Water pressure calculator
"Low pressure upstairs" is almost never the utility. It is elevation plus friction plus a partly-closed valve, and each of those is a number you can put a gauge on. This works the losses out in order so you know which one to go after before you open a wall.
Pre-filled with an upstairs shower on a 62 psi supply. Change any field.
Residual at the fixture
- Elevation loss
- 6.1psi
- Equivalent length
- 113ft
- Friction loss on the run
- 11.0psi
- Meter and valves
- 6.0psi
Comfortable working pressure. Most fixtures want 20 to 45 psi at the outlet; above 80 psi a pressure-reducing valve is required.
| Size | Loss at your flow | Residual |
|---|---|---|
| 1/2" | 43.9 psi | 6.1 psi |
| 3/4" | 11.0 psi | 38.9 psi |
| 1" | 3.4 psi | 46.6 psi |
| 1-1/4" | 1.6 psi | 48.3 psi |
How it sizes
Four losses stand between the meter and the shower head. Elevation is fixed geometry: 2.31 ft of rise costs 1 psi, so a second-storey fixture starts about 6 psi down before anything flows. The meter and its valves take a few more. Friction is the one that depends on what you did — it goes up roughly with the square of flow and comes down hard with pipe size.
The fourth loss is the one no calculator can see: a gate valve someone left three-quarters shut, a stop with a chewed washer, an aerator packed with scale. If the measured residual is far below what this returns, that is where it went.
Worked example
An upstairs shower, 14 ft above the meter, 75 ft of 3/4" run, drawing 8 gpm, on a 62 psi supply. Elevation takes 6.1 psi. The meter and stops take 6 psi. Seventy-five feet of pipe with fittings behaves like about 113 ft, and at 8 gpm three-quarter inch loses roughly 9.8 psi per 100 ft — call it 11 psi. Residual lands near 39 psi, which is comfortable.
Run the same job in 1/2" and the friction line alone takes over 40 psi. That is the whole argument for sizing the branch properly, in one line.
Above 80 psi is not a bonus
Code notes
IPC 604.8 Where the static pressure supplied to a building exceeds 80 psi, an approved pressure-reducing valve is required. IPC 607.3 A closed system created by that valve needs thermal expansion control. Your authority having jurisdiction (AHJ) may set a lower threshold; several coastal jurisdictions do.