Pump Power Calculator
Hydraulic power, shaft power, motor size and running cost from flow and head.
How it works
The useful work a pump does is P = ρ g Q H — density, gravity, volumetric flow and head. Everything else is loss. Divide by pump efficiency to get shaft power, then by motor efficiency to get what the meter sees.
Head is the term people get wrong. It is not the height you are lifting to; it is that height plus every friction loss in the system plus any pressure the discharge has to overcome. On a long or undersized pipe run the friction term routinely exceeds the static lift, which is why pipe sizing and pump sizing are the same problem and should not be done by different people.
Head expressed in metres is independent of density, which is why pump curves are drawn that way — the same impeller at the same speed produces the same head in metres whatever it is pumping. The power, though, is proportional to density, so switching from water to a fluid half again as dense needs half again the motor while the curve looks unchanged.
Efficiency is worth real money. A pump running 4000 hours a year at 5 kW shaft power costs several thousand over its life, and the gap between a 60%-efficient pump and an 80% one is 25% of that bill, every year, forever. Oversizing is the usual culprit — a pump throttled back to a duty far from its best efficiency point wastes the difference as heat and wears its bearings faster for the privilege.
Common questions
What head do I use?
Static lift from source level to discharge level, plus friction loss in all the pipework at the design flow, plus any pressure the discharge point requires. The pipe flow calculator on this site gives the friction term.
Why is my pump so far off its rated efficiency?
Almost certainly because it is oversized and running well left of its best efficiency point. A pump throttled by a valve is burning the difference as heat. Trimming the impeller or fitting a variable-speed drive is the fix.
Does a variable-speed drive help?
Substantially, on a system dominated by friction rather than static lift. The affinity laws put power at the cube of speed, so running at 80% speed uses about half the power. On a system that is mostly static lift the saving largely disappears.
What about NPSH?
Not covered here, and it is what actually stops pumps working. If the suction side cannot deliver enough absolute pressure above the fluid's vapour pressure, the pump cavitates regardless of how much power you give it.