Power from Torque and RPM Calculator

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Power from Torque and RPM Calculator + Formula, Examples & Applications

If you already know your motor's torque and speed, calculating real mechanical power is the next step before choosing a gearmotor, sizing your power supply, or building out a drivetrain. This calculator lets you quickly convert torque and RPM into shaft power, outputting watts, kilowatts, or horsepower. You can use Nm, oz-in, lb-ft, or lb-in—just match it to your datasheet. Below are the basic equations, some worked examples, conversion details, and useful engineering notes for typical actuator and motor tasks.

What Is Power from Torque and RPM?

Mechanical power is simply how quickly a rotating shaft can do work. With measured torque and RPM, you can figure out the output power in plain units.

Simple Explanation

If you’ve ever used a wrench, torque is the twisting force and RPM is how fast you turn it. Actual power combines these—a strong torque with zero speed gives zero power, as does spinning with no load. Power only happens when you have both torque and speed at the same time.

N (RPM) T (Nm) P (W) = T (Nm) × N (RPM) × 2π / 60 Torque Unit Conversion Factors Nm oz-in lb-ft lb-in ×1 ×0.007062 ×1.35582 ×0.11298

Power from Torque and RPM Calculator

Select unit below.
Motor or shaft speed in revolutions per minute.
Engineering calculation notice

This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.

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Power from Torque and RPM Interactive Visualizer

You can see directly how torque and RPM work together for mechanical power. Try different settings to visualize how the power changes—helpful when comparing actuators or gear motors for real design choices.

Torque (Nm) 2.0 Nm
Speed (RPM) 100 RPM

POWER (WATTS)

20.9 W

KILOWATTS

0.021 kW

HORSEPOWER

0.028 HP

FIRGELLI Automations — Interactive Engineering Calculators

🎥 Video — Power from Torque and RPM Calculator

Power from Torque and RPM Calculator

How to Use This Calculator

The calculator simply asks for torque and speed, and shows the output in three units. Steps are:

  1. Enter your torque value. Use the number from your datasheet, a test, or your sizing calculation.
  2. Select the torque unit. Pick Nm, oz-in, lb-ft, or lb-in, to match what you have. It auto-converts internally.
  3. Input the RPM (speed). Use the actual shaft speed or use the gearbox output RPM, as required for your design.
  4. Click "Calculate". You'll see results immediately in watts, kilowatts, and horsepower.
  5. Use "Try Example" for a basic 2 Nm / 100 RPM case, or "Clear" to empty the inputs.

Power from Torque and RPM Formula

The relationship between torque, speed, and power is direct: convert your input torque to newton-meters, then apply the equation.

Step 1 — Convert torque to Nm:
torqueNm = torqueValue × conversionFactor
Step 2 — Calculate power in watts:
P (W) = torqueNm × RPM × 2π / 60
Step 3 — Convert to kW and HP:
P (kW) = P (W) / 1000
P (HP) = P (W) / 745.7
Symbol Variable Unit
T Torque Nm (or oz-in, lb-ft, lb-in)
N Rotational Speed RPM
P Mechanical Power W, kW, or HP
2π / 60 RPM to rad/s conversion ≈ 0.10472

These are the conversion factors for common torque units. Multiply by the factor to get newton-meters:

Input Unit × Factor = Nm
Nm 1
oz-in 0.007062
lb-ft 1.35582
lb-in 0.11298

Simple Example

Scenario: You have a 12V DC gearmotor rated at 2 Nm of torque running at 100 RPM. How much mechanical power does it deliver?

Step 1 — Convert torque: The input is already in Nm, so the conversion factor is 1. torqueNm = 2 × 1 = 2 Nm.

Step 2 — Calculate power in watts:
P = 2 × 100 × 2π / 60
P = 2 × 100 × 0.10472
P = 20.94 W

Step 3 — Convert:
P = 20.94 / 1000 = 0.0209 kW
P = 20.94 / 745.7 = 0.0281 HP

What this means: This motor delivers about 21 watts of mechanical shaft power — typical for a light-duty linear actuator application like opening a vent, adjusting a monitor arm, or actuating a small hatch.

Engineering Applications

Power Is the Product of Torque and Speed

This isn't just theory—it's the first calculation for almost any motor-driven system. High RPM and low torque can create the same output power as low RPM and high torque. Gearboxes are built for this trade. Gearing down lets you turn a fast, weak motor into a slower, higher-torque drive, but the maximum power out (neglecting friction losses) stays similar. This is how you can often use a smaller, cheaper motor if you understand the real trade-off.

Where Do 12V DC Gearmotors Sit?

Most linear actuator motors in the 12V range sit in the 50–300 RPM bracket (after gearing). That usually means 5–60 watts at the shaft. So, for things like desk lifts, TV risers, or solar trackers, you're almost always in that window. If you need a lot more than 60 W, you're probably better off looking at bigger actuators or higher voltage motors.

Horsepower vs. Watts

1 HP is 745.7 W. You'll still see HP ratings in North America, even on electric motors, though it's rarely used for small actuators. For linear actuators, watts are the norm. If you need to convert between the two (for spec comparisons), this calculator already does that.

A Practical Benchmark

For quick reference, 3 Nm at 80 RPM means roughly 25 W mechanical power—enough for most light actuator jobs. If your design comes out under 10 W, you’re looking at micro-actuators. If you’re above 100 W, you’re in heavy-duty territory and need to pay extra attention to wiring and supply.

This Is Where Motor Selection Starts

Everything begins with a power requirement. This calculation just gives the mechanical output. Real motors aren’t 100% efficient—expect 20–40% losses in small, geared DC motors. Account for those losses, and add some safety margin, before locking in the final motor spec. If you size to run a motor continuously at its full output, you’ll usually shorten its reliable lifespan.

A Note on oz-in

Oz-in pops up everywhere on North American datasheets for small DC motors. For easy conversion, 1 Nm ≈ 142 oz-in. So, 200 oz-in is about 1.41 Nm. Not much torque, but it could be significant if the RPM is high. Use Nm for calculations whenever possible, or just plug your oz-in into this calculator.

Advanced Example

Scenario: You're designing a solar panel tracker that uses a 24V DC gearmotor. The motor datasheet lists a stall torque of 350 oz-in, but you know you'll operate it at roughly 50% of stall — call it 175 oz-in. The output shaft speed after gearing is 60 RPM. How much power does the motor deliver at this operating point, and is it reasonable for the application?

Step 1 — Convert torque to Nm:
torqueNm = 175 × 0.007062 = 1.236 Nm

Step 2 — Calculate power in watts:
P = 1.236 × 60 × 2π / 60
P = 1.236 × 60 × 0.10472
P = 1.236 × 6.2832
P = 7.77 W

Step 3 — Convert:
P = 7.77 / 1000 = 0.00777 kW
P = 7.77 / 745.7 = 0.0104 HP

Design interpretation: At roughly 7.8 W, this falls into the micro/light-duty actuator range. That’s more than enough for most solar tracker jobs, which run at low duty cycles. 175 oz-in at 60 RPM will give steady and controlled movement. If the motor draws 0.8 A from 24V, that's 19.2 W electrical input, so you're looking at about 40% efficiency—normal for small gear motors. The numbers add up for the intended use.

Frequently Asked Questions

Does this calculate electrical power or mechanical power? +

This calculates mechanical shaft power — the actual useful work output at the motor shaft. Electrical input power (volts × amps) will always be higher because no motor is 100% efficient. To find electrical power requirements, divide the mechanical power result by your motor's efficiency (typically 0.4–0.7 for small geared DC motors).

Should I use stall torque or rated torque in this calculator? +

Use the torque at your actual operating speed — not stall torque. At stall, RPM is 0 and power is 0. Most motors produce peak power at roughly 50% of no-load speed. Check your motor's torque-speed curve for the most accurate number at your expected operating point.

How do I convert oz-in to Nm in my head? +

Divide oz-in by roughly 142 to get Nm. So 142 oz-in ≈ 1 Nm, 284 oz-in ≈ 2 Nm, and so on. It's not precise (the exact factor is 141.6), but it's close enough for quick estimates before you reach for the calculator.

Can I use this for a linear actuator instead of a rotating motor? +

Yes — but you'd use the internal gearmotor's torque and RPM, not the actuator's linear force and speed. A linear actuator converts rotary motion to linear motion via a lead screw, so the torque-RPM relationship applies to the motor inside. If you already know the linear force and speed, use P = Force × Velocity instead for a more direct calculation.

Why does my motor draw more electrical power than this calculator shows? +

Because this calculator outputs mechanical power, not electrical. Every motor loses energy to friction, winding resistance, and magnetic losses. A typical small geared DC motor is 30–60% efficient, so the electrical input will be significantly higher than the mechanical output. Always size your power supply based on electrical draw, not mechanical output.

Does gearing change the power output? +

In theory, no — gearing trades speed for torque while preserving power. In practice, every gear stage introduces friction losses of roughly 5–15%. So the output shaft of a geared motor delivers slightly less power than the motor itself produces. Use the output shaft torque and RPM for the most accurate real-world power figure.

What RPM should I use if my motor has a variable speed controller? +

Use the RPM at the specific operating point you're designing for — typically the speed under your expected load. If you need to evaluate multiple operating points, run the calculator at each speed and compare. Peak power usually occurs at a specific torque-speed combination, not at maximum RPM.

About the Author

Robbie Dickson — Chief Engineer & Founder, FIRGELLI Automations

Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.

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