Torque Calculator

Torque calculator for motors and rotating shafts using power and RPM

This torque calculator is built for the most common real-world question in rotating equipment: “If I know the power and the RPM, what torque is on the shaft?” That shows up in motor selection, gearbox checks, belt and chain drives, couplings, mixers, pumps, fans, and almost anything with a rotating shaft.

You enter the load power in kilowatts (kW) and the rotational speed in revolutions per minute (RPM). The calculator then estimates the load torque in newton-metres (N·m). It also converts that torque to pound-feet (lb·ft) because many catalogues and datasheets still use imperial torque units.

Torque is the “twisting force” available at the shaft. Power is how fast that torque is being delivered over time. Speed links the two. For the same power, lower RPM means higher torque. For the same RPM, higher power means higher torque. This is why gearboxes exist: they trade speed for torque (and vice versa), with some losses.

The optional drivetrain efficiency input is there for one practical reason: you might know the power required at the load (for example, what the process needs), but you want a better estimate of what the motor must deliver. If you enter an efficiency below 100%, the calculator estimates the motor torque required to provide the same load power at the same RPM, accounting for losses.

The results are intentionally focused. You get the main torque figure first, then two supporting figures that help decisions: the same torque in lb·ft, and an equivalent force-at-radius reference (what force would create the same torque at a 1 m radius). That last line is useful as a quick sanity check, because it turns an abstract torque number into a more intuitive push or pull.

Assumptions and how to use this calculator

• Power input is treated as load (shaft) power in kW, not electrical input power to the motor.
• RPM is assumed to be the shaft speed at the point where you want torque (after any gearbox, if applicable).
• If you leave efficiency blank, the calculator assumes 100% (no losses) and motor torque equals load torque.
• Efficiency is applied as a simple scalar loss model (motor power = load power ÷ efficiency). It does not model speed slip, duty cycles, or dynamic transients.
• This is a steady-state estimate. Starting torque, peak torque, shock loads, and pulsating torque are not included.

Common questions

Is this the right torque calculator for force at a lever arm?

No. This page is locked to torque from power and RPM for rotating machinery. If your problem is “force times distance” (like tightening a bolt with a wrench or a lever), you need a lever torque calculator. Different inputs, different intent.

What power should I enter if I only know motor nameplate power?

Nameplate kW is usually the motor’s rated mechanical output at the shaft under specified conditions, not the electrical input. If your system has a gearbox, belt drive, or other losses, the load sees less than the motor shaft output. Use the load power if you know it. If not, you can start with motor rated kW and add a conservative efficiency in the calculator to estimate required motor torque for the load.

Why does torque go up when RPM goes down?

Because power depends on both torque and rotational speed. Delivering the same power more slowly requires more twist per revolution. That is the core relationship a gearbox exploits: reduce speed to increase torque (minus losses).

What efficiency value should I use if I do not know it?

Use 100% for a quick baseline, then refine. Typical combined drivetrain efficiencies vary by system. A single gearbox can be high, while multiple stages, belts, and couplings reduce it. If you are unsure, use a conservative estimate (for example, 90–95%) and compare the difference in required motor torque.

When will this calculator be misleading?

It can be misleading for applications dominated by transient conditions: high starting torque (inertia), frequent acceleration and braking, impact loading, or strongly pulsating torque. In those cases, you need a torque-time profile and equipment-specific service factors, not just a steady-state estimate.