Horsepower Calculator: Torque and RPM
This horsepower calculator estimates engine power three ways. If you know torque and RPM, it uses horsepower = torque × RPM ÷ 5,252. If you have a drag strip time slip, enter the vehicle weight with driver in pounds and either the quarter-mile elapsed time or the trap speed in mph, and it estimates wheel horsepower with the classic Hale formulas. Every answer is shown in horsepower, kilowatts and watts. The drag strip numbers are estimates, since traction, gearing and weather all matter, but they are a fast way to compare cars and mods.
Horsepower Calculator
Horsepower from torque and RPM, quarter-mile time or trap speed.
How to use the Horsepower Calculator
- Choose a method: torque and RPM, quarter-mile time, or trap speed.
- Enter the values in US units.
- Read horsepower plus kW and watts.
Horsepower formulas
hp = torque × RPM ÷ 5,252. From the drag strip: hp = weight × (5.825 ÷ ET)³, or hp = weight × (trap mph ÷ 234)³.
Features
- Horsepower from torque and RPM
- Quarter-mile ET method
- Trap speed method
- hp, kW and watts
- US units
- Copy and print
Frequently asked questions
How do you calculate horsepower from torque?
Horsepower = torque (lb-ft) × RPM ÷ 5,252. 400 lb-ft at 4,000 RPM is about 305 hp.
How accurate is the quarter-mile horsepower estimate?
It is a rough estimate of power at the wheels. Traction, launch, gearing and air density can shift it by 10% or more.
What is the trap speed formula?
hp = weight × (trap speed ÷ 234)³. A 3,500 lb car trapping 110 mph is about 363 hp.
How many watts is one horsepower?
One mechanical horsepower is about 745.7 watts, or 0.7457 kW.
Is wheel horsepower lower than engine horsepower?
Yes. The drivetrain loses roughly 10% to 20%, so wheel horsepower is lower than the crank rating.