This formula was established by James Watt in the 1800's and requires some known values:
Average horse walks at 21/2 MPH
Average horse pulls with a force of 150 pounds
1 mile = 5,280 feet
With this background, we will be able to establish the Horsepower formulas used today.
5,280 feet × 21/2 MPH = 13,200 FEET per HOUR
220 FT/MIN × 150 POUNDS = 33,000 FT LBS per MINUTE
2π = 6.2831853
Thus we get the familiar formula used today in calculating Hp.
= Radius from centerline of rotating shaft. Usually measured at a distance of one foot out from centerline.