Engine Horsepower Calculator
Compute engine horsepower three ways: from a dyno torque curve (HP = T × RPM / 5252), from quarter-mile trap speed (Huntington method), and from quarter-mile elapsed time (Patrick Hale method).
Horsepower (Torque × RPM)
495.0 hp
- In kilowatts
- 369.15 kW
- In metric horsepower (PS)
- 501.9 PS
- Estimate from trap speed (Huntington)
- 363.6 hp
- Estimate from elapsed time (ET method)
- 314.9 hp
Primary result uses HP = (Torque × RPM) / 5252, the exact mechanical horsepower from a measured torque curve. The trap-speed and ET estimates use empirical drag-strip formulas and assume all measurements are at the flywheel.
How to use this calculator
Enter your engine torque and the RPM at which it was measured to get the primary horsepower figure. If you have drag-strip data, enter the vehicle race weight and either the trap speed or the elapsed time to cross-check with the empirical Huntington and ET estimates. All three results are shown so you can compare a dyno number against real-world track performance.
How the calculation works
The dyno formula HP = (Torque × RPM) / 5252 is an exact unit conversion: one mechanical horsepower equals 33,000 ft·lbf per minute, and the 5252 constant comes from dividing 33,000 by 2π (the radians-per-revolution factor that converts rotational torque into linear work). Horsepower and torque curves on every dyno plot intersect at 5252 RPM — that is purely mathematical, not engine-specific. The MPH method (Roger Huntington, 1950s) uses HP = Weight × (MPH/234)³, calibrated empirically from drag-strip data. The ET method (Patrick Hale) uses HP = Weight × (5.825/ET)³ — equivalent to the MPH method when traction is consistent.
Worked example
A small-block V8 puts down 400 lb·ft at 6,500 RPM on the dyno. HP = (400 × 6,500) / 5252 = 495 hp. The same car weighs 3,500 lb on the scales and runs the quarter mile in 11.5 seconds at 121 MPH trap speed. Huntington estimate: 3,500 × (121/234)³ ≈ 484 hp. ET estimate: 3,500 × (5.825/11.5)³ ≈ 455 hp. All three agree within 10 % — typical accuracy for empirical formulas.
Frequently asked questions
Why do horsepower and torque always cross at 5252 RPM?
It is a mathematical artefact of the unit definitions, not a physical property of any engine. The formula HP = (Torque × RPM) / 5252 means that whenever RPM = 5252, the horsepower value equals the torque value numerically. So on any dyno chart with torque in lb·ft and RPM on the x-axis, the two curves must intersect at 5252 RPM — every time, for every engine. Engines that make peak torque below 5252 RPM will have peak HP at a higher RPM; engines making peak torque above 5252 RPM will peak HP earlier than peak torque.
Where does the constant 5252 come from?
One mechanical horsepower is defined as 33,000 foot-pounds-force of work per minute. Torque turning through one revolution does 2π radians of angular work, so the linear work per minute equals Torque × RPM × 2π ft·lbf. Setting that equal to one horsepower (33,000) and solving gives RPM = 33,000 / (2π × T), or rearranged for power: HP = T × RPM / (33,000 / 2π) = T × RPM / 5252.113. The 5252 is just 33,000 ÷ 2π rounded.
Is the MPH method (Huntington) accurate?
It is within roughly 5–10 % of the true flywheel horsepower for most cars, provided trap speed is measured cleanly and the run did not have wheelspin or aerodynamic issues. Roger Huntington calibrated the 234 constant from a large 1950s drag-strip dataset and it has held up surprisingly well because trap speed (unlike ET) is dominated by power-to-weight at the end of the run, when traction is no longer limiting. Modern variants tweak the constant slightly: 224 for very aerodynamic vehicles, 250+ for unfaired motorcycles, but 234 is the standard.
When should I trust ET over MPH or vice versa?
Trap speed (MPH) is the more reliable horsepower indicator. ET depends heavily on how well the car launches: tyre grip, suspension setup, driver reaction time and gearing all affect ET but barely affect trap speed. A car that bogs off the line might run a slow ET but still hit its proper trap speed. If your ET-based and MPH-based horsepower numbers disagree by more than ~15 %, suspect a traction problem rather than an engine measurement error. The MPH number is closer to the true flywheel hp.
Does this calculate wheel horsepower or flywheel horsepower?
The Torque × RPM formula simply converts whatever torque you enter — so the result is wheel hp if you measured wheel torque on a chassis dyno, or flywheel hp if you measured at the crankshaft on an engine dyno. The MPH and ET formulas were calibrated to estimate flywheel horsepower from track performance. Typical driveline losses for a rear-wheel-drive car are around 15 %, so divide flywheel hp by 1.15 to estimate wheel hp, or multiply wheel hp by 1.15 to estimate flywheel hp.
How do I convert engine horsepower to kilowatts or metric PS?
Multiply mechanical horsepower by 0.7457 to get kilowatts (1 hp = 745.6998716 W exactly under NIST SP 811). Multiply by 1.01387 to get metric horsepower (PS, ch, or CV — the unit on most European spec sheets). A 500 hp engine is 372.85 kW or 506.94 PS. The metric–imperial gap is about 1.4 % because they use slightly different "horse" definitions (75 kgf·m/s versus 550 ft·lbf/s).