What is horsepower?
Horsepower is a unit of measurement used to describe the rate at which work is done, specifically the power an engine or motor produces. The term originated in the 18th century when James Watt needed a way to compare the output of his steam engines to the pulling power of draft horses.
One mechanical horsepower represents the ability to move 550 pounds one foot in one second. Today, it remains the standard way to rate the power output of cars, trucks, lawnmowers, and many electric motors.
Horsepower formulas
The formula you use depends entirely on the data you already have.
Force and speed formula
When you know how much force is being applied and how fast an object is moving, use this formula:
HP = (Force × Velocity) / 550
In this equation, force is measured in pounds-force and velocity is measured in feet per second. The result is mechanical horsepower.
Torque and RPM formula
When you are working with rotating machinery like a car engine, you usually have torque and rotational speed:
HP = (Torque × RPM) / 5252
Torque is measured in pound-feet and RPM is revolutions per minute. The number 5252 is a mathematical constant. It comes from combining the definition of a horsepower (550 foot-pounds per second) with the circumference of a one-foot radius circle and converting minutes into seconds.
Watts to horsepower formula
When you are dealing with electrical motors, power is often rated in watts. To convert watts to mechanical horsepower:
HP = Watts / 746
There are exactly 746 watts in one mechanical horsepower.
How to use this calculator
- Choose your calculation method from the dropdown menu at the top of the tool.
- Enter your known values into the active input fields.
- Select the correct units for your measurements if the calculator offers unit toggles.
- Click the calculate button to see your result in mechanical horsepower.
Example: finding horsepower from torque and RPM
Imagine you are building a small go-kart and you know the engine produces 120 pound-feet of torque at 3,500 RPM. You want to know the horsepower at that specific engine speed.
First, select the torque and RPM method on the calculator and enter 120 for torque and 3,500 for RPM. The calculator will run the following math:
HP = (Torque × RPM) / 5252
HP = (120 × 3,500) / 5252
HP = 420,000 / 5252
HP = 79.97
The engine produces roughly 80 horsepower at 3,500 RPM. If your measurements were in metric units, such as Newton-meters for torque, the calculator handles the conversion to pound-feet automatically before applying the formula.
Different types of horsepower
Mechanical horsepower is the standard in the United States, but you might encounter other variations depending on where your equipment was manufactured.
Electrical horsepower is identical to mechanical horsepower in value, but it is used specifically when rating electric motors.
Metric horsepower, often abbreviated as PS or CV, is slightly smaller than mechanical horsepower. One metric horsepower equals about 735.5 watts, compared to the 746 watts in a mechanical horsepower. A 200 PS European car engine is slightly less powerful than a 200 HP American engine.
Boiler horsepower is an entirely different measurement used for steam boilers. It represents the energy required to evaporate 34.5 pounds of water per hour at 212 degrees Fahrenheit. You should never use boiler horsepower to rate a mechanical engine.
Frequently Asked Questions
Is 1 horsepower actually equal to the power of one horse?
No. A healthy horse can produce up to 15 horsepower for short bursts. James Watt designed the unit to be a highly conservative, continuous measure of work so his steam engines would look impressive compared to the average work output of a mine horse over a full shift.
Why does the torque formula use 5252?
The number 5252 is a constant derived from the geometry of a circle and unit conversions. Torque is measured in pound-feet, meaning a one-pound force at the end of a one-foot lever. A one-foot lever travels in a circle with a circumference of 6.2832 feet. When you divide 33,000 (which is 550 foot-pounds per second multiplied by 60 seconds in a minute) by 6.2832, you get exactly 5,252.
Can horsepower be negative?
No. Horsepower measures the rate at which work is being performed. If an engine is braking or absorbing power rather than producing it, it is not producing negative horsepower. The value simply drops to zero for the purposes of power calculation.
What is the difference between brake horsepower and standard horsepower?
Brake horsepower (BHP) measures power at the engine's output shaft before any power is lost to the gearbox, alternator, or other accessories. Standard horsepower ratings are often taken at the flywheel and may include some of those accessory losses depending on the specific testing standard the manufacturer uses.