Pressure Zone HGL Calculator
The hydraulic grade line sets the static pressure everywhere in a pressure zone. Work it both ways: pressure from HGL elevation, or HGL back-calculated from a gauge reading.
Static Pressure
| Item | Value |
|---|
How the calculation works
Static conditions (no flow, no friction loss)
Pressure head (ft) = HGL elevation − point elevation
Pressure (psi) = pressure head (ft) ÷ 2.31
HGL elevation (ft) = gauge elevation + gauge pressure (psi) × 2.31
1 psi = 2.31 ft of water column.
Pressure head (ft) = HGL elevation − point elevation
Pressure (psi) = pressure head (ft) ÷ 2.31
HGL elevation (ft) = gauge elevation + gauge pressure (psi) × 2.31
1 psi = 2.31 ft of water column.
| Condition | Typical minimum pressure |
|---|---|
| Normal operating conditions | 35 psi |
| Emergency / fire flow conditions | 20 psi |
Notes & limitations
2.31 ft of water per psi is the standard conversion. The 35/20 psi minimums follow the Ten States Standards (GLUMRB) for water works — confirm the governing values with your state primacy agency.
- This is static pressure only. Under flow, friction loss lowers the hydraulic grade line and the available pressure at the tap.
- A tank’s HGL drops as the water level drops — use the low-water level for the worst-case (minimum pressure) check and the overflow for the maximum.
- Back-calculating HGL from a gauge assumes the gauge reads static pressure (no flow in the service line) and is calibrated.
- Pressure zones are separated by PRVs, closed valves, or topography — verify which zone a point belongs to before applying its HGL.
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