Pump power calculator: hydraulic power, shaft power and motor input
This pump power calculator uses the formula from the KSB pump lexicon, P = ρ · g · Q · H / η. Enter the flow rate, the head, the density of the fluid and the pump efficiency from the curve: 200 gpm of water against 100 ft at 70 % efficiency need 7.23 hp at the shaft.
Example: Units: US (gpm, ft) · Flow rate Q 200 · Total head H 100 · Fluid density ρ 1,000 kg/m³ · Pump efficiency η 70 % → Pump shaft power: Pump power 7.23 hp = 5.39 kW. Source: KSB Centrifugal Pump Lexicon, “Power input”: P = ρ · g · Q · H / η (density, gravitational constant, flow rate, head, pump efficiency) (read 09.10.2026). Updated: .
How it is calculated
Pump power = density × g × flow rate × head ÷ efficiency. KSB writes it as P = ρ · g · Q · H / η, with ρ in kg/m³, g = 9.80665 m/s², Q in m³/s and H in m – the result is in watts. The part ρ · g · Q · H is the hydraulic power that ends up in the fluid; the pump needs more at its shaft because of its losses. Pump efficiency η is the ratio of the two.
Examples (water, 1000 kg/m³)
| Flow rate | Head | Efficiency | Hydraulic power | Shaft power |
|---|---|---|---|---|
| 50 m³/h | 30 m | 70 % | 4.09 kW | 5.84 kW |
| 10 m³/h | 20 m | 60 % | 0.54 kW | 0.91 kW |
| 36 m³/h | 10 m | 65 % | 0.98 kW | 1.51 kW |
| 100 m³/h | 50 m | 75 % | 13.62 kW | 18.16 kW |
Pump power in gpm and feet
In US units the calculator converts with the NIST factors 1 ft = 0.3048 m and 1 hp = 745.6999 W and with the exact US gallon of 231 in³ = 3.785411784 L. 200 gpm against 100 ft of head at 70 % efficiency give 5.06 hp of hydraulic power and 7.23 hp at the pump shaft. The rule of thumb BHP = gpm × ft × SG ÷ (3960 × η) gives 7.22 hp – the constant 3960 is rounded.
From shaft power to motor input
KSB distinguishes the pump efficiency from the efficiency of the whole pump set, measured at the motor terminal box. Enter the motor efficiency to see the electrical input: with 90 %, the 5.84 kW pump above draws 6.49 kW. Heavier fluids need more power – at 1200 kg/m³ the same pump needs 7.00 kW. For the energy bill use the electricity cost calculator; the flow in a pipe of a given size comes from the pipe flow rate calculator.
Frequently asked questions
What is the formula for pump power?
P = ρ · g · Q · H / η. With 50 m³/h (0.0139 m³/s), 30 m head, water and 70 % efficiency: 4.09 kW hydraulic power and 5.84 kW at the shaft.
How do I calculate pump horsepower from gpm and feet?
Convert to SI units or use gpm × ft × SG ÷ (3960 × η). 200 gpm at 100 ft and 70 % efficiency need about 7.23 hp.
What is the difference between hydraulic power and shaft power?
Hydraulic power ρ·g·Q·H is the energy put into the fluid per second. Shaft power is what the motor must deliver to the pump; the losses make it larger by the factor 1/η.
Which efficiency should I use?
The pump efficiency at your duty point from the manufacturer’s pump curve. A rough guess makes the result only an estimate.
Does the fluid density matter?
Yes, power rises in proportion to density: a fluid with 1200 kg/m³ needs 20 % more power than water at the same flow and head.
How big should the motor be?
Larger than the shaft power at the duty point, with a reserve for the operating range. The calculator gives the power at one point only.
Sources and legal basis
- KSB Centrifugal Pump Lexicon, “Power input”: P = ρ · g · Q · H / η (density, gravitational constant, flow rate, head, pump efficiency) (read 09.10.2026)
- KSB Centrifugal Pump Lexicon, “Efficiency”: pump efficiency = pump power output P_Q ÷ pump input power P; pump set efficiency = P_Q ÷ drive input power (DIN EN ISO 17769-1) (read 09.10.2026)
- NIST SP 811, Appendix B.8 – standard acceleration of free fall 9.806 65 m/s², 1 ft = 0.3048 m, 1 gal (US) = 3.785 412 L (exactly 231 in³ = 3.785 411 784 L), 1 hp (550 ft·lbf/s) = 745.699 9 W (read 09.10.2026)
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