Engine Horsepower Calculator
Calculate the horsepower of an engine using one of the three methods below. Learn more about how these methods work below.
Calculate Using Elapsed 1/4 Mile Time
Calculate Using 1/4 Mile Speed
Calculate Using RPM & Torque
Engine Horsepower Estimate:
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How to Calculate Engine Horsepower
Horsepower is a measure of the power output of an engine or a motor. The concept of horsepower was created by Scottish engineer James Watt, who made a number of major improvements to the efficiency of the earliest steam engines in the late 1700s.
Watt used horsepower to compare the output of steam engines to that of the draft horses that were used to do the same type of work. One horsepower was originally calculated to be the amount of work that a draft horse could do in one minute, or 33,000 foot-pounds per minute (ft-lb/min).
You can calculate the horsepower of an engine using several methods. Some common methods include the elapsed time method, the trap speed method, and the RPM and torque method.
Calculate Horsepower with the Elapsed Time Method
The elapsed time method is a way of finding engine horsepower by using the time it takes a vehicle to complete a quarter-mile or 400-meter run. By using the quarter-mile/400-meter time and the vehicle weight, it is possible to estimate the horsepower of the vehicle.
Use the following horsepower formula for the elapsed time method:
horsepower = weight ÷ (time ÷ 5.825)3
The weight is defined as the weight of the vehicle in pounds (including the driver, any passengers, and additional weight being carried) and the time is defined as the quarter-mile time in seconds.
For example, let’s calculate the horsepower for a vehicle weighing 3,750 pounds that completes the quarter-mile in 13 seconds.
horsepower = 3,750 ÷ (13 ÷ 5.825)3
horsepower = 3,750 ÷ 2.2323
horsepower = 3,750 ÷ 11.12
horsepower = 337.4 HP
Calculate Horsepower with the Trap Speed Method
The trap speed method is a way of estimating horsepower using the weight of the vehicle and its speed at the end of a quarter-mile or 400-meter run.
Use this formula for horsepower using the trap speed method:
horsepower = weight × (speed ÷ 234)3
The weight is defined as the weight of the vehicle in pounds (including the driver, any passengers, and additional weight being carried) and the speed is defined as the maximum speed reached at the end of the quarter-mile/400-meter run in miles per hour.
For example, let’s calculate horsepower for a vehicle weighing 3,750 pounds with a quarter-mile speed of 100 mph.
horsepower = 3,750 × (100 ÷ 234)3
horsepower = 3,750 × 0.4273
horsepower = 3,750 × 0.078
horsepower = 292.67 HP
Calculate Horsepower with the RPM & Torque Method
It is possible to calculate the horsepower of an engine using the RPM and torque method. While horsepower can be defined as the sheer power output of the engine, torque is defined as the amount of twisting force that is created by the engine.
Engine horsepower typically peaks at a specific RPM, so using that RPM number will give you the maximum horsepower of the engine.
The engine specifications should include the peak torque as well as the RPM at which the engine generates its maximum horsepower; this information is also likely to be published online.
Here is the formula for horsepower using the RPM and torque of the motor:
horsepower = (RPM × torque) ÷ 5,252
For example, let’s find the horsepower for an engine producing its maximum horsepower at 5,600 RPM and which also produces a peak torque of 350 foot-pounds.
horsepower = (5,600 × 350) ÷ 5,252
horsepower = 1,960,000 ÷ 5,252
horsepower = 373.19 HP
You might be interested in our horsepower conversion calculators or our carburetor size calculator.