RPM, Torque and Motor Power Calculator

This motor calculator converts rotational speed and torque into mechanical power, or calculates torque when power and RPM are known. Enter two of the three values—RPM, torque, and power—and use the matching formula below to solve for the third; the example calculator setup calculates power from RPM and torque.

Formula

For rotational motion, mechanical power equals torque multiplied by angular speed:

P = τ × ω = 2π × n × τ / 60

• P is mechanical power in watts (W).
• τ is torque in newton-metres (N·m).
• ω is angular speed in radians per second (rad/s).
• n is rotational speed in revolutions per minute (RPM).
• 2π/60 converts revolutions per minute into radians per second.

The calculator’s first result is watts. It also shows kilowatts and mechanical horsepower, using 1 kW = 1,000 W and 1 mechanical hp = 745.699872 W. To solve for torque instead, rearrange the same equation: τ = 60P / (2πn). To solve for RPM: n = 60P / (2πτ). Keep the units consistent: if power is in watts, torque must be in N·m for the result to be in RPM.

Worked Example

Using the calculator’s default values, speed is 1,750 RPM and torque is 10 N·m.

1. Convert RPM to radians per second: ω = 2π × 1,750 / 60 ≈ 183.26 rad/s.
2. Multiply by torque: P = 10 × 183.26 ≈ 1,832.6 W, which rounds to 1,833 W.
3. Convert to kilowatts: 1,832.6 / 1,000 = 1.833 kW.
4. Convert to mechanical horsepower: 1,832.6 / 745.699872 ≈ 2.46 hp.

Thus, at the entered operating point, 1,750 RPM and 10 N·m correspond to approximately 1.83 kW or 2.46 mechanical hp of mechanical power.

What RPM, Torque, and Power Mean

RPM describes how quickly a shaft rotates. Torque describes its turning effect: in SI units, one newton-metre is the torque produced by a one-newton force acting at a one-metre perpendicular distance from the axis. Power describes how quickly work is done or energy is transferred. The key relationship is that power depends on both torque and rotational speed, not either one alone.

This is why a motor can produce substantial torque at low speed without producing as much power as it would at higher speed with the same torque. Conversely, high RPM does not automatically mean high power if torque is small. Power and torque are related at a particular operating condition; they are not interchangeable descriptions of motor performance.

When to Use the Calculator

Use the calculation to check a motor or shaft operating point when you know its speed and torque, or to estimate the torque or speed implied by known mechanical power. It can help compare specifications stated in different units, understand a motor performance curve, or estimate the mechanical output needed by a driven machine. The calculation itself does not determine whether a motor can safely or continuously deliver that operating point.

For torque in pound-feet and speed in RPM, a common US customary form is:

Power (mechanical hp) = torque (lb·ft) × RPM / 5,252

For torque in pound-inches, use approximately hp = lb·in × RPM / 63,025. These constants are unit-conversion shortcuts for the same rotational-power equation, not different physical laws. Do not enter lb·ft as though it were N·m: 1 lb·ft is approximately 1.35582 N·m. Also distinguish mechanical horsepower from metric horsepower (often written PS); they are close but not identical.

Motor Ratings and Real Operating Conditions

The calculated number is mechanical power at the shaft, assuming the entered torque and RPM occur together. A motor’s nameplate power may be rated under specified conditions, while torque and speed vary with load, supply, control settings, and the motor’s design. For an induction motor, for example, synchronous speed inferred from supply frequency and pole count is not necessarily its actual loaded shaft speed. Use measured or specified operating RPM when you need an operating-point result.

Electrical input power is not the same as shaft output power. Some input energy becomes heat and other losses, so efficiency matters when estimating electrical demand. If efficiency is expressed as a decimal, an approximate relationship is electrical input power = mechanical output power / efficiency. The calculator does not estimate efficiency, current, voltage, starting performance, or energy consumption over time.

Common Errors and Limits

• Mixing units: Pair N·m with watts and RPM using the SI formula, or use the matching customary-unit formula. Convert before substituting if units differ.
• Combining unrelated values: Torque and RPM must describe the same shaft at the same moment or specified operating point. A peak torque figure and a speed from a different condition do not yield a meaningful rated power.
• Confusing power with energy: Watts measure power; watt-hours measure energy accumulated over time. This calculation does not tell you how much energy a motor uses during a task.
• Ignoring rounding: Displayed values are rounded. Use unrounded intermediate values for further calculations when precision matters, and remember that input measurements may be less precise than the displayed result.

At zero RPM, a shaft can have torque but the equation gives zero mechanical power because it is not rotating. The inverse formula for torque requires nonzero RPM, so do not use it at a stationary operating point. Real motors may also have limits on torque, speed, temperature, duty cycle, and mechanical loading; this simple equation does not verify those limits or select a motor for an application.

Frequently Asked Questions

How do I calculate motor power from RPM and torque?
For RPM and torque in N·m, calculate watts as 2π × RPM × torque / 60. Divide watts by 1,000 for kW or by 745.699872 for mechanical horsepower.

How do I calculate torque from motor power and RPM?
Use torque in N·m = 60 × power in watts / (2π × RPM). Convert the power to watts first, and RPM must be greater than zero.

Does higher RPM always mean more motor power?
No. Power rises with RPM only if torque stays the same. If torque falls as speed rises, the actual power depends on the product of torque and angular speed.

Why does my motor’s calculated power differ from its nameplate rating?
The torque and RPM may not describe the nameplate’s rated condition, or the stated power may refer to electrical input rather than mechanical shaft output. Check units, operating conditions, and the rating’s definition.

Can I use this formula for an electric motor?
Yes. The equation applies to rotational mechanical output from electric motors and other rotating machines. It calculates shaft power from torque and speed, not electrical input power or efficiency.

Related

• 1/4 Mile ET Calculator: Estimate Quarter-Mile Time
• Power RPM Calculator: Convert Torque and Speed
• Power-to-Weight Calculator: BHP per Tonne

in rpm | Rotational speed | rpm | 1750 | 0
in tq | Torque | N·m | 10 | 0
out power | Mechanical power | W | 2 * pi * rpm * tq / 60 | 0
out powerkw | Mechanical power | kW | power / 1000 | 3
out powerhp | Mechanical power | mechanical hp | power / 745.699872 | 2