Horsepower, Torque & Power-to-Weight Calculator

Frequently Asked Questions

• What is the relationship between horsepower and torque?+
  Power is torque multiplied by rotational speed. In SI units, power in watts equals torque in newton metres times 2π times rpm, divided by 60, so power in kilowatts is torque in N·m times rpm divided by about 9,549 (60,000 ÷ 2π). Torque is a turning effect, a force acting at a lever arm; power is how fast that turning effect does work, which is why power depends on rpm.
• How do I convert horsepower to kilowatts?+
  One mechanical horsepower, defined as 550 foot-pounds-force per second, is 745.699 87 watts in NIST Special Publication 811, so multiply hp by about 0.7457 to get kW, or divide kW by it to get hp. The calculator converts on the full-precision constant: type any one of hp, PS or kW and it fills in the other two.
• What is metric horsepower (PS)?+
  PS (Pferdestärke) is the metric horsepower, 75 kilogram-force metres per second. With the kilogram-force at its exact 9.806 65 N, that is 735.498 75 W, which NIST's table prints rounded to 735.4988 W. A PS is about 1.37% smaller than a mechanical horsepower, so the same engine's figure is about 1.4% larger in PS than in hp. Honda's Malaysian spec page for the Civic Type R, for one, prints its power in PS, which is why the tool has a PS field.
• Can I find torque from power, or power from torque?+
  Yes. Type two of power, torque and rpm and the tool solves the third: torque and rpm give power, power and rpm give torque, power and torque give rpm. If you type all three they must agree within 1%, or the tool shows the disagreement instead of an answer. Peak power and peak torque on a spec sheet are quoted at their own rpm, so they will not agree: Honda's Japanese sheet for the Civic Type R puts 243 kW at 6,500 rpm, and 243 kW at 6,500 rpm is 357 N·m, not the 420 N·m peak the same sheet prints.
• What is the 5,252 rpm fact about?+
  If torque is plotted in pound-force feet and power in mechanical horsepower on one numeric axis, the two numbers are equal at about 5,252 rpm, since 33,000 ÷ 2π is 5,252.11. It is a property of that pair of units, not of engines. In other units the meeting point moves: about 9,549 rpm for N·m against kW, 7,121 for N·m against hp, 7,024 for N·m against PS and 5,180 for lbf·ft against PS. The tool computes all five in its "How is this calculated?" panel.
• Why are there hp, PS and kW fields?+
  They are one quantity in three units. Type whichever your spec sheet prints. If you type more than one, they must agree within 0.5%, which allows for a figure rounded to a whole number; otherwise the tool says which values disagree and asks you to clear all but one.
• What does power-to-weight show?+
  Power divided by vehicle mass, as hp per tonne, PS per tonne and watts per kilogram, so two vehicles of different weight can be compared on one scale. The tool does not estimate a 0 to 100 km/h time: acceleration also depends on traction, gearing, drivetrain losses and aerodynamics, and we found no primary source for the weight-over-horsepower rule of thumb sometimes quoted.
• What is electric horsepower?+
  A horsepower of exactly 746 W. NIST SP 811 prints it in bold, its mark for an exact factor. It is 0.04% larger than the mechanical horsepower, so for most purposes the two read the same; this tool works in mechanical horsepower.
• Why does the tool refuse some numbers?+
  Power, torque, rpm and weight cannot be negative here, so a negative value is refused rather than carried into the result. Text that is not a plain number, including one with a thousands comma such as 1,234, is refused too, because a comma can be read two ways. A non-zero power at 0 rpm has no finite torque, so that is refused as well.
• Is the Horsepower & Torque Calculator free?+
  Completely free, with no account or usage limit. It runs entirely in your browser, collects no data, and works offline once the page has loaded.