Convert horsepower to GPM from pressure (hydraulic) or head (water pump) with typical pump efficiency, or find the HP needed for a target GPM.
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Formula
Hydraulic (pressure-based):
GPM = (HP × 1714 × η) / P
where HP = input horsepower, P = pressure in PSI, η = pump efficiency as a decimal.
Water pump (head-based):
GPM = (HP × 3960 × η) / H
where H = total dynamic head in feet, η = pump efficiency as a decimal.
Reverse (GPM → HP):
HP = (GPM × P) / (1714 × η)
where the output is the horsepower required to deliver that flow at pressure P.
Interpretation
The result tells you the volumetric flow a pump can deliver for the horsepower you’ve applied, after efficiency losses. The constants 1714 (hydraulic, HP·PSI basis) and 3960 (water, HP·ft basis) already bundle the unit conversions — you only need to supply the efficiency.
- Under 5 GPM: small hand pumps, compact hydraulic circuits, lab-scale flow.
- 5–20 GPM: typical mobile and light industrial hydraulics.
- 20–60 GPM: mid-size industrial pumps and machine tool systems.
- 60+ GPM: heavy industrial or large water-transfer pumps.
If your calculated GPM is far from the pump’s rated flow on its curve, check that you’re using the right pressure (system PSI, not peak relief setting) and a realistic efficiency — hydraulic pumps run 80–90%, centrifugal water pumps 60–80%.
Quick Reference Table
| HP | Pressure (PSI) | Efficiency | Flow (GPM) |
|---|---|---|---|
| 5 | 1500 | 85% | 4.86 |
| 10 | 2000 | 85% | 7.28 |
| 20 | 2500 | 85% | 11.66 |
| 25 | 3000 | 85% | 12.14 |
| 50 | 3000 | 90% | 25.71 |
FAQ
Which mode should I use — hydraulic or water pump?
Use a hydraulic mode when you know the system pressure in PSI (oil hydraulics, power units). Use a water pump mode when you know the total dynamic head in feet, meters, or PSI of lift — that’s how centrifugal pump curves are published.
What efficiency value should I enter?
For hydraulic gear and vane pumps, 80–85% is typical; piston pumps can reach 90–92%. For centrifugal water pumps, use 60–70% for small units and 75–80% for larger well-matched pumps. Enter 100 only for theoretical maximums.
Why do the formulas use 1714 and 3960?
They’re unit-conversion constants. 1714 converts HP × PSI to GPM for hydraulic fluid power. 3960 converts HP × feet-of-head to GPM for water (it includes water’s specific weight). Swapping them gives wrong answers by a factor of ~2.3.
Does this account for pipe friction or elevation losses?
No. The pressure or head you enter must already be the total the pump has to overcome — static lift plus friction losses plus discharge pressure. Calculate those separately and use the sum as your input.
