Pump Power Calculator
Pump power from flow and head or pressure: hydraulic, shaft and electrical kW and hp, next standard motor size, yearly energy cost and NPSH available.
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About this toolWhat is the Pump Power Calculator?
A pump turns electrical energy into fluid energy, losing some in the motor and more in the pump. This calculator follows that chain. It starts with the hydraulic power the liquid receives, ρgQH, divides by the pump efficiency to get the shaft (brake) power, and divides again by the motor efficiency to get the electrical input. With the defaults, 10 L/s of water against 30 m of head needs 2.94 kW of hydraulic power, 4.20 kW at the shaft with a 70% efficient pump, and 4.56 kW from the supply with a 92% motor.
You can enter the duty as total head in metres or feet, or as a pressure rise in kPa, bar, psi or MPa, and the tool converts using the liquid density. Presets cover water at 20 and 60 °C, seawater, a glycol mix and diesel. It then picks the next standard IEC motor rating in kW and NEMA rating in hp that covers the shaft power.
Add running hours and an electricity price to see energy use and cost per year. That figure often dwarfs the purchase price of the pump. A separate panel works out the net positive suction head available (NPSHa) from the surface pressure, suction lift or flooded head, suction losses and water temperature, so you can compare it with the pump's NPSH required.
- Hydraulic, shaft and electrical power in kW and hp
- Head or pressure-rise input with density-based conversion
- Next standard IEC (kW) and NEMA (hp) motor size
- Yearly energy and running cost in your currency
- NPSH available with water vapour pressure from 0 to 100 °C
How to use the Pump Power Calculator
- Enter the flow rate in any unit.
- Enter the total dynamic head, or switch to pressure rise and enter the difference between discharge and suction pressure.
- Pick the liquid or type its density.
- Enter the pump efficiency from its curve and the motor efficiency from its nameplate or efficiency class.
- Optionally add running hours per year and the price of electricity, and fill in the NPSH panel with the tank pressure, suction head, suction losses and water temperature.
Worked examples
Building water booster
Input
10 L/s water, 30 m head, 70% pump, 92% motor, 4,000 h/yr at 0.15 per kWh
Output
Hydraulic 2.94 kW, shaft 4.20 kW (5.63 hp), electrical 4.56 kW, IEC 5.5 kW or NEMA 7.5 hp motor, 18,240 kWh and 2,736 per year
Raising pump efficiency to 80% would save about 2,280 kWh a year at the same duty.
US process pump
Input
200 gpm, 120 ft, water, 75% pump, 93% motor, 6,000 h at $0.12
Output
Water power 6.06 hp, brake 8.08 hp, input 6.48 kW, about $4,664 a year
The 3960 rule gives 200 × 120 / 3960 = 6.06 hp, the same as ρgQH.
NPSH on a suction lift
Input
Open tank at sea level, 2 m lift, 0.5 m suction loss
Output
NPSHa = 7.6 m at 20 °C but only 3.2 m at 80 °C
Same pipework, very different margin. Hot water usually needs a flooded suction.
Background and insight
Hydraulic power is the rate of work done on the liquid: weight flow times head, P = ρgQH. In US units the same thing becomes water horsepower = gpm × ft × SG / 3960, the 3960 coming from 33,000 ft·lbf/min per hp divided by 8.33 lb per gallon. One mechanical horsepower is 745.7 W.
Pump efficiency comes from the manufacturer's curve at your duty point. Small end-suction pumps may reach 50 to 70% and large well-matched pumps 80 to 90%. Motor efficiency classes are set by IEC 60034-30-1, with IE3 (premium) motors roughly 85 to 96% depending on size. Standard motor ratings follow IEC 60072 in kW and NEMA MG 1 in hp.
Net positive suction head is about cavitation. NPSHa = (p_surface − p_vapour)/(ρg) + static suction head − suction losses. Hot water has a much higher vapour pressure (47.4 kPa at 80 °C versus 2.3 kPa at 20 °C), so a suction lift that works for cold water can starve a pump on hot water. Hydraulic Institute guidance (ANSI/HI 9.6.1) recommends an NPSH margin above NPSHr that depends on the application.
Common use cases
Motor sizing
Get the shaft power and the next standard IEC or NEMA motor rating for a pump duty.
Energy audits
Estimate yearly kWh and cost for an existing pump, and the saving from a more efficient pump or motor.
Cavitation checks
Work out NPSH available on a lift or with hot water before choosing a pump.
Turning pressure into head
Convert a gauge differential into metres or feet of head for a given liquid density.
Tips and limitations
- Size the motor for the highest power on the curve you might reach, usually at run-out (maximum flow), not only at the duty point.
- Head is independent of density, but power is not. A pump that lifts water 30 m lifts brine 30 m too, and needs about 20% more power to do it.
- Efficiency for viscous liquids is lower than the water curve shows. ANSI/HI 9.6.7 gives correction factors.
- Variable speed drives cut energy sharply at part load because power falls roughly with the cube of speed on systems dominated by friction.
- Keep NPSHa comfortably above NPSHr, add margin for hot or volatile liquids, and remember NPSHr rises with flow.
Frequently asked questions
How do you calculate pump power?
Hydraulic power P = ρ × g × Q × H. For water (1000 kg/m³) at 0.01 m³/s and 20 m: 1000 × 9.81 × 0.01 × 20 = 1.96 kW. Divide by pump efficiency for shaft power and by motor efficiency for electrical input.
What is the difference between hydraulic power and brake horsepower?
Hydraulic (water) power is the energy the liquid actually gains. Brake horsepower is what the pump shaft needs, which is higher because of losses inside the pump: BHP = water hp / pump efficiency.
How do I calculate pump horsepower from gpm and head?
Water hp = gpm × head (ft) × specific gravity / 3960. Divide by pump efficiency for brake hp. 100 gpm at 100 ft of water is 2.53 water hp, or 3.6 hp at 70% efficiency.
What is a typical pump efficiency?
Small centrifugal pumps often run 50 to 70%, medium ones 70 to 80%, and large pumps at their best efficiency point 80 to 90%. The exact figure comes from the pump curve at your flow and head.
What is NPSH and why does it matter?
Net positive suction head is the pressure margin above vapour pressure at the pump inlet, expressed as head. If NPSH available falls below what the pump requires, the liquid flashes to vapour in the impeller eye and the bubbles collapse violently: noise, lost head and eroded impellers.
How much does it cost to run a pump?
Electrical kW × hours × price per kWh. A pump drawing 4.56 kW for 4,000 hours at 0.15 per kWh costs about 2,736 a year. Over a 15-year life energy is usually the largest cost by far.
Is there an in-depth guide to the Pump Power Calculator?
Yes. “Pump Power, Head and Efficiency: From ρgQH to the Electricity Bill” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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