How to Use the Hydraulic Pump Flow Calculator
Before sizing a hydraulic system or selecting a pump, the first value engineers need is flow output. This calculator multiplies pump displacement (the theoretical volume moved per revolution) by pump speed to get theoretical flow, then applies volumetric efficiency — which accounts for internal leakage — to give you actual flow output.
Gear, vane, and piston pumps all list displacement (in³/rev) on their datasheets, and the formula is Qth(GPM) = displacement(in³/rev) × RPM ÷ 231. Real pumps always leak a small amount of oil back through internal clearances, so new-pump volumetric efficiency typically runs 90-95%, dropping further as components wear.
Actual flow output feeds directly into cylinder speed calculations, hydraulic motor RPM selection, and pipe velocity sizing. When pipe velocity gets too high, pressure drop, heat, and noise all increase, so knowing actual flow lets you pick the right pipe diameter too. Always compare results against the manufacturer's tested performance curve before finalizing a design, and have a fluid power engineer review critical hydraulic systems.
Frequently Asked Questions
If the catalog doesn't list it, use around 90% for a new gear pump, 92% for a vane pump, or 95% for a piston pump as a reference. Check the manufacturer's performance curve for an accurate design value.
If volumetric efficiency comes out below 85%, suspect internal pump wear or degraded oil viscosity. If it stays low consistently, consider servicing or replacing the pump.