Static lift is pure geometry — 0.433 psi per foot of height, and no pipe size changes it. If the budget is short and friction is already a small part of it, upsizing will not rescue the run, and the honest answer is a booster pump or a different service pressure. A lot of money gets spent on larger pipe for buildings whose real problem was height.
Velocity is a separate constraint
A pipe can satisfy the pressure budget and still be wrong. Water moving too fast erodes copper at the fittings and makes noise that carries through the structure. The usual ceiling is around 8 ft/s cold and lower on hot lines, where erosion is faster. Passing the pressure check does not excuse the velocity check.
What this tool will not do
It contains no code tables. It does not know which edition your jurisdiction has adopted, what local amendments apply, or anything about your job, and it makes no determination of compliance. A licensed professional and the authority having jurisdiction decide that, and where they or the published code differ from the page, they are right.