Motor Torque Calculator

About the Motor Torque Calculator

This motor torque calculator converts horsepower and RPM into motor shaft torque, in either imperial (lb-ft) or metric (Nm) units. Torque determines whether a motor can start and drive a given load — critical when sizing motors for conveyors, pumps, compressors, machine tool spindles, and any application where load varies with speed.

The calculator uses the standard NEMA constants (5252 for imperial, 9550 for metric) to give full-load running torque. Starting torque is a separate value that depends on the motor’s NEMA design code (B, C, or D).

How motor torque is calculated

The relationship between power, torque, and rotational speed:

Imperial: Torque (lb-ft) = (HP × 5252) ÷ RPM
Metric: Torque (Nm) = (kW × 9550) ÷ RPM

The constant 5252 comes from the definition of horsepower: 33,000 ft-lb/min divided by 2π radians per revolution. The metric equivalent (9550) comes from kW × 60 seconds × 1000 watts divided by 2π. These formulas give continuous full-load torque at the nameplate operating point — instantaneous starting torque is significantly higher.

Worked example

A 5 HP motor at 1,750 RPM produces (5 × 5252) ÷ 1750 = 15.0 lb-ft of full-load running torque. The metric equivalent: a 3.7 kW motor at 1,450 RPM produces (3.7 × 9550) ÷ 1450 = 24.4 Nm. To convert between the two: 1 lb-ft ≈ 1.356 Nm.

Starting torque vs running torque

Full-load running torque (what this calculator gives) is the torque the motor produces continuously at nameplate HP and RPM. Starting torque (also called locked-rotor torque) is much higher and depends on the NEMA design code printed on the nameplate:

• NEMA Design A: starting torque 100–200% of full-load (general purpose, low slip)
• NEMA Design B: starting torque 150–170% of full-load (most common general-purpose motors)
• NEMA Design C: starting torque 200–225% of full-load (high-inertia loads like conveyors, compressors)
• NEMA Design D: starting torque 275% of full-load (oil-well pumps, punch presses, hoists)

When to use this calculator

• Selecting a motor for a conveyor belt with known load and speed requirements
• Verifying that a replacement motor delivers the required starting torque
• Sizing gearmotors for slow-speed high-torque applications
• Calculating shaft torque on machine tool spindles for cutting force analysis
• Designing pulley/belt systems with correct torque transmission ratios
• Specifying motor couplers and shaft keyways with adequate strength
• Estimating drive train losses when comparing direct-drive vs gearbox configurations

Frequently Asked Questions

How do you calculate motor torque from HP and RPM?

Multiply horsepower by 5252, then divide by RPM. For a 5 HP motor running at 1,750 RPM: Torque = (5 × 5252) ÷ 1750 = 15.0 lb-ft. The metric formula uses 9550 instead of 5252 and gives the result in Newton-meters (Nm).

What is the metric formula for motor torque?

Torque (Nm) = (kW × 9550) ÷ RPM. For a 3.7 kW motor at 1,450 RPM: Torque = (3.7 × 9550) ÷ 1450 = 24.4 Nm. Multiply Nm by 0.7376 to convert back to lb-ft.

Where does the constant 5252 come from?

It’s derived from the definition of horsepower (33,000 ft-lb/min) divided by 2π radians per revolution: 33,000 ÷ (2 × 3.1416) ≈ 5252. Notice that at exactly 5,252 RPM, torque in lb-ft equals horsepower — that’s the speed where the two curves cross on a motor performance chart.

Is this starting torque or running torque?

This calculator gives full-load running torque at the nameplate operating point. Starting torque is higher and varies by NEMA design code: B = 150–170% of full-load, C = 200–225%, D = 275%.

How do I convert lb-ft to Newton-meters?

Multiply lb-ft by 1.356 to get Nm. For 100 lb-ft: 100 × 1.356 = 135.6 Nm. For the reverse (Nm to lb-ft), multiply by 0.7376.

What torque does my motor produce at variable speed (with a VFD)?

For a constant-torque load (conveyor, compressor): torque stays roughly constant from 0 to base speed, then drops as speed increases above base. For a variable-torque load (pump, fan): torque drops with the square of speed reduction. Always size the motor for full torque at the lowest operating speed.

What is breakdown torque?

Breakdown torque is the maximum torque a motor can develop without stalling. For NEMA Design B motors, it’s typically 200–250% of full-load torque. If load torque exceeds breakdown torque, the motor will stall and overload protection should trip within seconds.

How to Calculate Torque from Horsepower

How do you calculate torque from horsepower?

Multiply horsepower by 5,252 and divide by RPM: Torque (lb-ft) = (HP × 5252) ÷ RPM. For example, a 5 HP motor at 1,750 RPM produces (5 × 5252) ÷ 1750 = 15.0 lb-ft. In metric units, Torque (Nm) = (kW × 9550) ÷ RPM.

Where does the 5,252 constant come from?

It is the result of 33,000 ft-lb/min per horsepower divided by 2π (one revolution). At exactly 5,252 RPM, torque in lb-ft equals horsepower — that is why the HP and torque curves always cross at 5,252 RPM.

Motor Torque Quick Reference

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