Horsepower and Torque Converter
Convert between horsepower and torque at any RPM.
Calculate peak horsepower from torque curves, and understand the relationship between HP, torque, and engine speed.
The Relationship Horsepower = (Torque × RPM) / 5252 This constant (5252) comes from converting RPM and lb-ft to consistent units of power. 5252 = 33,000 ft·lb/min ÷ (2π) — derived from James Watt’s original definition of horsepower. James Watt (Scotland, 1782) defined 1 horsepower as the power of a horse lifting 33,000 lb × 1 ft in 1 minute.
HP and Torque Always Cross at 5252 RPM The horsepower and torque curves always intersect at exactly 5252 RPM (for any engine). Below 5252 RPM: torque curve is higher than HP curve. Above 5252 RPM: HP curve is higher than torque curve. This is a mathematical identity — not a coincidence.
What Torque and Horsepower Actually Mean Torque = twisting force. It’s what accelerates the car from a stop. High torque at low RPM = strong pulling power. Horsepower = rate of doing work. It determines top speed. HP = torque × angular velocity. At the same HP, a diesel with high torque at low RPM pulls harder; a gas engine with high RPM HP runs faster. A car accelerates based on torque at the wheels (torque × gear ratio × drivetrain efficiency).
Torque Units lb-ft (pound-feet): US standard. 1 lb-ft = 1.35582 N·m. N·m (Newton-meters): SI/metric standard. 1 N·m = 0.73756 lb-ft. kgf·m (kilogram-force meter): older metric. 1 kgf·m = 9.80665 N·m.
Power Units 1 mechanical horsepower (hp) = 550 ft·lb/s = 745.7 W 1 metric horsepower (PS or CV) = 75 kgf·m/s = 735.5 W 1 kW = 1.341 mechanical hp Engine dyno results are sometimes shown in kW (international) or PS (German/Japanese spec sheets).
Peak vs. Curve Most engines make peak torque at a lower RPM than peak HP. Diesel engines: peak torque 1,500–2,500 RPM. Peak HP 3,000–4,000 RPM. Naturally aspirated gas engines: peak torque 3,000–5,000 RPM. Peak HP 5,000–7,500 RPM. High-revving sports engines (e.g., Honda S2000): peak HP at 8,300 RPM.