Torque and horsepower are both measures of mechanical performance, but they represent different things. Understanding how to convert between them—and knowing when each matters—is fundamental for motor selection, drivetrain design, and troubleshooting.
The Basic Relationship
Torque is rotational force. Horsepower is the rate of doing work. They're related by speed:
HP = (Torque × RPM) / 5252
Where torque is in lb-ft, RPM is rotational speed, and 5252 is the constant that makes the units work out. This is derived from the definition of horsepower (33,000 ft-lb per minute).
For metric units:
kW = (Torque × RPM) / 9549
Where torque is in Newton-meters.
Working Through Examples
Example 1: Finding Horsepower
An engine produces 250 lb-ft at 4000 RPM:
HP = (250 × 4000) / 5252 = 190 HP
Example 2: Finding Torque
A 5 HP motor runs at 1750 RPM. What torque does it produce?
Torque = (HP × 5252) / RPM = (5 × 5252) / 1750 = 15 lb-ft
Convert Torque and Power Instantly
Convert between HP, kW, lb-ft, and Nm at any RPM. Includes common motor and engine calculations.
Why 5252 RPM Matters
At exactly 5252 RPM, horsepower equals torque (in lb-ft). This is where torque and horsepower curves cross on an engine dyno chart. Below 5252 RPM, torque is numerically higher. Above 5252 RPM, horsepower is numerically higher.
Torque vs. Horsepower: When Each Matters
Torque Matters For:
- Acceleration from a stop
- Climbing hills and towing loads
- Starting heavy machinery
- Fastener tightening specifications
Horsepower Matters For:
- Top speed and high-RPM performance
- Sustained work output over time
- Power transmission system sizing
- Energy consumption calculations
Motor Selection Guidelines
When selecting a motor, consider both torque and speed requirements:
- High torque, low speed: Gear motor or reducer on standard motor
- Variable speed: VFD (Variable Frequency Drive) with inverter-duty motor
- High starting torque: Consider motor type (NEMA Design C or D)
| Motor Speed | Torque at 1 HP | Torque at 5 HP |
|---|---|---|
| 3600 RPM | 1.46 lb-ft | 7.3 lb-ft |
| 1800 RPM | 2.92 lb-ft | 14.6 lb-ft |
| 1200 RPM | 4.38 lb-ft | 21.9 lb-ft |
| 900 RPM | 5.83 lb-ft | 29.2 lb-ft |
Common Conversion Mistakes
- Forgetting RPM: You cannot convert torque to horsepower without knowing speed
- Mixing units: Use lb-ft with the 5252 constant, Nm with 9549
- Ignoring efficiency: Motor nameplate HP is output; input power is higher
- Peak vs. continuous: Peak values are brief; size for continuous requirements
Quick Reference
- 1 HP = 746 watts = 0.746 kW
- 1 kW = 1.341 HP
- 1 lb-ft = 1.356 Nm
- 1 Nm = 0.738 lb-ft
The torque-power relationship is one of the most practical calculations in mechanical work. Once you internalize the formula, you can quickly evaluate motor specifications, check drivetrain designs, and diagnose performance issues.
