Motor Torque Calculator

This calculator converts a motor’s rated mechanical power and speed into shaft torque, expressed in both lb-ft and N-m. It answers a single question: given the horsepower or kilowatt rating on a nameplate and the running speed in rpm, how much twisting force does the shaft deliver at that speed? This is a power-to-torque conversion, not a motor sizing, starting-torque, or gearbox reduction calculation, and it does not evaluate breakaway torque, locked-rotor torque, or torque curves across the acceleration ramp.

The result is used in mechanical load reviews adjacent to electrical work: checking coupling and belt ratings, verifying that a driven load’s required torque falls within the motor’s continuous output, or cross-checking a mechanical spec sheet against a nameplate before wiring a branch circuit. It is a supporting figure for equipment selection, not a substitute for the motor manufacturer’s torque-speed data.

Inputs

• Power unit – toggle between Horsepower and Kilowatts. This sets the constant used in the formula.
• Mechanical power – the motor’s rated output in the selected unit. In the example, this is 10 (horsepower).
• Speed – rotational speed in rpm. In the example, this is 1750 rpm, a common full-load speed for a 4-pole induction motor on a 60 Hz supply.

Formula

Torque is derived from the standard power-speed relationship. For horsepower input:

Torque (lb-ft) = (HP × 5,252) / RPM

For kilowatt input, the metric form applies:

Torque (N·m) = (kW × 9,549) / RPM

The constant 5,252 comes from converting horsepower (33,000 ft-lb/min) into a torque-speed relationship at a given rpm; 9,549 performs the equivalent conversion for kilowatts (where 1 kW = 1,000 N·m/s) into N·m at a given rpm. The calculator computes both the horsepower/lb-ft pair and the kilowatt/N-m pair regardless of which unit was entered, using the standard hp-to-kW conversion (1 hp = 0.746 kW) to populate the second unit set.

Calculation Example

With Mechanical power = 10 hp and Speed = 1750 rpm:

• Torque (lb-ft) = (10 × 5,252) / 1750 = 30.0114 lb-ft
• Power used = 10 hp, converted to 7.457 kW
• Torque (N·m) = (7.457 × 9,549) / 1750 = 40.69 N·m

Both torque values describe the same physical shaft output; they differ only in unit system. The lb-ft figure is typically referenced when comparing against U.S.-manufactured coupling, sheave, or gearbox torque ratings, while the N·m figure aligns with metric equipment documentation.

Reading the Result

A torque value by itself does not confirm that a motor is correctly matched to a load. It confirms only the torque a motor produces at its rated speed and rated power, which is its continuous, full-load torque. Two motors with identical horsepower but different rated speeds will produce different torque: a 10 hp motor at 1750 rpm delivers roughly 30 lb-ft, while the same 10 hp at 3450 rpm (2-pole) delivers closer to 15 lb-ft, because torque is inversely proportional to speed for a fixed power. When comparing this output against a driven load’s torque requirement, use the load’s torque at its actual operating speed, not at a different speed point on the motor’s curve.

Field and Calculation Limits

This calculator performs arithmetic only on the power-speed-torque relationship at one operating point. It does not account for:

• Starting torque or locked-rotor torque, which can be several times the full-load torque calculated here and is set by motor design class (NEMA A, B, C, D), not by this formula.
• Torque-speed curve behavior during acceleration, where torque varies continuously from zero speed to rated speed.
• Gearbox or belt-drive ratios, which multiply or reduce torque and speed at the driven shaft relative to the motor shaft.
• Service factor, efficiency losses, or derating for ambient temperature, altitude, or duty cycle.
• Electrical sizing, including branch-circuit conductor ampacity, overload protection, or full-load current — these are governed separately by the motor’s nameplate FLA, NEC Article 430 tables, and the motor’s service factor, not by shaft torque.

Verify the driven equipment’s actual torque requirement at its rated speed against the manufacturer’s motor torque-speed curve before finalizing coupling, drive, or mechanical protection selections. For electrical branch-circuit and feeder sizing, use the motor’s nameplate full-load current and applicable NEC Article 430 requirements rather than the torque figure produced here.