What is Horsepower?
The idea of horsepower was created by the 19th-century engineer James Watt, who built some of the first steam engines. Watt's work was remarkable, and to honor him, his name was given to the unit of power, the watt.
Watt was working in a mine, using horses to pull wagons of coal. He wanted to determine how much coal a pony could pull in a wagon over a given length. He measured how many feet a horse could pull 22,000 pounds of coal in one minute. He then increased that amount to 33,000 foot-pounds in a minute, and called it horsepower.
It's a perfectly arbitrary amount, but it became the measure of how much work engines could do—no one had ever measured this before. So, imagine a horse pulling a wagon of coal out of a mine; with 1 horsepower of effort, the horse pulls 330 pounds of coal 100 feet in one minute.
Measuring Horsepower
Horsepower is measured by a dynamometer, which is a rotor in a housing. It takes a certain amount of power to make the rotor go around at a certain speed.
If you put a car into neutral, and then floor the engine while it is attached to a dynamometer, the device puts a load on the engine and measures whether it can turn the load or how fast it can turn the load around. If you run the engine at 5000 rotations per minute (rpm), you observe how much load turns on the dynamometer to calculate horsepower.
Every engine has a peak horsepower—an rpm value at which the power available from the engine is at its maximum. You will often see this expressed in a brochure or a review in a magazine as "320 HP @ 6500 rpm".
Gross vs Net Horsepower
Gross horsepower is a measure of the work output of an engine on just a dynamometer, when the engine is not connected to the usual accessories apparent in a running car. Net horsepower measures engine output when connected to belt-driven accessories such as water pumps, power steering pumps, and alternators. There are also parasitic losses in power caused by transmission drag and clutch or converter slippage that net horsepower takes into account. As a result of powering these moving parts, net horsepower can be considerably lower than gross horsepower.
Horsepower Versus Torque
Torque is defined as a rotating force. It's measured as the amount of force multiplied by the length of the lever through which it acts. For example, if you use a one-foot-long wrench to apply 10 pounds of force to a bolt head, you're generating 10 pound-feet of torque. Note that torque is measured in pound-feet, and horsepower is measured in foot-pounds per minute.
Torque is the force that can be applied to push the vehicle forward. At a given vehicle weight, a high torque means the vehicle can accelerate faster and is more responsive. Although not always true, generally, the more torque produced by an engine, the more work potential it has. Similarly, an engine that produces more horsepower generally has a greater ability to generate higher torque.
To understand the relationship between horsepower and torque clearer, think of the difference between a racing car and a tractor. A racing car is light, and so its high level of horsepower puts the torque through the gearing system to make it go fast. A tractor, on the other hand, is a massive, heavy piece of machinery that is intended to do work. The tractor can't go fast, but its gearing applies torque so that it can push and pull. If you put the same high horsepower engine on a racing car and a tractor, the result would be a speedy racing car, but not a vehicle that could break down a concrete wall. The slow tractor applies its work to pressure on the wall and breaks it down.
Horsepower and Performance
What we call a "high-performance" car is just a vehicle with a lot of horsepower and a low weight; the power-to-weight ratio is the essential determination for a high-performance car. A typical example would be a Ferrari, which might have 800 horsepower on a 3,500-pound car. This puts the power-to-weight ratio at about 0.229. In comparison, an average SUV might have about 300 horsepower to move about 4,500 pounds. Its power-to-weight ratio is much lower (about 0.067). The sports car will go from zero to 60 miles per hour much faster.
How to Use This Calculator (The Formulas)
This calculator provides two standard mathematical methods commonly used to estimate vehicle horsepower based on quarter-mile runs.
The Elapsed Time (ET) Method
This method uses the total vehicle weight (including driver) and the elapsed time (ET) to finish a quarter mile (402.3 meters).
The Trap-Speed Method
This method uses the vehicle weight and the speed at which the vehicle finished a quarter mile. The speed used should be the speed attained exactly at the quarter-mile point, not the average speed over the entire run.
Frequently Asked Questions
No, the results are estimations only. While these formulas are widely accepted in drag racing to give a good ballpark figure of wheel horsepower (or sometimes estimated crank horsepower depending on drivetrain loss assumptions), a dynamometer provides the most accurate measurement.
Yes. The vehicle weight should include not only the vehicle itself but also the driver, passengers, and anything else of significant weight inside the car. The formulas calculate the power required to move the total mass.
Absolutely not. Do not measure elapsed time or trap-speed on a quarter mile on public streets or highways. Driving over the speed limit is illegal, and it endangers you and others. Always use legal places like drag strips or raceways, and ensure your vehicle is in perfect working order.
Trap-speed is generally considered a better indicator of actual engine horsepower because it is less affected by traction (wheelspin) off the starting line. A poor launch will ruin your Elapsed Time, resulting in a lower estimated horsepower, but you can still achieve a high Trap-Speed indicative of your true engine power.
One mechanical horsepower is equivalent to approximately 745.7 watts. To convert, simply multiply your horsepower figure by 745.7. Our calculator automatically provides the watt conversion in the results dashboard.