Horsepower Calculator
Calculate from Torque & RPM
Calculate from Weight & 1/4 Mile Time
How Does This Work?
This calculator provides two different methods to estimate an engine's horsepower (HP):
- From Torque & RPM: This is a direct mathematical conversion. It uses the standard formula: Horsepower = (Torque × RPM) / 5252. This calculation is precise, assuming your torque and RPM figures are accurate. The number 5252 is a constant that makes the units work out correctly.
- From Weight & Quarter Mile Time: This method provides a real-world estimate based on a vehicle's performance. It uses a common physics-based formula developed by Patrick Hale: Horsepower = Vehicle Weight / (Elapsed Time / 5.825)3. This estimates the power needed to accelerate a certain weight over a specific distance in a given time, but it can be affected by factors like traction, aerodynamics, and driver skill.
The Surprising History of "Horsepower"
The term "horsepower" was coined by the Scottish engineer **James Watt** in the late 18th century. Watt made significant improvements to the steam engine and needed a way to market his powerful new machines to customers who were still using horses for work, especially in mines and mills.
To create a relatable unit, he observed horses at work and estimated that one horse could lift 550 pounds at a rate of one foot per second. This became the definition of one horsepower (1 HP = 550 ft-lb/s). It was a brilliant marketing move. By comparing his engines directly to the familiar power of a horse, he could easily demonstrate their value. Interestingly, Watt's estimate was quite generous; most horses cannot sustain that level of work for an extended period. His unit, however, became the enduring standard for measuring the power of engines everywhere.
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Frequently Asked Questions (FAQ)
What is the difference between Wheel HP (WHP) and Crank HP (BHP)?
Crank Horsepower (BHP or Brake Horsepower) is the power measured directly at the engine's crankshaft, before any losses from the drivetrain. Wheel Horsepower (WHP) is the power that actually reaches the wheels and makes the car move. WHP is always lower than BHP due to power loss through the transmission, driveshaft, and differential (typically 10-20% loss). The quarter-mile calculation gives an estimate closer to WHP.
Why do torque and horsepower always cross at 5252 RPM?
This is a mathematical constant. Because horsepower is calculated as (Torque × RPM) / 5252, when the RPM is exactly 5252, the RPM value and the 5252 constant cancel each other out, leaving Horsepower = Torque. On any dyno graph where torque and horsepower are plotted, the lines will always cross at this point.
Is the Quarter Mile ET (Elapsed Time) the only factor?
No, trap speed (the car's speed at the end of the quarter mile) can also be used to estimate horsepower, often with more accuracy as it's less affected by the initial launch. However, ET is a more commonly known and accessible metric for most car enthusiasts.