Pressure pushes water out of an opening at a speed set by Bernoulli’s principle; the flow rate is that speed times the opening area. Real outlets lose energy to friction and contraction, captured by the discharge coefficient C_d (about 0.97 for a smooth nozzle, 0.6 for a sharp-edged hole).
How to use the PSI to GPM Calculator
- Pressure inside
- Pressure at the exit
- Opening diameter
- Discharge coefficient
- Every result field is editable: type into Flow rate and Pressure inside plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
The pressure difference is converted to pascals, the ideal outlet velocity is found from √(2ΔP/ρ) with water’s density, and it is multiplied by the opening area and C_d.
Inputs explained
| Field | What to enter |
|---|---|
| Pressure inside | Units: psi, kPa, bar |
| Pressure at the exit | Units: psi, kPa, bar |
| Opening diameter | Units: in, cm, mm |
| Discharge coefficient | A number |
What each result means
- Flow rate = [Discharge coefficient] × pi × [Opening diameter]^2 ÷ 4 × sqrt(2 × max([Pressure inside] – [Pressure at the exit], 0) ÷ 998.2)
- Exit velocity = sqrt(2 × max([Pressure inside] – [Pressure at the exit], 0) ÷ 998.2)
Example values
How the main result changes when Pressure inside changes and everything else stays at the example values above:
| Pressure inside (psi) | Flow rate (GPM) |
|---|---|
| 30 | 39.649 |
| 45 | 48.56 |
| 60 | 56.072 |
| 75 | 62.691 |
| 90 | 68.674 |
| 120 | 79.298 |
Formula
Q = C_d × A × √(2 × ΔP ÷ ρ)Worked example
Inputs — Pressure inside: 60 psi; Pressure at the exit: 0 psi; Opening diameter: 0.5 in; Discharge coefficient: 0.97.
Results — Flow rate = 56.072 GPM; Exit velocity = 94.46 ft/s.
Frequently asked questions
How many GPM through a 1/2″ opening at 60 psi?
About 22 GPM in theory; hoses and fittings reduce this.
Does this include pipe friction?
No. Long pipes lose pressure; use a pipe-friction calculator for those.
What formula does the PSI to GPM Calculator use?
Q = C_d × A × √(2 × ΔP ÷ ρ).