Horsepower dates back to the late 18th century when Scottish engineer James Watt was looking for an ingenious way to market his new and improved version of the 1712 Newcomen steam engine.
Inspired by Thomas Savery’s analogy between the working capacity of engines and horses, in his 1702 book titled The Miner’s Friend, Watt went on to pioneer the unit as we know it today.
The calculations he used to derive the value of 1 horsepower involved an actual horse rotating a 12 ft radius mill-wheel 144 times in an hour (2.4 times in a minute), which means it covered a distance (arc length) of 2.4 × 2π × 12 ft.
Watt assumed that the horse could pull with a force of 180 ft-lb/min. No one knows how healthy or motivated that horse was. But he had to make an assumption for the calculations to be complete.
Using the following equations:
- Power = Force x Velocity
- Torque = Force x Radius
- Velocity = Distance / Time
He derived that 1 horsepower is equal to one horse moving 32,572 pounds of mass, 1 ft in 1 minute, which was rounded to 33,000 ft-lb/min. Effectively, 1 horsepower is the power required for moving 33,000 pounds of mass by 1 foot, in 1 minute.
Despite the questionable nature of Watt’s calculations, horsepower went on to become the industry standard for measuring engine power output.
The 5252 Constant
Fun fact: This also explains the 5252 constant — ever wondered why on almost every dyno sheet you look at, the hp and torque curves intersect at 5252 rpm?
That’s because 33,000 / 2π = 5252.
Side note: 2π is needed here because torque is measured by radians, and there are 2π radians per revolution (360).
Have questions about the different units used to measure horsepower? Check out this guide to BHP vs HP vs WHP.