🛠️ Gear Pump Size Calculator

Gear pumps are the workhorses of hydraulic and lubrication systems. Two meshing gears trap oil in the spaces between their teeth and carry it from the suction side to the discharge side. The gear pump size calculator applies the fundamental relationship flow = displacement × speed, where displacement is the volume delivered per revolution (cc/rev) and speed is in RPM. The theoretical flow in liters per minute is displacement × RPM / 1000, and the real flow is a little less because some oil leaks back through the clearances.

When you spec a hydraulic system you usually know the flow you need—to move a cylinder at a given speed, or to feed a motor at the right RPM. Working backwards from the formula gives the required displacement, which is how the pump is categorized: a 12 cc/rev pump at 1,500 RPM delivers about 18 L/min theoretically, around 17 L/min in practice. The calculator handles both directions and folds in the volumetric efficiency automatically.

The theoretical flow is displacement times speed. Volumetric efficiency ηv, typically 88–96%, accounts for internal leakage through the gear clearances; the real flow is theoretical flow × ηv. The tool lets you solve for flow (given displacement and speed) or for displacement (given a target flow and speed), then reports the theoretical and actual flows side by side.

A handy derived metric is the drive power: the motor must deliver pressure × flow / 600 in kilowatts, or use the horsepower calculator with the same inputs. A pump rated 20 L/min at 150 bar needs about 5 kW before efficiency losses. That number decides the electric motor frame, the coupling size and the inlet line diameter.

1. Choose what you know: displacement and speed, or target flow and speed.
2. Enter the displacement in cc/rev, or the flow in L/min.
3. Enter the operating speed in RPM.
4. Enter the volumetric efficiency (default 90%).
5. Click Calculate for flow rate and required drive power.

What does displacement per revolution mean?

Displacement is the volume of fluid the pump moves in one full revolution of its shaft, usually stated in cubic centimeters per revolution (cc/rev). Multiply by speed to get flow: 10 cc/rev at 1,500 RPM delivers 15 L/min theoretical.

Why is actual flow lower than theoretical?

Because some fluid leaks backward through the clearances between the gear teeth, side plates and the casing. This leakage is called slip, and the ratio of actual to theoretical flow is the volumetric efficiency, usually 88–96%.

How do I choose between a gear pump and a piston pump?

Gear pumps are simple, cheap and tolerant of contamination, ideal for fixed-flow duties up to about 200 bar. Piston pumps handle higher pressures, variable displacement and better efficiency, but cost more and need cleaner oil. Match the pump type to pressure, flow control needs and oil cleanliness.

What happens if I run a gear pump too fast?

Excessive speed raises wear, noise and cavitation risk, and the slip losses grow. Every pump has a maximum rated speed set by the manufacturer, typically 2,000–3,600 RPM depending on size and inlet conditions. Stay below it.

How do I convert L/min to gpm?

Divide liters per minute by 3.785 to get US gallons per minute. A 60 L/min pump delivers about 15.9 gpm. The calculator accepts either unit for the target flow.

Can the same formula size a pump for a motor drive?

Yes—reversing the reasoning, a hydraulic motor receives Q L/min and turns at n RPM, so its consumption per revolution is Q/n. Motors are rated by the same displacement parameter, so the tool works for both directions.

Volumetric efficiency drops as pressure rises because leakage through gear clearances increases with pressure. For a first pass use 90%—it matches most new industrial gear pumps at rated pressure. The pump inlet must be adequately sized and flooded; cavitation from a starved suction line is the most common cause of premature gear pump failure. Always size for the maximum flow the system will ever demand, then confirm the speed is within the pump's published range.