Motor Torque Calculator

The Motor Torque Calculator computes the mechanical torque produced at a motor shaft given the output power and rotational speed. Torque is the rotational force that drives mechanical loads such as pumps, fans, conveyors, compressors, and machine tools, making this calculation central to motor sizing and mechanical system design.

Torque and power are intimately related through rotational speed. The relationship is T = P/ω, where T is torque in newton-meters, P is power in watts, and ω is angular velocity in radians per second. In practical engineering, this is expressed as T (N·m) = P (kW) × 9549 / N (RPM), where the constant 9549 combines the unit conversions (1000 W/kW × 60 s/min / 2π).

Understanding torque is critical for several reasons. First, the mechanical coupling between motor and load must be rated for peak torque, not just rated torque. Starting torque for induction motors can be 150-300% of rated torque. Second, gearbox selection depends on input torque — a gear reducer multiplies torque by the gear ratio while reducing speed proportionally. Third, shaft and keyway design must withstand both steady torque and dynamic torque spikes.

The relationship between torque, power, and speed is also key to understanding why lower-speed motors produce more torque for the same power. A 10 kW motor at 500 RPM produces twice the torque of a 10 kW motor at 1000 RPM. This is why high-torque, low-speed applications (mixers, crushers, hoists) favor direct-drive motors with many poles or geared solutions.

In variable speed drive applications, torque remains roughly constant through the constant-torque speed range (below base speed with V/Hz control), while power increases linearly with speed. Above base speed in the field-weakening region, torque decreases as speed increases while power remains constant. This torque-speed envelope defines the operating region for VFD-driven motors.

The imperial unit for torque (foot-pounds) remains common in US industry, particularly for engine and gearbox ratings. The conversion factor is 1 N·m = 0.7376 ft·lb. This calculator provides both SI and imperial values for cross-referencing with equipment datasheets.

Motor torque can also be verified against current draw using the equation T ≈ k × I_q in vector-controlled drives, where I_q is the torque-producing current component. Modern drive systems measure and display torque in real time through this current-based estimation, enabling precise process control without a physical torque sensor.