HP & Torque Calculator
Convert between HP, PS, BHP and kW. Convert between Nm and lb-ft. Calculate power from torque and RPM. Enter engine data to get specific power, specific torque and mean piston speed — with an estimated power curve.
Understanding Power and Torque Units
Horsepower exists in several variants. SAE horsepower is the US standard measured at the flywheel. PS — Pferdestärke — is the metric horsepower used across continental Europe and is very slightly weaker than SAE HP. One PS equals 0.9863 SAE HP. BHP or brake horsepower is the British standard and is effectively equivalent to SAE HP for most purposes. kW is the SI unit and the most technically correct way to express power.
Torque is simpler — the only conversion needed is between Newton metres and pound-feet. One lb-ft equals 1.3558 Nm. European manufacturers use Nm universally while American manufacturers traditionally use lb-ft.
FAQ
What is the difference between HP and PS?
One PS equals 0.7355 kW while one SAE HP equals 0.7457 kW. The difference is less than 1.5% — a 500 PS engine produces 493 SAE HP. In everyday conversation the two are used interchangeably.
What is mean piston speed?
Mean piston speed is the average speed at which the piston travels inside the cylinder in metres per second. Formula: stroke (mm) × RPM × 2 / 60,000. Values above 20 m/s are high performance territory. Formula One engines operate above 25 m/s at peak RPM.
What does specific power mean?
Specific power is power output per litre of displacement. A naturally aspirated engine at 100 HP/L is considered high performance. Turbocharged engines routinely exceed 150 HP/L. An OM606 at 518 HP from 3.0 litres produces approximately 173 HP/L.
Why does the HP formula use 7121 and not 9549?
The constant 9549 gives kilowatts, not horsepower. To get SAE horsepower directly from Nm and RPM the correct constant is 7121. Using 9549 gives a result roughly 25% too high, which is a common mistake in online calculators.
How does the power curve relate to the torque curve?
Power is mathematically derived from torque and RPM — HP = (Nm × RPM) / 7121. The HP curve shown is calculated directly from the torque curve, so they are always mathematically consistent. The torque curve peaks first, then the HP curve peaks at a higher RPM as rising RPM compensates for falling torque.