Motor Torque From Power Calculator

A motor torque from power calculation converts mechanical horsepower and rotational speed into shaft torque. The result identifies how much turning force is available at the motor shaft at the entered speed.

For motor-driven equipment, torque is used alongside horsepower, RPM, and load characteristics to review whether a motor can turn the connected machine. It is particularly useful when comparing a motor nameplate to a pump, fan, conveyor, gearbox, compressor, mixer, or other rotating load specification.

Torque does not size branch-circuit conductors, feeders, overcurrent protection, or raceway fill directly. Those electrical design decisions require motor current, voltage, wiring method, terminal rating, insulation temperature rating, ampacity, adjustment factor, correction factor, and applicable code requirements. Torque is instead a mechanical output value that helps establish the load the electrical system must serve.

Power and Speed Inputs

The calculator uses two inputs:

Input	Field Purpose	Unit
Power	Mechanical power delivered by the motor shaft	hp
Speed	Shaft rotational speed at the operating condition	rpm

Power is entered in horsepower. For this calculation, horsepower is mechanical shaft horsepower, not electrical input power. Motor electrical input can exceed shaft output because real motors have losses and operate at less than 100% efficiency.

Speed is entered in revolutions per minute. A motor described as 1,750 rpm is commonly associated with a nominal 1,800 rpm synchronous-speed family, but the actual rated speed must come from the motor nameplate or manufacturer data. Shaft speed changes the calculated torque directly: at the same horsepower, lower RPM produces higher torque, while higher RPM produces lower torque.

The calculator applies the entered values exactly as provided. It does not determine motor slip, full-load current, service factor, starting torque, breakdown torque, gearbox ratio, load inertia, or drive efficiency.

Torque Calculation

The calculator returns torque in both lb-ft and N-m.

\(\displaystyle \text{Torque lb-ft} = \frac{5252 \times \text{horsepower}}{\text{speed rpm}}\)

\(\displaystyle \text{Torque N-m} = \text{Torque lb-ft} \times 1.3558179483314\)

The constant 5252 converts the relationship between horsepower, rotational speed, and pound-feet of torque.

Torque and horsepower describe different aspects of motor output:

• Horsepower describes the rate of mechanical work.
• Torque describes rotational force at the shaft.
• RPM describes how fast the shaft rotates.

A motor can produce the same horsepower at different speeds, but the torque will not be the same. For a fixed horsepower value, reducing shaft speed increases torque proportionally.

Calculation Example

Enter the following values:

Field	Entered Value
Power	20 hp
Speed	1750 rpm

\(\displaystyle \text{Torque lb-ft} = \frac{5252 \times 20}{1750}\)

\(\displaystyle \text{Torque lb-ft} = 60.0229\ \text{lb-ft}\)

\(\displaystyle \text{Torque N-m} = 60.0229 \times 1.3558179483314\)

\(\displaystyle \text{Torque N-m} = 81.3801\ \text{N-m}\)

The calculated motor shaft output is 60.0229 lb-ft, or 81.3801 N-m, at 1,750 rpm and 20 hp.

That value can be compared with equipment torque requirements only when both values refer to the same shaft location and operating condition. A gearbox, belt drive, chain drive, coupling, or variable-frequency drive can change the torque and speed delivered to the driven equipment.

Motor and Drive Coordination

Motor torque is commonly reviewed during equipment selection and motor-control planning.

A pump or fan may be specified by required shaft horsepower at a given speed. A conveyor or mixer may be characterized by running torque and starting torque. A gearbox may list its allowable input torque, output torque, speed range, and service factor. The torque calculation provides a common mechanical value for comparing these requirements.

For variable-speed applications, torque behavior depends on the motor and drive system. A VFD can change motor speed, but the resulting torque capability depends on the motor design, drive programming, cooling method, load type, and operating frequency. A constant-torque conveyor load and a variable-torque centrifugal fan load should not be evaluated as if they impose the same mechanical demand.

The calculated shaft torque also does not establish starting performance. A motor may have adequate running horsepower while still requiring sufficient locked-rotor, pull-up, or breakdown torque to accelerate a high-inertia or heavily loaded machine.

Electrical Design Boundary

Use the torque result as a mechanical conversion value, then perform electrical design from verified motor and equipment data.

Motor branch-circuit conductor ampacity, feeder sizing, disconnect rating, overload protection, short-circuit and ground-fault protection, voltage-drop review, and raceway fill are based on the applicable electrical design method and verified equipment information—not on calculated shaft torque alone.

Confirm the following separately:

• Motor nameplate voltage, full-load current, horsepower, speed, phase, and duty.
• Actual driven-equipment torque and speed requirement.
• Motor efficiency, service factor, thermal rating, enclosure, and cooling arrangement.
• Starting method, VFD settings, acceleration requirements, and load inertia.
• Gearbox ratio, belt or chain losses, coupling limits, and manufacturer torque ratings.
• Branch-circuit and feeder conductor ampacity after applicable terminal rating, insulation temperature rating, correction factor, and adjustment factor review.
• Voltage drop under starting and running conditions where conductor length or motor starting current makes it relevant.
• AHJ, equipment manufacturer, and project-specific requirements.

The calculation is a direct horsepower-to-torque conversion. Verify the motor nameplate, actual operating speed, duty, drive arrangement, and manufacturer data before using the result for equipment selection or electrical system decisions.