Pressure is height

The drain side runs on gravity. The supply side fights it — and the exchange rate is fixed.

0.433 psi per foot

A column of water one foot tall pushes down with 0.433 psi (62.4 pounds per cubic foot, spread over 144 square inches). That single number is the whole story of building water pressure:

• Climb a foot, lose 0.433 psi. It costs pressure to lift water.
• Drop a foot, gain 0.433 psi. Basements read higher than the street.

Street pressure of 60 psi becomes 56.5 psi at a first-floor faucet 8 ft up, and 49.2 psi at a top-floor shower 25 ft up. Nothing is wrong with any of those numbers — that's just height doing what height does. The water pressure tool runs the arithmetic for any elevation, up or down.

The two limits

Code brackets the usable range from both ends:

• Too little: most fixtures need roughly 8 psi flowing to work at all (some, like flushometers, need much more). If the street pressure minus the climb can't clear the fixture's minimum, the top floor gets a booster pump — not a bigger pipe.
• Too much: above 80 psi static, code requires a pressure-reducing valve (PRV). High pressure hammers valves, blows washing machine hoses, and turns small leaks into sprays. If a house sits at the bottom of a hill, expect the PRV.

Static vs. flowing

A gauge on a closed system reads static pressure — the ceiling. Open a tap and the reading drops: friction in every foot of pipe and every fitting eats pressure as water moves. Static tells you what you have to spend; what's left at the fixture while it runs is what actually matters. That's why supply pipe sizing is about keeping friction losses small enough that the worst fixture still clears its minimum on a busy morning.

Why this matters

"No pressure upstairs" complaints are usually not a mystery — they're 0.433 × height plus friction, calculated in ten seconds. Knowing the exchange rate tells you immediately whether the fix is a PRV setting, a booster, or just managed expectations.