How to Calculate Brake Horsepower

by Richard RoweThe steam engine was undoubtedly one of the most important inventions of all time -- but James Watt had a hard time selling it. Farmers, used to measuring work in terms of how much a horse could do, simply couldn't wrap their minds around the potential of Watt's creation. At least, not until Watt created one of today's most household of engineering terms: "horsepower."

### Calculating Horsepower

Horsepower is a measure of how much work something can get done in a certain period of time. When Watt wanted to describe "one horsepower," he observed that a draft horse could turn a 12-foot-diameter mill wheel with a force of 180 pounds 144 times in an hour. Working out the math, he calculated out that the horse did about 33,000 foot-pounds per minute of work -- which he called "one horsepower." The horse's actual peak output was close to 15 horsepower, but only very briefly; since horsepower is calculated over time, and the time measured was an hour, the peak output didn't really matter. The formula for horsepower we use today was derived from Watt's observation of how much work a horse could do in an hour. Horsepower is equal to foot-pounds of torque, multiplied by rpm, divided by 5,252. If you're measuring electrical output, one horsepower is equal to 746 watts of power. The Watt Constant of "5,252" in that formula makes for a signature feature on all engine dynomometer charts: horsepower and torque are always equal at 5,252 rpm. So if you ever get into an internet argument, and someone shows you a dyno sheet where the horsepower and torque lines don't cross at exactly 5,252 rpm, then you know it's a bogus dyno chart.