Pump Horsepower for Flow

HP = (GPM × ft head) / (3960 × pump efficiency).

Inputs

Centrifugal: 60-75%. Positive displacement: 80-90%.

Result

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How to use this calculator

  • Enter required GPM and head.
  • Pump efficiency from spec or 65% default.

About this calculator

Pump sizing: HP = (GPM × ft head) / (3960 × η). 3960 is unit conversion constant (water specific gravity built in). 30 GPM at 50 ft head with 65% efficient pump: ~0.58 HP. Add safety factor (10-25%) and round up to next standard size (1/2, 3/4, 1, 1.5, 2 HP). Centrifugal pumps run 60-75% efficient at design point; positive displacement 80-90%. Oversize hurts efficiency; undersize fails to meet demand.

Frequently asked

Where does 3960 come from?+
33,000 ft·lb/min/HP / 8.33 lb/gal = 3960 ft·gal/min/HP.
What's "head"?+
Vertical lift + friction loss in pipes + pressure at outlet. Total dynamic head (TDH).
Friction loss?+
Hazen-Williams or Darcy-Weisbach. Rule: 100 ft of 1" pipe at 30 GPM ≈ 30-40 ft head loss.
Variable-frequency drive?+
VFD adjusts pump speed to demand. Saves 30-50% energy on variable loads.
Cavitation?+
Pressure drops below vapor pressure at pump inlet → bubbles → damage. Ensure adequate NPSH (net positive suction head).

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