Pump Horsepower Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A calculator can remove arithmetic without removing judgment. For pump horsepower, mechanical sizing is rarely just arithmetic; ratios, efficiencies, diameters, speeds, and load paths must all refer to the same operating condition. Use the result as a transparent model of the fields on this page, not as a substitute for knowing what those fields mean.

What this calculator does

This page focuses on shaft power from Discharge (Q), Differential head (H), Density of fluid (ρ). The calculation stays deliberately narrow: it uses the fields tied to this relationship and reports related quantities only when they follow from those same inputs.

How to use it

Enter Discharge (Q), Differential head (H), Density of fluid (ρ), Revolutions (N), Acceleration due to gravity (g) from the same physical setup, keeping every unit consistent with the selector shown on the page. After the result appears, make one small input change as a sanity check rather than relying on the first number simply because it has several decimal places.

How the calculation works

Hydraulic power is Ph = ρgQH. Required shaft power is Ps = Ph/η, with efficiency entered as a fraction. The page also reports a specific-speed expression based on RPM, flow, and head.

Example

Using the default example on the page (Discharge (Q) = 0.01 m³/s; Differential head (H) = 20 m; Density of fluid (ρ) = 1000 kg/m³; Revolutions (N) = 1500 rpm), the calculator returns shaft power of 2,451.6625 W. Use the default result as a hand-check point, then test one input at a time so an inverted ratio or unit mistake becomes obvious.

How to interpret the result

Use the shaft power for estimation and comparison within this mechanical model. A number that looks reasonable can still be wrong if a ratio is inverted or a diameter, RPM, pressure, efficiency, or force uses a different convention than the page expects.

Limitations and notes

The page converts hydraulic power to shaft power with one entered efficiency and uses a simplified specific-speed expression. Pump curves, system curves, viscosity, impeller diameter, motor efficiency, cavitation margin, off-design operation, and unit-system conventions for published specific speed are not automatically accounted for.