Interactive Calculator · 2026
Pump Size Calculator
Size a pump from flow and head. Enter the flow rate, the total head it must lift against and the pump efficiency — get the shaft power in kW, the hydraulic power in HP and the motor rating to specify.
Hydraulic power → recommended motor rating (with 20% margin)
Your duty point
Volume the pump must deliver per hour.
Static lift plus friction losses.
Wire-to-water efficiency of the pump — typical centrifugal pumps sit around 60–75%.
Hydraulic power vs motor rating
The motor is sized above the hydraulic duty so it never runs at 100% load.
The hydraulic power is what the water actually needs. Motors are picked one standard frame up — here a 3 HP motor — so the pump runs comfortably inside its rating with headroom for start-up and viscosity swings.
Always confirm the motor against the manufacturer's pump curve at your exact duty point.
How this is calculated
- Shaft power = (9.81 × flow × head) ÷ (3600 × efficiency) = (9.81 × 20 × 15) ÷ (3600 × 0.65) = 1.26 kW.
- Hydraulic power = shaft power × 1.341 = 1.26 × 1.341 = 1.69 HP.
- Recommended motor = ⌈hydraulic power × 1.2⌉ = ⌈1.69 × 1.2⌉ = 3 HP.
Indicative sizing for concept planning. A detailed selection must also fix NPSH, the system curve and starting method — confirm with a qualified consultant before procurement.
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