How to use this calculator
Replace the example values with your own measurements or specifications. Keep the units shown beside each label. Results update as you type; reset restores the example.
Use torque measured at the same rpm, not peak torque combined with peak-power rpm. Wheel and crankshaft measurements are different; this tool does not estimate drivetrain loss. Mechanical hp differs from metric PS.
The formula
A worked example
300 lb-ft at 5,000 rpm produces about 285.60 mechanical hp, 212.97 kW, or 289.56 PS. In the other direction, 100 kW at 3,000 rpm corresponds to 318.31 N·m (234.77 lb-ft). Keep the power or torque and RPM from the same operating point.
Common questions
How do I calculate torque from horsepower?
Choose Power (mechanical hp), enter the power and its corresponding RPM, and read the torque in lb-ft and N·m. The equation is lb-ft = hp × 5,252.113122… ÷ rpm.
Can I use newton meters or kilowatts?
Yes. Choose Torque (N·m) to calculate power, or Power (kW) to calculate torque. RPM is revolutions per minute in every mode.
Are horsepower and PS the same?
No. Mechanical horsepower is about 745.70 watts; metric horsepower (PS) is about 735.50 watts. For the same physical power, the PS number is about 1.4% higher. Select the unit stated in your specification.
Can I multiply peak torque by redline RPM?
Only if that torque is actually available at redline. Otherwise the calculated power is misleading.
Why do torque and horsepower cross near 5,252 rpm?
Their numerical values match there only when torque is in lb-ft and power is in mechanical horsepower.
Why is zero RPM rejected when I enter power?
At zero speed, power does not uniquely determine torque. Dividing by RPM is undefined. When torque is the known input, zero RPM instead gives zero rotational power.
Sources & calculation method
NIST Guide to the SI, Appendix B.9 lists mechanical horsepower as 550 ft·lbf/s and gives conversions for horsepower to watts and pound-force feet to newton meters. Its separate metric-horsepower entry explains why PS and mechanical hp are not interchangeable.
The 5,252.113122 divisor comes from 33,000 ÷ (2π): one revolution is 2π radians and one mechanical horsepower is 33,000 ft·lbf/min. Thus torque × rpm × 2π ÷ 33,000 gives mechanical horsepower. The calculator derives conversion factors from standard gravity and the defined foot and pound: approximately 0.745699872 kW per mechanical hp, 0.73549875 kW per metric PS, and 1.355817948 N·m per lb-ft. It retains precision until displaying results.
References accessed September 9, 2026. These sources support the stated relationships and units; they have not reviewed or endorsed AutomotiveCalc.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.