Horsepower Calculator
Work out engine horsepower from torque and RPM, or convert any power figure between hp, kW, watts, and metric PS with the full formula shown.
🏎️ What is Horsepower?
Horsepower is a unit of power that measures how quickly work is done. It was coined by James Watt in the 1780s to compare the output of steam engines with the draft horses they replaced, defining one horsepower as 33,000 foot-pounds of work per minute. In rotating machinery, horsepower is produced by torque turning at a given speed, so it depends on both the twisting force and how fast the shaft spins.
Horsepower ratings appear everywhere: car and motorcycle engines, electric motors, pumps, generators, tractors, and power tools are all rated in horsepower or its metric cousins. When a manufacturer quotes a 200 hp engine, they mean it can sustain that rate of doing work at a specified RPM. Dynamometers measure torque across the RPM range and compute horsepower from it, which is why every dyno chart shows both a torque curve and a horsepower curve.
A common misconception is that horsepower and torque are the same thing, or that one is simply better than the other. They are related but distinct: torque is the rotational force, while horsepower is the rate at which that force does work. Because power equals torque multiplied by angular speed, an engine raises horsepower either by making more torque or by spinning faster. The two curves always cross at exactly 5252 RPM, a direct result of the defining constant.
This calculator computes horsepower from torque and RPM in either lb·ft or N·m, and also converts any power figure between mechanical horsepower (hp), metric horsepower (PS), kilowatts, and watts. Every result is shown with the exact formula and full arithmetic so you can follow the conversion.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Engine Power in lb·ft
A performance engine making 400 lb·ft at 5,000 RPM
Example 2 — Engine Power in N·m
A diesel engine making 500 N·m at 3,000 RPM
Example 3 — Convert kW to Horsepower
A 150 kW electric motor
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate horsepower from torque and RPM?
Multiply torque in pound-feet by engine speed in RPM and divide by 5252: hp = torque × RPM / 5252. For example, 400 lb·ft at 5000 RPM gives 400 × 5000 / 5252 = 380.8 hp. The constant 5252 comes from 33,000 ft·lb per minute per horsepower divided by 2π.
Why is the constant 5252 used in the horsepower formula?
One horsepower is defined as 33,000 foot-pounds of work per minute. Converting rotational work to that unit means dividing by 2π radians per revolution: 33,000 / (2 × π) = 5252. That is why hp = torque (lb·ft) × RPM / 5252, and why horsepower and torque curves always cross at 5252 RPM.
How do you convert kW to horsepower?
Divide kilowatts by 0.7457 to get mechanical horsepower, since 1 hp = 0.7457 kW (745.7 watts). A 150 kW motor equals 150 / 0.7457 = 201.2 hp. To convert horsepower back to kilowatts, multiply hp by 0.7457.
What is the difference between hp and PS?
Mechanical horsepower (hp) and metric horsepower (PS, from the German Pferdestärke) are slightly different units. 1 hp equals 745.7 watts, while 1 PS equals 735.5 watts. That makes 1 hp equal to about 1.014 PS, so a 200 hp engine is roughly 202.8 PS. European makers often quote PS, US and UK makers quote hp.
What is the difference between torque and horsepower?
Torque is the rotational force an engine produces, measured in lb·ft or N·m. Horsepower is the rate at which that torque does work, and it depends on both torque and RPM. An engine can have high torque but modest horsepower if it turns slowly, because power = torque × angular speed.
How do you calculate horsepower from N·m?
Convert to power in kilowatts first: kW = torque (N·m) × RPM / 9549, then divide by 0.7457 for horsepower. For example, 500 N·m at 3000 RPM gives 500 × 3000 / 9549 = 157.1 kW, which is 157.1 / 0.7457 = 210.6 hp. The 9549 constant is 60,000 / 2π.
What is metric horsepower (PS)?
Metric horsepower, written PS or cv, is defined as the power to raise 75 kilograms by 1 metre in 1 second, which equals 735.5 watts. It is slightly smaller than mechanical horsepower (745.7 watts). Many European car specifications list engine output in PS rather than hp or kW.
Is wheel horsepower the same as engine horsepower?
No. Engine (crank) horsepower is measured at the crankshaft, while wheel horsepower is measured at the drive wheels after drivetrain losses. Those losses are typically 10 to 15 percent for a manual gearbox and higher for automatics, so wheel horsepower is always lower than the quoted crank figure.
How much is 1 horsepower in watts?
One mechanical horsepower equals exactly 745.7 watts (more precisely 745.6999 W). One metric horsepower (PS) equals 735.5 watts. So a 100 hp engine produces about 74.6 kilowatts, and a 100 PS engine produces about 73.5 kilowatts of mechanical power.
Can this calculator convert between all power units?
Yes. Use the Convert Power mode to enter a value in horsepower, kilowatts, watts, or metric PS, and the calculator returns the equivalent in every other unit at once. The Engine mode instead derives power from torque and RPM, which is how dynamometers rate engines.