How it works
A positive-displacement pump moves a fixed volume each revolution, so its flow is simply displacement times speed, less internal leakage: Q = displacement · n · η_vol With displacement in cm³/rev and speed in rpm, dividing by 1000 gives litres per minute. The volumetric efficiency η_vol accounts for the flow that leaks back internally — it falls as pressure and wear rise.
The useful hydraulic power is flow against pressure. In practical
units that reduces to the fluid-power shortcut
P_hyd (kW) = Q (L/min) · p (bar) / 600
where the 600 constant carries the L/min·bar → kW conversion. The driving motor has
to supply more than this, because the pump is not perfect: the
shaft power is the hydraulic power divided by the overall efficiency,
P_shaft = P_hyd / η_overall. Size the motor to the shaft power, not the
hydraulic power.
Worked example
An 18 cm³/rev pump driven at 1500 rpm against 160 bar (16 MPa), with a volumetric
efficiency of 92% and an overall efficiency of 85%. The flow is
18 × 1500 ÷ 1000 × 0.92 ≈ 24.8 L/min. The hydraulic power is
24.8 × 160 ÷ 600 ≈ 6.6 kW, so the shaft power required is
≈ 6.6 ÷ 0.85 = 7.8 kW (about 10.5 hp) — pick the next standard motor up.
Load this page and the calculator shows these numbers.
Frequently asked questions
- How do I calculate hydraulic pump flow from displacement and RPM?
- Flow Q = displacement (cm³/rev) × speed (rpm) ÷ 1000 × volumetric efficiency, giving L/min. For example an 18 cm³/rev pump at 1500 rpm with 92% volumetric efficiency delivers 18 × 1500 ÷ 1000 × 0.92 ≈ 24.8 L/min. Enter the displacement, speed and efficiency above.
- How do I calculate hydraulic power?
- Use the fluid-power shortcut: power (kW) = flow (L/min) × pressure (bar) ÷ 600. So 24.8 L/min at 160 bar is 24.8 × 160 ÷ 600 ≈ 6.6 kW of hydraulic (useful) power. The 600 constant bakes in the L/min·bar → kW conversion.
- What size motor do I need to drive the pump?
- Size the motor to the shaft power = hydraulic power ÷ overall efficiency. At 6.6 kW hydraulic and 85% overall efficiency that is 6.6 ÷ 0.85 ≈ 7.8 kW (about 10.5 hp), so you would pick the next standard motor up.
- What is volumetric efficiency?
- It is the fraction of the geometric (theoretical) flow the pump actually delivers — the rest leaks internally past the clearances, and the loss grows with pressure and wear. Gear pumps are typically 0.85–0.93; piston pumps 0.92–0.97.
- How do I get the flow in GPM?
- Toggle Imperial in the header and the flow shows in US gallons per minute; the example above is about 6.6 GPM. Power switches to horsepower at the same time.
- Does this work in metric and imperial?
- Yes — toggle SI/Imperial in the header. Displacement switches between cm³ and in³, pressure between bar and psi, flow between L/min and GPM, and power between kW and hp.
Method & assumptions
- Positive-displacement pump (gear, vane or piston) — flow is proportional to displacement and speed.
- The 600 constant bakes in the L/min·bar → kW conversion for hydraulic power; it is exact, not an approximation.
- Efficiencies are typical values — use the pump's own efficiency curves at your operating pressure for a firm number; both fall as pressure rises.
- Ignores line and valve pressure losses, fluid compressibility and inlet (cavitation) limits; pressure is the working pressure at the pump outlet.
Related calculators
- Hydraulic Cylinder Force Calculator — Push and pull force with the rod-area differential shown explicitly. Standard ISO bores.
- Pneumatic Air Consumption Calculator — Free-air consumption per cycle and per minute for a pneumatic cylinder.