Horsepower Calculator computes mechanical power from the two most common inputs engineers and mechanics actually have on hand: a rotating shaft's torque and RPM, or a linear load's force and speed. Unlike drag-strip horsepower estimates, both formulas here are exact unit conversions, not curve-fit approximations.

The torque/RPM relationship — horsepower equals torque times RPM divided by 5252 — comes directly from the definition of horsepower (550 ft-lb per second) combined with the geometry of rotation (2π radians per revolution, RPM converted to revolutions per second). Because it's exact rather than fitted, it produces the well-known fact that a torque curve (in lb-ft) and a horsepower curve for the same engine always intersect at precisely 5252 RPM, regardless of the engine.

The force/speed method applies the same underlying definition of power (force times velocity) to straight-line motion instead of rotation — useful for towing capacity, conveyor belts, or anything where the "RPM" framing doesn't apply. The HP↔kW tab handles the separate, purely definitional conversion between horsepower and the SI kilowatt unit.