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How to Size a Hydraulic Motor: Displacement, Torque & RPM
June 5, 2026
Hydraulic motor sizing comes down to three numbers that pull on each other. Displacement sets the torque. Flow sets the speed. Pressure links the two. Get displacement and pressure right and you hit your torque; match the flow to that displacement and you hit your RPM. Use the calculator below, check it against the formulas, then pick the motor that fits.
The hydraulic motor sizing calculator
Enter your displacement, pressure, flow, and a rough efficiency. The tool returns torque, shaft speed, and horsepower, so you can weigh the trade-off before you shop the Hydraulic Motors category.
How to use the calculator
Three inputs do the work. Knowing where each one comes from is the difference between a motor that drives your load and one that stalls or overspins.
- Displacement (in³/rev)
- How much oil the motor takes in per turn. It is the biggest lever: it sets torque per PSI, and it divides your flow to set speed. Orbital units in the Gerotor & Geroler torque motors group run from a few cubic inches up past 30 in³/rev.
- Pressure (psi)
- Your working pressure, not the relief setting. Use the pressure the load actually draws; the motor only makes torque against the load in front of it, up to its rated maximum.
- Flow (GPM)
- What your pump delivers to the motor. Sizing a pump or cylinder instead? The Hydraulic Cylinder Force Calculator handles the linear side of the same system.
- Efficiency (%)
- Two losses matter. Friction trims the torque you get — call it 90% for a good orbital. Leakage trims the RPM, because a little oil slips past the shaft instead of turning it. We cut the torque for friction, then trim the RPM for the unit we pick.
The torque and speed formulas
Both numbers come from two short equations that every fluid-power handbook shares. They line up with the Engineering ToolBox pump and motor formulas and the Danfoss (Eaton Char-Lynn) orbital motor line we stock.
Torque (lb-in) = Displacement (in³/rev) × Pressure (psi) ÷ (2π)
Speed (RPM) = Flow (GPM) × 231 ÷ Displacement (in³/rev)
Power (HP) = Torque (lb-in) × Speed (RPM) ÷ 63,025
The torque equation is why you can trade displacement for pressure. The same torque comes from a big motor at low pressure or a small motor at high pressure. That trade is the whole idea behind a low-speed, high-torque motor. The chart below plots torque against displacement at 1,000 psi — read your size across, then scale up for your real pressure.
Worked example: sizing a motor from your numbers
Say you are driving a conveyor and the numbers below are your starting point. They are an example, not a spec for any one motor — swap in yours.
- Set the torque target. The conveyor needs about 3,000 lb-in (250 lb-ft) to start under load. That is your torque number.
- Pick a pressure and solve for displacement. At 2,500 psi, with friction trimmed, the torque formula gives about 8.4 in³/rev. Round up to a stocked 9 in³/rev frame for margin.
- Check the speed your flow produces. With 10 GPM into 9 in³/rev, the speed formula gives about 257 RPM. If that is too fast, drop the flow or step up a displacement size — the speed chart below shows the trade.
- Confirm the power. About 3,223 lb-in at 257 RPM works out to roughly 13 HP at the shaft — well inside a 10 GPM, 2,500 psi circuit.
- Match it to a real unit. A 9 in³/rev result points straight at an orbital motor like the Char-Lynn 101-1001 H-Series motor. Confirm the exact displacement and port size on the product page before ordering.
These formulas give paper values. Real motors lose a little torque to friction and a little speed to leakage, and every unit has a rated pressure, speed, and load limit. Stay inside the datasheet ratings, add margin for cold starts and shock loads, and guard the rotating shaft and driven parts per OSHA machine-guarding rules.
Match your numbers to the right hydraulic motor
Your displacement result maps cleanly to a motor family. Here is how the math lands on the lines we keep in stock.
| Your result | Motor family | Where it fits | At Northern Hydraulics |
|---|---|---|---|
| Low speed, high torque (large displacement, under ~500 RPM) | Geroler / gerotor orbital | Augers, conveyors, wheel and winch drives | Char-Lynn H-Series & the Geroler torque-motor sub-category |
| Very high torque, heavy continuous load | Large orbital / gear motor | High-load drives, mixers, shredders | Dynamic Fluid Components BMK6-400-CC-G2-SF5 (RolorTorc gear design) |
| Moderate torque and speed, compact mount | Compact orbital (H-Series) | General-purpose drives and replacements | Char-Lynn 101-series motors |
For most low-speed, high-torque jobs, start in the Gerotor & Geroler torque motors group and confirm displacement against your calculator result. Replacing an existing unit? The Char-Lynn 101-1008-009 H-Series motor and the wider Char-Lynn motors listing cover the common H-Series footprints.
High-torque or obsolete unit? Lead with a quote. A heavy-frame motor like the Dynamic Fluid Components BMK6-400-CC-G2-SF5 is a bulk- and spec-match buy, not a one-click checkout — request a quote and an NH specialist will confirm displacement, ports, and shaft against your numbers. Cross-references are a fit guide based on published specs; verify bore, port, shaft, and pressure rating before ordering.
If it were our call
Size displacement for torque first, every time. Torque decides whether the machine moves at all, and it is set by displacement and pressure. Speed you can still tune later by changing flow. So we set the displacement to clear the torque at the pressure we have, then trim the pump flow to land the RPM. If the result sits between two stocked sizes, we round up — an oversized motor loafs, an undersized one overheats. When the unit is high-ticket, heavy, or a swap for something discontinued, skip the guesswork. Send the numbers through the Hydraulic Motors category or an RFQ, and a specialist confirms the fit. We have matched motors to machines since 1963, and five minutes on the phone beats a return.
Frequently Asked Questions (FAQ)
Quick answers to the sizing questions buyers ask most before choosing a hydraulic motor.
What size hydraulic motor do I need?
Work it backward from the load. Find the torque the machine needs, divide by your pressure to get displacement (torque = displacement × pressure ÷ 2π), then check that your flow gives an acceptable RPM at that displacement. If torque and speed pull in opposite directions, fix displacement to the torque and adjust flow for the speed. Stuck between sizes? Send the numbers and a parts specialist will confirm the unit from the Geroler torque-motor sub-category.
How do I calculate hydraulic motor torque?
Multiply displacement in cubic inches per revolution by pressure in PSI, then divide by 2π (about 6.28). That gives torque in inch-pounds on paper. A 9 in³/rev motor at 2,500 psi makes about 3,580 lb-in. Cut about 10% for friction — here roughly 3,200 lb-in — for the torque you get. Confirm the rating on the motor’s own datasheet, such as the Char-Lynn 101-1001 H-Series motor page.
How do I work out hydraulic motor speed in RPM?
Multiply your flow in GPM by 231, then divide by displacement in cubic inches per revolution. Ten GPM into a 9 in³/rev motor is about 257 RPM on paper; trim a little for leakage to get the real figure. Want more speed from the same motor, send more flow; want less, use a bigger size. The speed chart above plots this for small, medium, and large sizes across the hydraulic motors we stock.
How much horsepower does a hydraulic motor make?
Output horsepower is torque in inch-pounds times RPM, divided by 63,025. The example above — about 3,223 lb-in at 257 RPM — is roughly 13 HP at the shaft. That has to fit inside your circuit’s input power, which is GPM times PSI divided by 1,714, so 10 GPM at 2,500 psi supplies about 14.6 input HP. The gap between the two is your overall efficiency. Browse units by output in the Hydraulic Motors category.
Can I replace a motor with a different displacement?
Only if you keep the same torque and speed behavior. A different displacement changes both: a larger unit makes more torque but spins slower on the same flow, and a smaller one does the reverse. Match displacement, pressure rating, mount, and shaft to the original, or accept a known change in performance. Not sure what you have? Send a photo or part number and we will cross-reference it from the Char-Lynn motors line, or request a quote for the exact equivalent.
