Whether you’re designing a hydraulic power unit (HPU) from scratch or selecting a pump out in the field, one question always comes up first: “If I run a 20 cc/rev pump at 1,750 RPM, how much flow do I get — and what size motor do I need to drive it?” You know the formulas exist, but unit conversions always seem to get in the way.
“Enter your pump specs and working pressure — get flow rate, motor power, and horsepower all at once.”
This post covers the hydraulic pump flow rate (Q) formula and the required motor power (kW) calculation, along with an interactive calculator where you plug in your numbers and get instant results. We also wrap up with practical motor selection tips straight from the field.
1. The Theory Behind the Calculator
Before jumping to the calculator, let’s quickly run through the two formulas it uses.
- Flow Rate (Q): Multiply the pump’s displacement per revolution (cc/rev) by the motor speed (RPM). The result is the total output flow in liters per minute (L/min). Simple as that.
- Required Power (kW): Multiply pressure (bar) by flow rate (L/min), then divide by 600 and the pump’s overall efficiency. The result is the minimum motor power needed to hit your target pressure.
No pump converts 100% of motor input into pressurized flow. Losses come from two places: internal fluid leakage (volumetric loss) and mechanical friction between moving parts. As a rule of thumb — gear pumps typically run at 80–85% overall efficiency, while piston pumps reach 85–90%.
2. Hydraulic Pump Flow & Power Calculator
Enter the pump’s displacement per revolution and the connected motor speed. The calculator handles the rest.
3. Real-World Tips for Motor Selection
The “Required Power (kW)” shown above is the theoretical minimum to reach your target pressure. When it comes time to actually spec and purchase a motor, keep these two points in mind.
- Build in a startup margin: When a stopped motor kicks in and starts pushing fluid, it sees a brief inrush load well above steady-state. Standard practice is to select a motor rated at at least 1.2–1.3× the calculated value (20–30% margin).
- Round up to the next standard rating: If the calculation gives you 4.3 kW, you can’t buy a 4.3 kW motor off the shelf. Move up to the next standard size — in this example, 5.5 kW.
Getting the theory right, then multiplying by a design margin, is a simple discipline that prevents two of the most common field failures: insufficient pressure buildup and burned-out motors. Do the math — then give yourself some breathing room.
Related Article
If you’re sizing a hydraulic pump, you’ll likely need to calculate cylinder thrust force as well. The article below walks through the formula and provides an instant calculator — just enter bore diameter and pressure.
📄 Related ArticlePneumatic / Hydraulic Cylinder Force Calculator — Donaenam.com
Enter bore diameter and operating pressure to instantly calculate push and pull force (kgf & N). Covers both pneumatic and hydraulic cylinders.