Pump Power Calculator

Sizing a pump's drive motor starts with the hydraulic power — the actual useful work rate needed to lift a given flow of fluid against a given total head:

Pₕᵧᵨ = ρ × g × Q × H

where ρ is fluid density, g is gravitational acceleration (9.80665 m/s²), Q is volumetric flow rate, and H is total head — the total energy per unit weight of fluid the pump must add, expressed as an equivalent height, combining static lift, friction losses in the piping, and any pressure difference between the source and destination.

No pump converts 100% of the mechanical energy delivered to its shaft into useful hydraulic work — internal losses from fluid recirculation, disk friction, and mechanical (bearing/seal) friction mean the shaft power (also called brake power, the power the driver must deliver to the pump shaft) is always higher than the hydraulic power, by a factor set by the pump's efficiency:

Pₛₕₐᶠₜ = Pₕᵧᵨ / ηₚ

Finally, if the pump is driven by an electric motor, the motor itself also has losses (electrical and mechanical), so the motor input power — the actual electrical power drawn from the supply — is higher again than the shaft power, by the motor's own efficiency:

Pₘₒₜₒᵣ = Pₛₕₐᶠₜ / ηₘ

Pump efficiency itself is not a fixed number for a given pump — it varies across the pump's operating curve, peaking at its Best Efficiency Point (BEP) and dropping off at flow rates well above or below that point. For motor sizing, the shaft power should be checked not just at the design duty point but across the pump's full expected operating range, since running far from BEP both wastes energy and can shorten pump life through increased vibration and bearing/seal wear.

1. Using static lift alone as "head" and ignoring friction losses. Total head is the static lift plus all friction losses in the suction and discharge piping (and any pressure head difference) — using static lift alone significantly understates the true head the pump must overcome, and therefore understates power.

2. Sizing the motor to shaft power at the design point only. Centrifugal pump power typically increases toward the low-head, high-flow end of its curve — a motor sized exactly to the design-point shaft power can be overloaded if the system operates further right on the curve (e.g. during low static head conditions).

3. Forgetting motor efficiency when comparing to motor nameplate ratings. Shaft power is not the same as motor input (electrical) power — comparing a calculated shaft power directly against a motor's kW nameplate without accounting for motor efficiency slightly understates actual electricity draw.

4. Using a generic efficiency figure instead of the actual pump curve. Pump efficiency varies with flow rate across the pump's operating curve and is different for every pump model and impeller trim — a rule-of-thumb efficiency is fine for early estimating, but final sizing should use the specific pump's published performance curve.

5. Mixing up density for non-water fluids. Pumping a fluid denser or lighter than water (brine, oil, slurry) changes hydraulic power proportionally — using water's density (1000 kg/m³) for a different fluid gives a power figure that's wrong by the same ratio as the density difference.

6. Not adding a safety margin on the selected motor size. Even after correctly calculating motor input power, it's common practice to select the next standard motor frame size above the calculated requirement, rather than the exact calculated value, to allow for pump wear, minor system changes, and starting torque requirements.