This calculator works out horsepower from torque and engine speed. Enter the torque in pound-feet and the rpm at which it is measured, and you get mechanical horsepower, kilowatts and the torque in newton-metres.
Formula
Power = torque × angular speed. With torque T in lb-ft and speed in rpm, hp = T × rpm ÷ 5252.113, where 5252.113 = 33,000 ÷ 2π (33,000 ft-lbf per minute is one mechanical horsepower). 1 hp = 0.745699872 kW and 1 lb-ft = 1.355818 N·m.
- T: torque at the crank or shaft, lb-ft
- rpm: revolutions per minute at that torque
Worked Example
300 lb-ft at 5,000 rpm.
- 300 × 5000 = 1,500,000
- 1,500,000 ÷ 5252.113 = 285.6 hp
- 285.599 × 0.745699872 = 212.97 kW
- 300 × 1.355818 = 406.7 N·m
Limits and common mistakes
- Torque and horsepower curves always cross at 5,252 rpm when plotted in lb-ft and hp.
- Use torque and rpm from the same point of the curve; peak torque and peak power happen at different speeds.
- Metric: kW = N·m × rpm ÷ 9549.3.
- Brake (crank) figures are higher than wheel figures because of drivetrain losses.
Formula used
in t | Torque | lb-ft | 300 | 0 | 100000 in rpm | Engine speed | rpm | 5000 | 0 | 100000 out hp | Horsepower | hp | t * rpm / (33000 / (2 * pi)) | 1 out kw | Power | kW | hp * 0.745699872 | 2 out nm | Torque (metric) | N·m | t * 1.3558179483314004 | 1
Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.
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