Quick answer
Pump flow in L/min = displacement (cc/rev) × rpm × volumetric efficiency ÷ 1,000. A 20 cc/rev pump at 1,500 rpm and 92% efficiency delivers 27.6 L/min (7.29 US GPM).
Enter the displacement, speed and volumetric efficiency. The calculator returns flow in litres per minute and gallons per minute.
Related tools: Gallons per Minute Calculator: Flow in gallons per minute from pipe size and velocity, or volume and time. Friction Loss Calculator: Pipe friction head loss with the Hazen–Williams equation.
How to use the Pump Flow Calculator
- Enter displacement (cc/rev) (above 0).
- Enter speed (rpm) (above 0).
- Enter volumetric efficiency (%) (between 0 and 100). It starts at 95.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Displacement | A number, in cc/rev | > 0 |
| Speed | A number, in rpm | > 0 |
| Volumetric efficiency | A number, in % | > 0, ≤ 100, starts at 95 |
Formula and method
Formula
Q (L/min) = displacement × rpm × efficiency ÷ 1000
IEEE-754 double precision; shown to the decimals indicated.
Theoretical flow reduced by volumetric efficiency.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Displacement = 20; Speed = 1500; Volumetric efficiency = 92.
| Result | Value |
|---|---|
| Flow | 27.6 L/min |
| US GPM | 7.291 |
Example 2
Inputs: Displacement = 45; Speed = 2200; Volumetric efficiency = 95.
| Result | Value |
|---|---|
| Flow | 94.05 L/min |
| US GPM | 24.845 |
Example 3
Inputs: Displacement = 10; Speed = 1000; Volumetric efficiency = 100.
| Result | Value |
|---|---|
| Flow | 10 L/min |
| US GPM | 2.642 |
The first line is the headline result (flow). Below it the pump Flow Calculator lists uS GPM, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Efficiency
Volumetric efficiency is the share of the ideal flow that actually leaves the pump. It falls as pressure rises and oil heats and thins. A gear pump may be 85 to 95%.
Sizing
Choose displacement and speed to reach the flow your actuators need. The ideal flow here is 30 L/min (20 × 1,500 ÷ 1,000), and the actual 27.6 L/min is 92% of it.
How the result changes with displacement
Holding every other input at the first worked example, this table changes only displacement. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Displacement | Flow | US GPM |
|---|---|---|
| 10 | 13.8 L/min | 3.646 |
| 20 | 27.6 L/min | 7.291 |
| 40 | 55.2 L/min | 14.582 |
| 80 | 110.4 L/min | 29.165 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Displacement: entering "0" shows "Displacement must be greater than 0." beside that field and no result.
- Volumetric efficiency: entering "0" shows "Volumetric efficiency must be greater than 0." beside that field and no result.
Choosing between the Pump Flow Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Gallons per Minute Calculator](/engineering/gallons-per-minute/) | use | Flow |
| [Friction Loss Calculator](/engineering/friction-loss/) | flow, inside diameter and pipe length | Head loss |
| [Hydraulic Cylinder Force Calculator](/engineering/hydraulic-cylinder-force-calculator/) | units, pressure and bore diameter | Extend force |
| [Pipe Pressure Drop Calculator](/engineering/pipe-pressure-drop-calculator/) | flow, inside diameter and length | Pressure drop |
| [Flow Rate Calculator](/science/flow-rate-calculator/) | use | Flow rate |
Background: fluid flow
Volumetric flow rate is Q = A × v (cross-sectional area times average velocity). Bernoulli's equation, P + ½ρv² + ρgh = constant, holds along a streamline for steady, incompressible, frictionless flow, so real pipes need friction corrections. The Reynolds number Re = ρvL/μ compares inertia with viscosity.
Laminar flow in a pipe is usually quoted below Re of about 2,100 to 2,300 and fully turbulent above about 4,000, with an uncertain transition between; treat the thresholds as approximate.
Assumptions and limitations
- Efficiency depends on pressure and temperature
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for displacement and volumetric efficiency to confirm this.
Frequently asked questions
What is displacement?
The volume pumped per revolution.
How do I get the drive power?
Flow times pressure, divided by the pump's overall efficiency.
Does speed change flow?
Yes, flow is directly proportional to speed: doubling the rpm doubles the flow at the same efficiency.
What does the Pump Flow Calculator calculate?
Pump output from displacement, speed and volumetric efficiency. It returns Flow and US GPM.
What formula does the Pump Flow Calculator use?
Q (L/min) = displacement × rpm × efficiency ÷ 1000. Theoretical flow reduced by volumetric efficiency.
What do I need to enter in the pump Flow Calculator?
You enter Displacement (cc/rev), Speed (rpm) and Volumetric efficiency (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Efficiency depends on pressure and temperature.
What is an example of a pump Flow result?
With displacement 20, speed 1500 and volumetric efficiency 92, the flow is 27.6 L/min.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Efficiency depends on pressure and temperature.
Related calculators
- Gallons per Minute Calculator — Flow in gallons per minute from pipe size and velocity, or volume and time.
- Friction Loss Calculator — Pipe friction head loss with the Hazen–Williams equation.
- Pipe Pressure Drop Calculator — Pressure drop in a straight pipe from the Darcy–Weisbach equation.
- Flow Rate Calculator — Volumetric flow rate from area and velocity, or volume and time.
- Flow Rate Converter — Convert litres, gallons and cubic metres or feet per second, minute and hour.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.