Enter any two of the three pump variables — horsepower, total head, or flow rate — along with the fluid’s specific gravity and pump efficiency to solve for the third.
Related Calculators
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- Horsepower To GPM Calculator
- Head Flow To Horsepower Calculator
- Pump Brake Horsepower Calculator
Formula
The calculator uses the standard pump hydraulic equation, rearranged for each mode:
HP = (Q × H × SG) / (3960 × η)
Q = (HP × 3960 × η) / (H × SG)
H = (HP × 3960 × η) / (Q × SG)
where HP = pump shaft horsepower, Q = flow in GPM, H = total head in feet, SG = specific gravity of the fluid, and η = pump efficiency as a decimal (1.0 = 100%). The constant 3960 converts GPM × ft of water into horsepower.
Interpretation
When efficiency is set to 100%, the horsepower output is water horsepower (WHP) — the ideal hydraulic power delivered to the fluid. At any efficiency below 100%, the result is brake horsepower (BHP) — the shaft power the motor must supply to move that flow against that head.
- Under 1 HP: fractional-HP pumps (small circulators, aquarium, light transfer).
- 1–5 HP: residential well, pool, and booster pumps.
- 5–50 HP: commercial HVAC, irrigation, and process pumps.
- 50–500 HP: industrial and large irrigation pumps.
- Over 500 HP: municipal water, mining, and power plant service.
Typical centrifugal pump efficiencies run 60–85%. If you size a motor using WHP (η = 100%), you will undersize the drive — always divide by the actual pump efficiency to get BHP, then add a service factor.
Quick Unit Reference
| From | To | Multiply by |
|---|---|---|
| m³/h | GPM | 4.40287 |
| L/s | GPM | 15.8503 |
| meters of head | feet of head | 3.28084 |
| feet of head (water) | psi | 0.4335 |
| kW | HP | 1.341 |
FAQ
What is "total head" — is it the same as the vertical lift?
No. Total dynamic head (TDH) is the sum of static lift, friction losses in the piping, and any pressure differential between the source and discharge. Using only vertical lift will undersize the pump.
Do I use water horsepower or brake horsepower to pick a motor?
Use brake horsepower (BHP), which includes pump efficiency. Then select a motor rated at or above BHP — often with a 10–25% service factor for safety and transient loads.
Why does specific gravity matter if I'm pumping water?
For water, SG = 1.00 and the term drops out. For heavier fluids like brine (1.25) or sulfuric acid (1.84), the pump must do proportionally more work at the same head and flow, so HP scales directly with SG.
What efficiency should I use if I don't know the pump curve?
For a rough estimate, use 65–75% for standard centrifugal pumps at their best efficiency point. Small pumps (under 5 HP) trend lower, around 50–60%. For precise sizing, always pull efficiency from the manufacturer's curve at your operating point.
