Hydraulic power is the useful power added to the fluid; dividing by pump efficiency gives the shaft (brake) power the driver must deliver. The recommended motor applies the API 610 rated-power margin on top.
Worked example
50 m³/h of water at 60 m head, 70% pump / 95% motor efficiency. ≈ 11.7 kW shaft power, and an API 610 margin recommends a ~14.6 kW (≈ 20 hp) motor.
Inputs
Flow rate (m³/h)
Differential head (m)
Specific gravity — Relative to water (1000 kg/m³).
Pump efficiency (%)
Motor efficiency (%)
Frequently asked questions
What is the difference between hydraulic and shaft power?
Hydraulic power is the energy actually delivered to the fluid. Shaft (brake) power is what the pump draws from the driver — always larger, because pump efficiency is below 100%.
Why is the recommended motor larger than the shaft power?
API 610 specifies a margin so the motor isn't run at its limit: 125% of pump power up to 22 kW, 115% from 22–55 kW, and 110% above 55 kW. Then you round up to a standard motor frame.
Does this size the pump for NPSH?
No. Power and NPSH are separate checks — use the NPSH Available tool and compare against the pump's required NPSH with margin per ANSI/HI 9.6.1.
Assumptions
Differential head is the pump duty point — system static + friction head must be determined separately.
Efficiencies are at the rated point; check the actual pump curve.
NPSH is not evaluated here — verify NPSH available > NPSH required with margin per API 610 / ANSI-HI 9.6.1.
References
API Std 610 / ISO 13709 — Centrifugal Pumps for petroleum, petrochemical and natural gas industries.
Hydraulic power P = ρ g Q H is fundamental; motor margins per API 610 rated-power table.
Pipe Pressure Drop — Frictional ΔP and head loss for single-phase flow in a circular pipe (Darcy–Weisbach, Swamee–Jain friction factor).
Fittings Pressure Drop (K-factors) — Minor losses through elbows, tees, and valves by the ΣK excess-head method — the companion to straight-pipe friction.
Pipe Velocity & Reynolds — Line velocity, Reynolds number, and flow regime from flow rate and pipe inside diameter.
Friction Factor (Moody) — Darcy (and Fanning) friction factor from Reynolds number and relative roughness — Colebrook–White solved exactly, with the Swamee–Jain explicit fit for comparison.