Water Pressure and Pipe Sizing

Static and flow pressure

Water pressure is measured in pounds per square inch (psi). Two numbers matter:

• Static pressure: with every tap closed. A gauge on an outdoor hose bibb with nothing running reads static.
• Flow pressure: what’s left at a fixture while water is running. It’s always lower, because moving water loses pressure to friction and equipment.

Fixtures care about flow pressure. Wisconsin’s plumbing code sets minimums at the fixture outlet, and every supply system has to be designed to deliver them ¹.

Height costs pressure

Water has weight. A column of water one foot tall presses on its base with about 0.433 psi: pressure in a fluid rises with depth as density times gravity times height ². Run that backward and every foot you lift water costs 0.433 psi.

So a second-floor shower 20 feet above the basement meter starts out about 8.7 psi behind before a drop flows. Flip it around and 2.31 feet of water makes one psi, which is why water towers are tall and why a well pump’s pressure switch has to account for the height of the house.

Where the rest goes

Wisconsin’s code lists what a supply design has to account for ¹:

1. the fixture load on each pipe (in water supply fixture units or gallons per minute),
2. the minimum pressure available at the main or pressure tank,
3. pressure lost to elevation,
4. pressure lost through the water heater, water treatment devices, meter, and backflow preventers,
5. the minimum flow pressure the controlling fixture needs,
6. pressure lost to friction in pipe, fittings, and valves.

Friction rises steeply with flow: double the flow through the same pipe and the friction loss roughly triples or quadruples. That’s what pipe sizing fights. Bigger pipe means slower water and less friction. The code’s tables give the maximum load each size of copper, PEX, CPVC, or steel pipe can carry, and larger-than-minimum piping must still flush effectively at 8 feet per second ¹.

What the fixture needs

Wisconsin’s minimum flow pressures at the fixture supply outlet ¹:

A shower that turns weak when someone flushes downstairs is a house running near those minimums.

Too much pressure

High pressure is a problem too: it hammers valves, makes pipes bang when taps close, and wastes water. If the pressure from the main or a private supply exceeds 80 psi, Wisconsin requires a pressure reducing valve and strainer, installed downstream of the meter and upstream of every fixture ¹. A screw-on gauge at a hose bibb tells you where you stand for a few dollars.

Try it

Thread a pressure gauge onto a basement hose bibb. Read it with everything off (static). Then open an upstairs shower and read again (flow, plus height). The difference shows what your pipes, meter, softener, and stairs cost you.

What to take from this

Fixtures need flow pressure: 8 psi for most, 20 for one-piece toilets and pressure-balance shower valves. Pressure from the main gets spent on height (0.433 psi per foot), the meter, softener, water heater, and friction, which grows fast with flow; pipe sizing balances these against the fixture load. Over 80 psi requires a pressure reducing valve. A $10 gauge tells you static and flow pressure.