Horsepower From Torque Calculator

This calculator turns a torque reading and the engine speed it was measured at into horsepower, kilowatts and metric horsepower (PS). Enter torque in pound-feet or newton-meters; it also shows the torque in the other unit so you can compare US and European spec sheets.

It is useful when a dyno sheet, workshop manual or spec table gives you torque at a given RPM but not power, when you want to check a manufacturer's claim, or when you are comparing a high-revving gasoline engine with a low-revving diesel. Power is what ultimately determines acceleration and top speed, so it pairs well with a 0-60 time estimate.

The result is only as good as the torque figure. Peak torque and peak power almost never happen at the same RPM, so multiplying peak torque by redline overstates power. For general rotational work outside engines, the torque calculator handles force × lever arm.

Power is torque times angular speed (P = T × ω). The 5,252 constant comes from James Watt's definition of one horsepower as 33,000 foot-pounds of work per minute, divided by 2π radians per revolution. Metric horsepower (PS) is slightly smaller at about 735.5 W, so the PS figure is roughly 1.4% higher than the hp figure for the same engine.

The formula is exact physics; any error comes from the inputs. Wheel-dyno torque gives wheel horsepower, which is lower than the crank figure manufacturers quote because the transmission and driveline absorb some power.

Rearrange the formula and at exactly 5,252 rpm, hp = torque × 1, so the two curves on any dyno chart plotted in hp and lb-ft intersect there. Below 5,252 rpm the lb-ft number is larger; above it, horsepower is larger.

If a chart shows the curves crossing somewhere else, either the axes use different scales or the units are mixed (for example kW against Nm, which never cross at 5,252). This makes the crossover a useful check that a chart or a set of quoted numbers is consistent.

Spec sheets mix units by region: US sources use hp and lb-ft, European sources use kW or PS and Nm, and older Japanese and German sources often quote PS (also written cv or ch).

• Crank (brake) horsepower is measured on an engine dyno at the flywheel. Manufacturers rate engines this way, typically to SAE J1349 in the US.
• Wheel horsepower comes from a chassis dyno and is lower because the gearbox, differential and tires absorb power. Losses are commonly estimated at roughly 10–20%, but they vary with drivetrain type and dyno, so treat any fixed percentage as an approximation.
• Gear ratio does not change power. A lower gear multiplies torque at the wheels but divides wheel speed by the same ratio, so the product stays the same apart from friction losses. Use the RPM to speed calculator to see how gearing maps engine speed to road speed.