Horsepower Calculator
Calculate engine horsepower using torque and RPM, or estimate vehicle horsepower from 1/4 mile drag racing times and trap speeds.
How Horsepower is Calculated
Horsepower (HP) is a unit of measurement for power, representing the rate at which work is done. James Watt originally defined it to compare steam engines to the power of draft horses.
Standard Mechanical Formula
The most accurate way to calculate horsepower for an engine is using its torque and RPM (Revolutions Per Minute).
HP = (Torque × RPM) / 5252
Why 5252? It is a constant derived from the definition of horsepower (33,000 ft-lbs/min) and the conversion from RPM to radians. At 5252 RPM, torque and horsepower are always equal.
Estimating Horsepower from 1/4 Mile
If you do not have a dyno chart, you can estimate flywheel horsepower using the weight of the vehicle (including driver) and its performance on a drag strip.
- Elapsed Time (ET) Method: Uses the time it takes to finish the 1/4 mile.
HP = Weight × (5.825 / ET)³ - Trap Speed Method: Uses the speed at the finish line. This is often considered more accurate as it is less affected by wheel spin at the start.
HP = Weight × (Speed / 234)³
? Frequently Asked Questions
The Torque/RPM formula calculates power at whatever point you measure torque (usually flywheel). The 1/4 mile formulas estimate Flywheel (Crank) Horsepower, accounting for typical drivetrain losses.
ET (Elapsed Time) is heavily dependent on traction and driver skill at the launch. Trap Speed (finish line speed) is a better indicator of raw power-to-weight ratio.
Torque is the rotational force an engine produces (the 'pull'). Horsepower is how fast that force can be applied (the 'speed').