How to Calculate a Car's Horsepower

Calcuate a car's horsepower using a simple mathematical formula.

Horsepower is a unit for measuring power, and is the standard unit used to express the output power of a car's engine. According to Web Cars, the unit was invented by James Watt. It was originally used to compare steam engines to the horses they replaced in industrial work. While many people are familiar with the term horsepower, not many people know how to calculate a car's horsepower. Fortunately, by collecting some information about your car you can use a mathematical formula to calculate your car's horsepower with no special equipment.

Things You'll Need

• Stopwatch
• Calculator

Instructions

1. Drive your car on a drag strip that is a quarter-mile long. Time the car with the stopwatch to determine how long it takes the car to run the entire quarter-mile at its maximum speed. This is called the elapsed time (ET) and is one number needed to calculate a car's horsepower.

2. Weigh your car. If you know someone with automotive scales, you can weigh the car to get a definite number. If you do not have access to automotive scales, you will find the manufacturer's official numbers for the weight of the car in your owner's manual. Unless you have made major modifications to your car that would affect its weight, the manufacturer's figure will still give you a good estimate of a car's horsepower.

3. Calculate the horsepower of the car using a calculator and the ET method formula. According to DSM.org, this formula can be written as "horsepower = weight / (ET / 5.825)³." This will give you the horsepower of the car.

Tips & Warnings

• Be sure to include the weight of the driver along with the car when using this method to calculate a car's horsepower.

• If not at a drag strip, avoid public roads when timing your car in the quarter mile and always obey all traffic laws.

• While using this formula to calculate a car's horsepower is effective, special equipment like a dynamometer is necessary to get an exact figure.

References

• Web Cars: What is Horsepower?
• DSM.org: Simple Horsepower Calculator