Motor Horsepower From Torque Calculator

The Motor Horsepower From Torque Calculator converts measured or specified shaft Torque and rotational Speed into mechanical Power in horsepower and kilowatts.

This calculation is used when equipment data is expressed as torque and rpm rather than motor output. Common applications include reviewing a driven load, comparing a reducer or belt-drive requirement to a motor selection, checking whether a proposed motor has sufficient mechanical output, and documenting process-equipment demand. The result describes power at the shaft under the entered operating condition; it does not establish branch-circuit ampacity, feeder size, raceway fill, overload settings, or motor conductor requirements by itself.

Shaft Torque and Rotational Speed

Torque is the turning force delivered at a shaft. In this worksheet, Torque is entered in pound-feet (lb-ft). Speed is the shaft rotation rate, entered as Speed in revolutions per minute (rpm).

Mechanical power depends on both values:

• A shaft can produce high torque at low rpm with modest horsepower.
• The same torque at a higher rpm produces more horsepower.
• A motor, gearbox, sheave arrangement, or variable-frequency drive can change the relationship between torque and speed at different points in the mechanical system.

Use torque and speed from the same shaft location. For example, do not combine motor-shaft rpm with torque measured at a gearbox output shaft unless the gear ratio and mechanical losses have already been accounted for. A gearbox commonly reduces speed and increases output torque, while the mechanical power remains lower than the input power after losses.

Power Calculation

The calculator produces two values:

Result	Meaning
Power (hp)	Mechanical horsepower corresponding to the entered Torque and Speed
Power (kW)	The same mechanical power expressed in kilowatts

The horsepower calculation uses the standard mechanical relationship:

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

The kilowatt conversion is:

\(\displaystyle \text{kilowatts} = \text{horsepower} \times 0.745699872\)

The 5252 constant incorporates the conversion between foot-pounds per minute and mechanical horsepower. At 5,252 rpm, torque in lb-ft and mechanical horsepower have the same numeric value. For example, 10 lb-ft at 5,252 rpm equals 10 hp.

The calculator reports mechanical output power, not electrical input power. Actual electrical input to a motor will normally be higher than shaft output because motor efficiency and other losses must be considered separately.

Calculation Example

Enter the following operating values:

Input	Entered value
Torque	100 lb-ft
Speed	1,750 rpm

Calculate horsepower:

\(\displaystyle \text{hp} = \frac{100 \times 1750}{5252} = 33.3206 \text{hp}\)

Convert horsepower to kilowatts:

\(\displaystyle \text{kW} = 33.3206 \times 0.745699872 = 24.8472 \text{kW}\)

Result: 100 lb-ft at 1,750 rpm requires 33.3206 hp, or 24.8472 kW, of mechanical shaft power.

A motor selection based on this result must allow for the actual driven-load demand, expected operating speed, service conditions, starting requirements, duty cycle, and available motor ratings. A nominal 25 kW motor is not automatically an equivalent field selection merely because the calculated mechanical result is near 24.85 kW.

Motor and Drive Review

Torque-to-horsepower conversion is especially useful when reviewing driven equipment whose mechanical specifications are listed in torque, speed, or both. It can support planning for:

• Motor replacement when the original nameplate is missing or does not match the driven load.
• Gearmotor and reducer evaluation using output-shaft torque and output rpm.
• Belt-drive or chain-drive checks where speed changes between motor and driven equipment.
• VFD applications where motor speed varies and available torque may be limited by the motor, drive, control mode, or application.
• Mechanical load comparisons for pumps, fans, conveyors, compressors, mixers, and process equipment.

For a VFD-operated motor, calculate the required power at the actual operating speed and torque. Constant-torque and variable-torque loads do not behave the same way. A conveyor may require substantial torque at low speed, while a centrifugal fan or pump often follows a changing load curve as speed changes. The calculator converts the entered operating point; it does not model the entire load curve.

Electrical Design Boundary

The calculated hp or kW can guide later motor-circuit work, but it is not a substitute for electrical design inputs. Motor branch-circuit conductors, feeder conductors, disconnecting means, overcurrent protection, motor overload protection, and controller ratings require separate evaluation using the applicable electrical code, equipment instructions, motor data, and AHJ requirements.

Verify these conditions independently:

• Motor nameplate voltage, phase, full-load current, rated horsepower, rated speed, service factor, and duty rating.
• Motor efficiency, power factor, and actual input current.
• Starting current, locked-rotor conditions, acceleration time, and driven-load inertia.
• VFD output capability, carrier-frequency effects, cable length limits, motor insulation suitability, and control method.
• Branch-circuit and feeder ampacity after applicable adjustment factors, correction factors, terminal rating limits, and installation conditions are evaluated.
• Voltage drop under running and starting conditions, particularly on long branch circuits and feeders.
• Equipment manufacturer torque, speed, reducer, coupling, belt, sheave, and bearing limits.

Do not use calculated mechanical kW as direct electrical demand without confirming motor efficiency and operating load. Likewise, do not use calculated horsepower alone to select AWG or kcmil conductors, raceway fill, breaker size, fuse size, overload settings, or disconnect ratings.

Field Verification

Use actual shaft data whenever possible. Torque may come from a torque transducer, dynamometer test, equipment manufacturer data, calculated load demand, or a drivetrain specification. Speed may be obtained from a tachometer, encoder, VFD display, controller feedback, or verified sheave and pulley ratios.

Confirm that Torque is in lb-ft and Speed is in rpm before using the result. Torque values in lb-in, N·m, or other units require conversion before entry. A unit mismatch can create a large horsepower error even when the formula is applied correctly.

The calculator performs the torque-to-power conversion only. Verify the motor nameplate, actual speed, motor efficiency, duty, drive characteristics, and manufacturer requirements before making an equipment or electrical installation decision.