Engine Horsepower Calculator
Turn dyno numbers into horsepower. Enter torque (lb-ft or N·m) and RPM for the exact conversion, or estimate output from engine displacement and tune level — with results in hp, kW, and metric PS.
Torque & RPM gives power at that RPM — use the values from the same point on the dyno curve. Peak power usually arrives well above peak torque.
Torque, RPM, and the 5,252 formula
Power is torque times rotational speed. In imperial units the constants collapse to one famous number: HP = torque (lb-ft) × RPM ÷ 5,252. The 5,252 comes from Watt’s definition of a horsepower (33,000 ft·lb per minute) divided by 2π — and it’s why every dyno chart’s horsepower and torque curves cross at exactly 5,252 RPM. Below that RPM torque is the bigger number; above it, horsepower.
The displacement method is a planning rule of thumb: modern naturally aspirated engines typically deliver about 60–80 hp per liter, older economy designs less, and forced-induction engines 100–140+. It brackets what a build can make, not what a specific engine does.
How it’s calculated
HP = T × RPM ÷ 5,252 (T in lb-ft; N·m ÷ 1.3558 first). kW = hp × 0.7457; PS = hp × 1.0139. Displacement: hp ≈ liters × specific output (hp/L), shown with a ±15% band; CID × 0.016387 = liters.
Manufacturer ratings are net (SAE J1349, with accessories); marketing figures and wheel-dyno readings can differ by 10%+ either way.
Worked example
A dyno pull showing 310 lb-ft at 5,600 RPM is 310 × 5,600 ÷ 5,252 = 331 hp (246 kW / 335 PS) at that engine speed. In metric, that torque is 420 N·m. The estimate mode: a 5.0-liter modern NA V8 at ~70 hp/L suggests roughly 350 hp (about 300–400 with the ±15% band) — right where most factory 5.0s land.
Common mistakes
- Multiplying peak torque by peak RPM — use torque and RPM from the same point on the curve.
- Forgetting N·m → lb-ft (÷1.3558); 420 N·m is 310 lb-ft, not 420.
- Comparing a wheel-dyno number to a factory crank rating without accounting for ~10–15% drivetrain loss.
- Treating hp/L as a quality score — a lazy 6.2L pushing 420 hp beats a stressed 2.0L at 300 hp for towing.
Where it is used
- Reading dyno sheets: converting the torque curve to a power curve.
- Estimating what a planned engine build or swap can produce.
- Translating European kW/PS specs to hp (and back).
- Checking a tuner’s claimed gains against physics.
Frequently asked questions
Why do horsepower and torque cross at 5,252 RPM?
Because HP = T × RPM ÷ 5,252 — at exactly 5,252 RPM the equation forces them equal. It is a unit artifact (33,000 ft·lb/min ÷ 2π), not a property of engines; plotted in kW and N·m the curves cross elsewhere.
How do I convert N·m to lb-ft?
Divide by 1.3558 (or multiply by 0.7376). 420 N·m = 310 lb-ft. The calculator does it automatically when you pick N·m.
How accurate is the displacement estimate?
It is a bracket, not a measurement. Naturally aspirated production engines mostly fall between 55 and 85 hp/L depending on tune and rev ceiling; turbocharged engines commonly reach 100–140 hp/L. Cam, compression, fuel, and boost move real output across that whole range.
What is the difference between gross and net horsepower?
Net (the modern SAE J1349 standard) is measured with production intake, exhaust, and accessories fitted; gross (pre-1972 US ratings) removed them and read much higher. That is much of why muscle-car-era numbers look inflated today.
Is 1 PS the same as 1 hp?
Almost: 1 PS (metric horsepower, a.k.a. CV or ch) = 735.5 W while 1 hp = 745.7 W, so PS figures read about 1.4% higher for the same engine. 300 PS = 296 hp = 221 kW.