Most engines use horsepower to describe how much work they can do in a given amount of time. The constant 1 horsepower equals 550 foot-pounds per second. In other words, 1 horsepower is the amount of work required to move a load of 550 pounds over 1 foot, in 1 second. Because horsepower, like wattage (no coincidence that James Watt invented the measurement), bears a direct relationship to both power and to time, it can be calculated a number of different ways. When the load that needs to be moved, the distance that it needs to move, and the amount of time in which it needs to move are known, use a variation of a simple formula to calculate horsepower.

1 horsepower = 550 foot-pounds/second 1 horsepower = 746 watts

If P = FD/T, then F = PT/F and T = DF/P.

Simplify the equation for power: 1 horsepower (hp) = 550 foot-pounds / 1 second. More generally, power = force times distance, divided by time.

Use the abbreviations F for "force" (pounds); D for "distance" (feet); and T for "time" (seconds), in the following general formula: P = F * D / T.

Plug in the amount of force (in pounds) that needs to be moved. For example, if the amount of force is 550 pounds, plug in "550 lbs" for F: P = 550 lbs * D / T.

Plug in the distance (in feet) that the load needs to move. For example, if the distance needed is one foot, plug in "1 foot" for D: P = 550 lbs * 1 foot / T.

Plug in the amount of time (in seconds) that the movement requires. For example, if the load needs to move the required distance in one second, plug in "1 second" for T: P = 550 lbs * 1 foot / 1 second.

With all these variables plugged in to the formula, complete the calculation. Keep all the units of measurement in the equation. For example, multiply F (550 lbs) by D (1 foot) to get 550 foot-pounds. Divide this figure by T to get 550 foot-pounds/second.

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- horsepower image by Kevin Chesson from Fotolia.com