Match the torque to its RPM
With torque in Nm, multiply by RPM and 2π/60 to obtain watts. Divide by 1,000 for kilowatts or by 745.6998716 for mechanical horsepower. A peak torque number paired with the RPM of peak power usually describes two different operating points and cannot be used as one measurement.
P(W) = T(N·m) × 2π × RPM / 60Use torque and RPM from the same operating point.
Work through an example
400 lbf·ft at about 5,252.11 rpm produces 400 mechanical hp. The familiar numeric crossover applies to these units only. In Nm and kW, the numeric crossover occurs at another RPM because the unit scales differ.
- Torque
- 400 lbf·ft
- Engine speed
- ≈ 5,252.11 rpm
- Mechanical power
- ≈ 400 hp
Use torque and RPM from the same operating point
Read a torque curve at the RPM being evaluated, then use P = torque × angular speed. Combining peak torque with the RPM of peak power usually mixes two points. A complete curve gives a more useful picture than either peak number alone.
Keep mechanical horsepower and SI units distinct
With torque in newton-meters and speed in rpm, watts equal torque × RPM × 2π/60. Convert watts to kW or mechanical hp after this step. The familiar 5,252 crossover is a numeric consequence of lbf·ft and mechanical horsepower units.
Build a torque and power operating-point table
Hold torque at 200 N·m and vary shaft RPM. This intentionally constant-torque example illustrates the power relationship; it is not a dynamometer curve for a real engine. A real curve uses the torque actually measured at each listed RPM.
P(W) = T(N·m) × RPM × 2π/60- Choose a torque value and the RPM at which that torque applies.
- Convert lbf·ft to N·m if using the SI power equation.
- Multiply torque by RPM × 2π/60 to obtain watts.
- Divide watts by 1,000 for kW or by 745.6998716 for mechanical hp, keeping the measurement location identified.
| Shaft RPM | Torque (N·m) | Power (kW) | Mechanical hp |
|---|---|---|---|
| 2000 | 200 | 41.888 | 56.17 |
| 4000 | 200 | 83.776 | 112.35 |
| 6000 | 200 | 125.664 | 168.52 |
With torque held constant, doubling shaft speed doubles power. This follows from doing the same rotational work per revolution twice as often. In an actual engine, torque may rise or fall with RPM, so power does not necessarily continue along this straight proportional relationship.
Check the inputs before interpreting a difference
Peak torque and peak horsepower often refer to different operating points. Do not pair the peak torque magnitude with the RPM at which peak power occurs unless the torque curve supports that combination.
Before you apply the result
Engine power and wheel power use the same physical definition but refer to different measurement locations. Applying a selected loss percentage produces a scenario, not a measured drivetrain efficiency or a substitute for a dyno test.
Measurement point
Label whether power is measured at the engine, hub or wheel.
Loss assumptions
A selected loss percentage is a scenario, not a measured efficiency.
Units
Keep N·m, lbf·ft, W and mechanical horsepower distinct.
Operating point
Match the torque value to the RPM at which it occurs.
Try the calculation
Enter your own values in the related calculators. Compare one input at a time and keep units and operating conditions visible with the result.
Frequently asked questions
Why do torque and horsepower cross near 5,252 RPM?
When torque is in lbf·ft and power in mechanical hp, their numerical conversion factor makes the values equal near 5,252.11 rpm. The crossover is unit-dependent.
Can high torque at zero RPM produce shaft power?
At zero angular speed, torque multiplied by angular speed is zero shaft power, even when the applied torque is nonzero.
Does a loss percentage measure wheel horsepower?
No. Applying a selected loss creates a scenario. Measured wheel power requires an appropriate measurement at that location.
Why can horsepower rise after engine torque starts falling?
Power is torque multiplied by angular speed. If RPM increases proportionally faster than torque decreases, their product can still rise. Use values from the same operating point to calculate it.
Reference and method
Power and rate of work; mechanical horsepower conversion follows 550 ft·lbf/s.
NASA Glenn — What is power? ↗These explanations and examples describe the documented model. No manufacturer testing, named expert authorship or independent professional review is claimed. Read the method policy or report a correction.