Horsepower and Torque Converter
Convert between horsepower and torque at any RPM.
Calculate peak power from torque curves and understand the HP, torque, and engine speed relationship.
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), which traces straight back to James Watt’s original definition.
Watt (Scotland, 1782) defined 1 horsepower as the power of a horse lifting 33,000 lb through 1 ft in 1 minute.
HP and Torque Always Cross at 5252 RPM The horsepower and torque curves always intersect at exactly 5252 RPM, on any engine ever built. Below 5252 RPM the torque number is the larger of the two. Above it, the horsepower number is.
There is nothing physical happening at 5252. It falls out of the arithmetic: at that engine speed the RPM/5252 multiplier equals one, so the two figures are numerically identical. Plot the same engine with torque in Newton-metres and the crossing moves, which is the giveaway. It is a property of the units, not of the motor.
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.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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