This free pump power calculator works as a hydraulic power calculator, a pump kW calculator and a brake horsepower (BHP) calculator for centrifugal water and process pumps. Enter the flow rate, total head, fluid specific gravity and the pump and motor efficiencies, and it returns the hydraulic power, shaft (brake) power, motor input power and a suggested standard motor — in both pump kW and HP.
How to calculate pump power
Pump power always starts from the energy you actually add to the fluid — the hydraulic power. For a flow rate Q in m³/h and a total head H in metres:
- Hydraulic power:
P(kW) = ρ · g · Q · H / 3.6e6, whereρis fluid density (kg/m³),g= 9.81 m/s². The 3.6×10⁶ factor converts m³/h and the result into kilowatts. - Density from specific gravity:
ρ = SG × 1000. Water (SG 1.0) is 1000 kg/m³. - Shaft (brake) power:
shaft kW = hydraulic kW / (pump eff%). - Motor input power:
motor kW = shaft kW / (motor eff%)— this is what you pay for at the meter.
That chain — hydraulic → shaft → motor — is the heart of pump sizing. Each step is bigger than the last because every device in the line loses some energy as heat and friction.
Worked example. Pump 50 m³/h of water (SG 1.0) to 30 m head, with a 70% pump efficiency and a 90% motor efficiency. Density is 1000 kg/m³, so hydraulic power = 1000 × 9.81 × 50 × 30 / 3,600,000 ≈ 4.09 kW. Shaft power = 4.09 / 0.70 ≈ 5.84 kW (about 7.8 HP). Motor input = 5.84 / 0.90 ≈ 6.49 kW. The next standard frame above 7.8 HP is a 7.5 HP motor on the minimum, though continuous duty usually steps up to 10 HP.
Hydraulic vs shaft vs motor power
These three numbers are often confused, but they answer different questions:
- Hydraulic power is the useful work delivered to the liquid — the theoretical minimum. It ignores all losses.
- Shaft power (also brake power or brake horsepower / BHP) is the power the motor must put into the pump shaft. It is the hydraulic power divided by the pump efficiency, and it is the figure you compare against the motor rating.
- Motor input power is the electrical power drawn from the supply — shaft power divided by motor efficiency. It drives your running cost and cable/breaker sizing.
| Quantity | Formula | Example (50 m³/h, 30 m) |
|---|---|---|
| Hydraulic power | ρ · g · Q · H / 3.6e6 | ≈ 4.09 kW |
| Shaft / brake power (BHP) | hydraulic / pump eff | ≈ 5.84 kW (7.8 HP) |
| Motor input power | shaft / motor eff | ≈ 6.49 kW |
Sizing the motor
To size the driver, take the shaft (brake) power, convert to HP (HP = kW / 0.7457), and choose the next standard motor frame above it — with a service-factor margin so the motor never runs into overload at the worst-case duty point. The table below shows typical shaft power for common water-pump duties at 70% pump efficiency.
| Flow (m³/h) | Head (m) | Hydraulic kW | Shaft kW @ 70% | Typical motor |
|---|---|---|---|---|
| 10 | 20 | 0.55 | 0.78 | 1 – 1.5 HP |
| 25 | 25 | 1.70 | 2.43 | 3 – 5 HP |
| 50 | 30 | 4.09 | 5.84 | 7.5 – 10 HP |
| 100 | 40 | 10.90 | 15.57 | 20 HP |
| 200 | 50 | 27.25 | 38.93 | 50 HP |
Always confirm the pump efficiency from the published curve at your duty point, not a generic figure, and account for viscosity if the fluid is not water — a higher specific gravity (heavier liquid) raises every number proportionally.
From sizing to production
A power calculation answers one line of a quotation. Building the pump means turning that duty point into a bill of materials, a costed quote, a work order and a GST invoice — and getting it right on every order. Inside OEMup ERP, the sizing data flows into the item master and BOM: motor frame, impeller, casing and seals roll up into a costed assembly, the work order drives production, and the e-invoice and e-way bill are generated at dispatch — no re-keying between engineering and accounts. It is purpose-built for pump & valve manufacturers. Start a free trial and run a pump order end to end.
Frequently asked questions
How do I calculate pump power in kW?
Hydraulic power in kW = ρ × g × Q × H / 3.6e6, where ρ is density (kg/m³), g is 9.81, Q is flow in m³/h and H is head in m. For water at 50 m³/h and 30 m that is about 4.09 kW. Divide by pump efficiency for shaft power, then by motor efficiency for motor input power.
What is the difference between hydraulic and shaft power?
Hydraulic power is the useful power given to the fluid. Shaft power (brake power / BHP) is what the motor must deliver to the pump shaft, and is higher because the pump is not 100% efficient: shaft = hydraulic / pump efficiency. At 4.09 kW hydraulic and 70% efficiency, shaft power is about 5.84 kW.
How do I size a pump motor?
Work out the shaft (brake) power, convert to HP (HP = kW / 0.7457), and pick the next standard motor frame above it with a service-factor margin. A 5.84 kW shaft power is about 7.8 HP, so the minimum standard size is 7.5 HP and continuous-duty designs usually step up to 10 HP. The calculator suggests the next standard HP automatically.
What pump efficiency should I assume?
Use the value from the pump curve at your duty point. As a guide, small centrifugal pumps run 40–60%, medium 60–75% and large well-matched pumps 75–85%. A 70% pump and 90% motor efficiency is a reasonable default for a first estimate, but confirm against the actual curve before finalising the motor.
More shop-floor tools: All Calculators · Motor HP Calculator · Tank Volume Calculator.
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