Power from Torque and RPM Calculator

Posted by Robbie Dickson on

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

You know the torque and speed of your motor — but how much power does it actually deliver? That's the question you need to answer before selecting a gearmotor, sizing a power supply, or designing any drivetrain. This calculator converts torque and RPM into mechanical shaft power in watts, kilowatts, and horsepower. It supports Nm, oz-in, lb-ft, and lb-in — so you can work in whatever units your motor datasheet provides. Below you'll find the formula, worked examples, unit conversions, and practical guidance for actuator and motor applications.

What Is Power from Torque and RPM?

Mechanical power is the rate at which a rotating shaft does work. If you know how hard it twists (torque) and how fast it spins (RPM), you can calculate exactly how many watts it produces.

Simple Explanation

Think of torque as the "push" on a wrench and RPM as how fast you're spinning it. Power combines both — it tells you how much work the shaft does every second. A motor with huge torque but zero speed produces zero power. Same goes for a motor spinning fast with no load. You need both torque and speed together to get useful work out of any rotating system.

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.

🎥 Video — Power from Torque and RPM Calculator

How to Use This Calculator

This calculator takes your motor's torque and speed and gives you shaft power in 3 useful units. Here's how to use it:

  1. Enter your torque value. Pull this number from your motor datasheet, test bench reading, or design requirement.
  2. Select the torque unit. Choose Nm, oz-in, lb-ft, or lb-in — whatever matches your source data. The calculator handles the conversion automatically.
  3. Enter the rotational speed in RPM. This is the motor shaft speed or the output speed after any gearing. Make sure you use the correct value for your point of interest.
  4. Click "Calculate." You'll get power in watts, kilowatts, and horsepower — all 3 displayed at once.
  5. Use "Try Example" to see a worked calculation with 2 Nm at 100 RPM. Hit "Clear" to reset everything and start fresh.

Power from Torque and RPM Formula

The core relationship between torque, speed, and power is straightforward. First, convert your torque into newton-meters using the appropriate conversion factor, then apply the power 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

And here are the torque unit conversion factors — multiply your input value by the factor below to get Nm:

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 is the fundamental relationship that drives every motor selection decision. Power equals torque multiplied by angular speed. That means higher RPM with lower torque can produce the same power as low RPM with high torque. A small, fast motor through a gear reduction can match the output power of a larger, slower motor — the gearing trades speed for torque while keeping power roughly constant (minus friction losses). Understanding this trade-off is how you right-size a motor instead of over-specifying one.

Where Do 12V DC Gearmotors Sit?

Most 12V DC motors used in linear actuators run between 50 and 300 RPM after gearing. That puts the typical output power of FIRGELLI actuator gearmotors in the 5–60 W range. That's plenty for applications like sit-stand desks, TV lifts, solar tracker adjustments, agricultural equipment, and industrial access panels. If your calculation shows you need more than 60 W, you're likely looking at a higher-voltage motor or a different actuator class entirely.

Horsepower vs. Watts

1 horsepower equals 745.7 watts. North American motor catalogs still widely use HP ratings, especially for AC motors and larger DC motors. For small actuator applications, watts make more sense — you'll rarely see a linear actuator rated in horsepower. But if you're comparing a gearmotor against an HP-rated specification, this calculator handles that conversion instantly.

A Practical Benchmark

A gearmotor producing 3 Nm at 80 RPM delivers about 25 W of mechanical power. That's enough for most light-duty FIRGELLI actuator applications. It's a good mental benchmark — if your numbers come in near that range, you're in well-proven territory for 12V and 24V actuator systems. Go much below 10 W and you're looking at micro-actuator territory. Go above 100 W and you're entering heavy-duty industrial territory that demands more robust power supply and wiring considerations.

This Is Where Motor Selection Starts

Before you pick a motor or actuator, you need to know your power requirement. This calculator gives you that starting number. From here, you'll factor in efficiency losses (typically 20–40% for a geared DC motor), duty cycle, and ambient temperature to arrive at the motor rating you actually need. Always select a motor with some overhead — running a motor at 100% rated power continuously shortens its life dramatically.

A Note on oz-in

The oz-in (ounce-inch) unit appears constantly on small motor datasheets in North America. It's a common source of confusion. For reference, 1 Nm equals approximately 141.6 oz-in. So if your motor datasheet says 200 oz-in, that's about 1.41 Nm. Not a lot of torque — but at high RPM, it can still produce meaningful power. Always convert to Nm first if you're doing power calculations, or just let this calculator handle it.

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 of mechanical output, this motor sits comfortably in the micro to light-duty range. For a solar tracker that only adjusts position a few times per hour, this is more than adequate. The low duty cycle means the motor won't overheat, and 175 oz-in at 60 RPM provides smooth, controlled movement. If you're running this motor from a 24V supply drawing 0.8 A, that's 19.2 W of electrical input — giving you a motor efficiency of about 40%, which is typical for small geared DC motors. Everything checks out.

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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