How to Calculate Shaft Horsepower From Torque

by Richard Rowe
horse image by milemarsovac from

Horsepower is a mathematically derived number meant to give some perspective on how much work an engine can produce relative to a horse. The idea of horsepower is a fairly arbitrary one, and was originally developed by James Watt to market his new and improved steam engine. In actuality, an average horse can produce about 15 horsepower in short bursts and about 5 horsepower continuously. One horsepower is actually closer to the output of a medium-sized monkey.

Step 1

Multiply the engine RPM by the torque (measured in foot/pounds). Example: If you record 200 foot/pounds of torque at 5000 RPM, the result is 1,000,000.

Step 2

Record the number found in Step 1.

Step 3

Divide the number recorded by 5,252 to find the horsepower. Example: if you divide 1,000,000 by 5,252 you get 190, which is the engine's horsepower.

Step 4

Repeat steps 1 through 3 for as many RPM points as you'd like to measure, and record them on a graph, with RPM as the horizontal axis and horsepower as the vertical.

Draw a line connecting your data points, which will give you the engine's basic "horsepower curve."