Horsepower and wattage (both developed by steam engine pioneer James Watt) are two ways of quantifying how much work an engine or person can perform over a given period of time, and both have everything to do with torque and rpm. Wattage and horsepower are interchangeable, and knowing the variable interplay can be valuable in assessing a motordriven vehicle or appliance's potential performance.

Calculate known torque at a given rpm to horsepower. Multiply known rpm by known torque in footpounds, then divide by 5,252. Example: A motor produces 200 foot pounds at 3,000 rpm. Multiply torque (200) by rpm (equals 600,000), and divide by 5,252 to end up with a horsepower figure of 114.2.

Convert horsepower to watts by multiplying known horsepower by 746 (the number of watts per horsepower). The 114.2 horsepower engine in the above example produces 85,193.2 watts. You can convert watts back to horsepower by multiplying it by 0.001341.

Convert wattage to footpounds of torque per second by multiplying the wattage by 0.737562. This comes out to 62,835.26 footpounds per second.

Calculate rpm using a known horsepower and torque figure. Multiply horsepower by 5,252, and divide by the torque in footpounds. Using our example engine, we'll multiply known horsepower (114.2) by 5,252 (equals 599,788.4), then divide by the known torque (200). This comes out to 2,998 rpm, which is very close to the known 3,000 rpm.

Calculate torque in footpounds at a given rpm and wattage. First, convert wattage to horsepower by multiplying it by 0.001341. Then, multiply horsepower by 5,252, and divide by rpm. For our example engine, we know it produces 85,193.2, which multiplied by 0.001341 equals the known 114.2 horsepower. Then, we'll multiply 114.2 by 5,252 (equals 599,778), and divide by the known rpm (3,000). This comes out to 199.92 footpounds of torque, which rounds up to our known 200 footpounds.
References
 "The Engineering Handbook, Second Edition"; Stephen Dorf; 2004
 Electricians Toolbox: Motor Formulas
 Belden: Conversion Factors