Pump Power Calculator
Size a pump and estimate the electrical power it will draw at a given duty point.
Electrical input power
11.57 kW
- Hydraulic power
- 8.16 kW
- Shaft power
- 10.87 kW
- Flow in SI units
- 0.027778 m³/s
- Standard gravity used
- 9.80665 m/s²
Calculated from the values you entered. Formula version 1; last reviewed 2026-09-15.
What this calculator does
Hydraulic power from density, gravity, flow and head, divided by the efficiencies the engineer supplies. Standard gravity is the only constant used. Density and both efficiencies are entered by the engineer; no typical pump or motor efficiency is assumed.
Inputs and what they mean
- Flow rate
- — number value.
- Flow unit
- — choice value.
- Total head
- — number value.
- Fluid density
- — number value.
- Pump efficiency
- — From the pump curve at this duty point..
- Motor efficiency
- — From the motor data sheet..
How to use it
- Enter your own figures — the calculator never fills in a rate, price or benchmark for you.
- Press Calculate to see the result.
- Read the formula, variables, assumptions and source below the result before you rely on it.
Formula
Ph = ρ × g × Q × H; Shaft power = Ph ÷ pump efficiency; Electrical input = Shaft power ÷ motor efficiency
Worked example
100 m³/h of water against 30 m of head
Pump at 75% efficiency driven by a 94% efficient motor.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
Results depend entirely on the figures you enter and are rounded for display. They are for general information and education, not professional advice.
Reference: U.S. Department of Energy — Improving pumping system performance: a sourcebook for industry
Last reviewed:
Common questions
Formula, source and verification
Hydraulic power from density, gravity, flow and head, divided by the efficiencies the engineer supplies.
The question it answers: How much power will this pump draw at the flow and head we need?
The formula
Ph = ρ × g × Q × H; Shaft power = Ph ÷ pump efficiency; Electrical input = Shaft power ÷ motor efficiency
- Q — Flow rate
- (m3_s). Volumetric flow at the duty point.
- H — Total head
- (m). Total dynamic head.
- ρ — Fluid density
- (kg/m³). Density of the pumped fluid, entered by the engineer.
- g — Gravitational acceleration
- (m/s²). 9.80665 m/s², the standard value.
- ηp — Pump efficiency
- (%). From the pump curve at the duty point.
- ηm — Motor efficiency
- (%). From the motor data sheet.
Units: m³/h or L/s in, metres of head, watts and kilowatts out.
What kind of calculation this is
Deterministic formula. The same inputs always give the same answer. The maths is fixed and does not depend on judgement.
Method
Standard gravity is the only constant used. Density and both efficiencies are entered by the engineer; no typical pump or motor efficiency is assumed.
Assumptions built into the result
- Operational: Incompressible fluid at the stated density.
- Industry: Standard gravity of 9.80665 m/s².
Figures this calculator will never guess for you
- No default pump efficiency, motor efficiency, safety factor or energy price is applied.
Limitations
- Power at one duty point only. It does not model system curves, NPSH or transient behaviour.
Source and version
- Standard or reference
- Pumping system power relationships — U.S. Department of Energy (Improving Pumping System Performance: A Sourcebook for Industry, U.S. Department of Energy.)
- Published source
- U.S. Department of Energy — Second edition
- Formula version
- Version 1
- Verification
- Reviewed against the cited source on
- How much weight the source carries
- Standards or government
- Applies to
- Currency
- The result is a ratio or index, so it does not depend on currency.
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