Enrichment · STEM Explorations · Physics

Torque, speed and
horsepower are one
equation apart.

Type two of torque, speed and power in the units you have. The converter works out the third and shows all three in every common unit.

ConvertWhat you get

The torque and power converter

The worked example is a made-up pickup engine making 300 lb·ft at 4,000 rpm. Nothing you type leaves this browser.

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What you get

Enrichment · STEM ExplorationsFree

What you finish with
The missing quantity, all three in metric and US units, and power across speeds at that torque. It prints as a summary and downloads as a spreadsheet.
Who it is for
Physics and engineering students, automotive and trades students, and robotics teams sizing motors.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The STEM Explorations space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose the torque and power units.
  2. Fill in two of torque, speed and power.
  3. Read the third and the conversions.
  4. Use the bars to see power rise with speed.
  5. Print the summary for a lab or a build log.

Convert

How the conversion works

Power is torque times angular speed.

The worked example: 300 lb·ft at 4,000 rpmTorque406.7 N·m300 lb·ft × 1.3558Angular speed418.9 rad/s2π × 4,000 ÷ 60Power170.4 kWtorque × angular speedHorsepower228.5 hp÷ 745.7 WThe worked example: 300 lb·ft at 4,000 rpmTorque406.7 N·m300 lb·ft × 1.3558Angular speed418.9 rad/s2π × 4,000 ÷ 60Power170.4 kWtorque × angular speedHorsepower228.5 hp÷ 745.7 W
Torque is twisting force; power is how fast that twisting does work. The same torque at twice the speed is twice the power.

The arithmetic

Angular speed in radians a second is 2π × rpm ÷ 60. Power in watts is torque in newton-metres times that. Any one of the three follows from the other two. In US units this becomes hp = lb·ft × rpm ÷ 5,252, because 5,252 ≈ 33,000 ÷ 2π.

Exact units

A pound-foot is 1.355818 N·m. A mechanical horsepower is 550 foot-pounds a second, 745.70 W; the metric horsepower (PS) is 735.50 W. Engine ratings may be measured at the crankshaft or at the wheels, and with different standards, so compare like with like.

Sources

  1. National Institute of Standards and Technology. Special Publication 811, Guide for the Use of the International System of Units, appendix B conversion factors (nist.gov).
  2. OpenStax. University Physics Volume 1, section 10.8: work and power for rotational motion (openstax.org).
  3. SAE International. J1349, Engine Power Test Code: how engine power and torque are rated (sae.org).
  4. U.S. Department of Energy. Motor efficiency, torque and speed basics, Advanced Manufacturing Office motor tip sheets (energy.gov).

The worked example is made up. Your figures stay in this browser. Support: team@caveatacademic.com.