Horsepower Calculator
Find horsepower from torque and RPM, from an electric motor's volts and amps, or from a quarter mile time slip, with kW, metric PS and every step shown.
Drivetrain loss and vehicle weight (optional)
Motor speed for shaft torque (optional)
Crankshaft output from 400 lb·ft at 5,000 RPM, using 1 hp = 550 ft·lbf/s.
Torque needed for 450 hp
That is 72.7 lb·ft (98.6 N·m) more than you entered. At 400 lb·ft, 450 hp arrives at 5,909 RPM.
Your calculation
The same steps the calculator runs, with your numbers.
- Constant: 33,000 ÷ 2π = 5,252.11.
- Horsepower: 400 × 5,000 ÷ 5,252.11 = 380.80 hp.
- Kilowatts: 380.80 × 0.7457 = 283.96 kW.
- Metric horsepower: 283,962 W ÷ 735.49875 = 386.08 PS.
- Torque for 450 hp: 450 × 5,252.11 ÷ 5,000 = 472.69 lb·ft.
Horsepower at other engine speeds
The same 400 lb·ft held at each speed. A real engine makes less torque away from its peak, so treat the faster rows as an upper limit.
| RPM | Horsepower | kW | Relative |
|---|---|---|---|
| 2,500 | 190.4 | 142.0 | |
| 3,750 | 285.6 | 213.0 | |
| 5,252 (crossover) | 400.0 | 298.3 | |
| 6,250 | 476.0 | 355.0 | |
| 7,500 | 571.2 | 425.9 |
This is crank (brake) horsepower. A chassis dyno reads lower wheel horsepower because the drivetrain absorbs part of the power; add a loss percent under the optional inputs to estimate it.
How the Horsepower Calculator Works
This horsepower calculator has three modes. Torque and RPM turns torque at a given speed into horsepower, kW and metric PS, and finds the torque or speed for a target output. Electrical finds motor shaft horsepower from volts, amps, efficiency and power factor, and the current a motor of a given size draws. Quarter mile estimates engine horsepower from race weight with elapsed time (ET), trap speed or both. Each mode shows the exact arithmetic so you can check it by hand.
What Is Horsepower?
Horsepower is a unit of power, the rate at which work is done. James Watt adopted the value in the late 18th century after experiments with dray horses: one horsepower equals 33,000 foot pounds of work per minute, the power needed to lift 33,000 pounds one foot in one minute. Dividing by 60 gives the form used in modern standards, 550 foot pound force per second. Encyclopaedia Britannica notes that this figure is about 50 percent more than an average horse can sustain over a working day.
NIST Special Publication 811 lists 1 hp (550 ft·lbf/s) as 745.6999 watts. Other horsepower units exist, and mixing them up causes small errors:
- Mechanical horsepower (hp): 550 ft·lbf/s = 745.69987 W.
- Metric horsepower (PS, CV): 75 kilogram force meters per second = 735.49875 W.
- Electrical horsepower: exactly 746 W, used in motor formulas.
- Boiler horsepower: 9,809.5 W, or 13.15 mechanical hp.
BHP meaning: brake horsepower is horsepower measured at the engine's output shaft with a brake dynamometer. Indicated horsepower is calculated from cylinder pressure and is higher, because it includes the power the engine spends on its own internal friction.
Horsepower Formula: Torque × RPM ÷ 5,252
Power is torque multiplied by rotational speed. One revolution turns the shaft through 2π radians, so a torque of T pound feet at N revolutions per minute does 2π × T × N foot pounds of work each minute. Dividing by Watt's 33,000 foot pounds per minute gives horsepower, and the two constants combine into the familiar 5,252.
The calculator uses the exact 33,000 ÷ 2π and 60,000 ÷ 2π, so it can differ from a hand calculation with 5,252 in the second decimal. The horsepower to torque form is the same equation solved for torque, which gives the torque needed for a target horsepower.
How to calculate horsepower manually
- Read the torque and the RPM at which it occurs from the same point on a dyno chart. Peak torque and peak horsepower usually occur at different speeds.
- If the torque is in newton meters, multiply by 0.73756 to get pound feet, or use the kW formula directly.
- Multiply torque by RPM and divide by 5,252.
- Multiply by 0.7457 for kilowatts, or by 1.01387 for metric PS.
Why Torque and Horsepower Cross at 5,252 RPM
Torque × RPM ÷ 5,252 equals the torque only when RPM is 5,252. So on any dyno chart that plots pound feet and horsepower on the same vertical scale, the curves meet at 5,252 RPM. The crossing comes from the units chosen for the chart, not from anything inside the engine.
| Engine speed | Torque | Horsepower | Larger number |
|---|---|---|---|
| 3,000 RPM | 300 lb·ft | 171.4 hp | Torque |
| 5,252 RPM | 300 lb·ft | 300.0 hp | Equal |
| 6,500 RPM | 300 lb·ft | 371.3 hp | Horsepower |
The crossover moves with the units. With newton meters and kilowatts, the numbers match at 9,549 RPM. With newton meters and mechanical horsepower, they match at 7,121 RPM. A diesel truck engine that redlines near 3,000 RPM never reaches the crossover, so its torque figure in pound feet is always larger.
Horsepower Conversions: hp to kW, PS, BTU/h and ft·lbf/s
To convert hp to kW, multiply by 0.7457. To convert kW to hp, multiply by 1.341. Each row of the table gives one unit expressed in the others, using the NIST conversion factors.
| 1 unit of | Watts | Mechanical hp | PS | BTU/h | ft·lbf/s |
|---|---|---|---|---|---|
| Mechanical hp | 745.70 | 1 | 1.01387 | 2,544.43 | 550 |
| Metric hp (PS) | 735.50 | 0.98632 | 1 | 2,509.63 | 542.48 |
| Electrical hp | 746 | 1.00040 | 1.01428 | 2,545.46 | 550.22 |
| Boiler hp | 9,809.5 | 13.1548 | 13.3372 | 33,471.4 | 7,235.12 |
| Kilowatt (kW) | 1,000 | 1.34102 | 1.35962 | 3,412.14 | 737.56 |
Common values: 100 hp = 74.57 kW, 150 hp = 111.85 kW, 200 hp = 149.14 kW, 300 hp = 223.71 kW and 500 hp = 372.85 kW. Going the other way, 50 kW = 67.05 hp, 100 kW = 134.10 hp, 200 kW = 268.20 hp and 300 kW = 402.31 hp.
Metric horsepower vs mechanical horsepower
Metric horsepower is about 1.4 percent smaller than mechanical horsepower. European manufacturers often quote engines in PS or CV, so the same engine shows a slightly larger number there: a car rated at 300 PS makes 295.9 mechanical hp, or 220.6 kW. The kilowatt is the SI unit and the safest figure to compare across markets.
Electric Motor Horsepower from Volts and Amps
A motor's horsepower rating is the mechanical power at its shaft. The electrical input is larger, because some power becomes heat inside the motor, and for AC supplies only the real part of volts times amps does work. The U.S. Department of Energy fact sheet on motor load gives three phase input power as volts × amps × power factor × √3 ÷ 1,000 kilowatts, and defines efficiency as output divided by input. Multiplying input power by efficiency and dividing by 746 gives output horsepower.
For three phase motors, V is the line to line voltage, which is why √3 appears. Using 745.7 W instead of 746 W changes the result by only 0.04 percent. The amps to horsepower direction checks a clamp meter reading; the reverse estimates full load current.
Horsepower from Quarter Mile ET and Trap Speed
Roger Huntington fitted an early trap speed formula from road and strip tests in the 1950s, and in 1973 physics professor Geoffrey Fox described the theory behind it in the American Journal of Physics. The calculator uses the later versions from engineer Patrick Hale of Racing Systems Analysis, which tie trap speed and ET to the ratio of weight to power.
Weight means race weight: the car with fuel and the driver. The result estimates net engine horsepower at the flywheel, not wheel horsepower. Because power enters the formulas as a cube, small timing changes move the estimate a lot: a 10 percent slower ET implies about 25 percent less power, and doubling power cuts ET by only about 21 percent.
Fox listed the factors that change real performance: tire grip, aerodynamic drag, rotating inertia, gearing, shifting, center of gravity and driver skill. Weight and power dominate trap speed, while ET depends far more on the launch, so the calculator leads with the trap speed estimate when both are present. A large gap between the two usually means time lost at the start.
Brake, Crank and Wheel Horsepower
Manufacturers rate engines at the crankshaft. This is brake horsepower, also called crank or flywheel horsepower, measured on an engine dynamometer. In the United States, SAE J1349 is the engine power test code for net power and torque ratings, intended to reflect the engine as installed in customer service.
Wheel horsepower is measured on a chassis dynamometer at the driven wheels. It is always lower, because the transmission, driveshaft, differential and tires absorb part of the power. The loss depends on the drivetrain layout, transmission type and dyno, so no single percentage fits every car. If you know yours, enter it under the optional inputs in Torque and RPM mode to see a wheel horsepower estimate.
How to Use the Calculator
- Torque and RPM. Enter torque in lb·ft or N·m and the engine speed. A target horsepower shows the torque or RPM it needs.
- Electrical. Pick the supply, then enter volts, amps, efficiency in percent and power factor.
- Quarter mile. Enter race weight with the driver, then ET, trap speed or both.
Common Mistakes
- Mixing N·m and lb·ft. 400 N·m is only 295 lb·ft.
- Pairing peak torque with peak RPM. Use torque and RPM from the same point on the curve.
- Dropping √3 on three phase. The result comes out 42 percent low.
- Entering efficiency as 0.85. The field takes a percent, so enter 85.
- Leaving out the driver. Race weight includes everyone and everything in the car.
Frequently Asked Questions
How do you calculate horsepower from torque and RPM?
Multiply torque in pound feet by engine speed in revolutions per minute and divide by 5,252. An engine making 400 lb·ft at 5,000 RPM produces 400 × 5,000 ÷ 5,252 = 380.8 horsepower. For metric units, multiply torque in newton meters by RPM and divide by 9,549 to get kilowatts, then divide by 0.7457 to get horsepower: 350 N·m at 4,500 RPM gives 164.9 kW, or 221.2 hp. To find torque from horsepower, reverse the formula: torque = hp × 5,252 ÷ RPM, so 300 hp at 6,000 RPM needs 262.6 lb·ft.
Why do torque and horsepower cross at 5,252 RPM?
One horsepower is 33,000 foot pounds of work per minute, and each revolution turns a torque through 2π radians, so horsepower equals torque × RPM ÷ (33,000 ÷ 2π), which is torque × RPM ÷ 5,252.11. At 5,252 RPM the two numbers are equal, so a dyno chart that plots lb·ft and hp on the same scale shows the curves crossing there. Below 5,252 RPM the torque figure is larger, and above it the horsepower figure is larger. The crossing comes from the units, not from the engine. With newton meters and kilowatts the curves cross at 9,549 RPM instead.
How do I convert hp to kW?
Multiply mechanical horsepower by 0.7457 to get kilowatts, and multiply kilowatts by 1.341 to get horsepower. The exact factor is 745.69987 watts per horsepower, which follows from the definition 1 hp = 550 foot pound force per second. So 150 hp = 111.9 kW, 300 hp = 223.7 kW and 150 kW = 201.2 hp. Electric motors often use the electrical horsepower of exactly 746 watts, which differs by 0.04 percent. Metric horsepower (PS) is smaller at 735.49875 watts, so 1 kW = 1.36 PS and 300 PS = 295.9 hp.
What is the difference between hp, bhp and PS?
Hp usually means mechanical horsepower: 550 foot pound force per second, or 745.7 watts. Bhp, brake horsepower, is the same unit measured at the engine output shaft with a brake dynamometer, before transmission losses, so it reads higher than the wheel horsepower a chassis dynamometer reports. PS, from the German Pferdestärke, is metric horsepower: 75 kilogram force meters per second, or 735.49875 watts. European brochures also write it as CV. One PS equals 0.9863 hp, so a car rated at 300 PS makes 295.9 hp or 220.6 kW.
How do you calculate electric motor horsepower from volts and amps?
Multiply volts by amps, efficiency and power factor, then divide by 746. For a three phase motor, also multiply by √3, about 1.732, and for DC use a power factor of 1. A single phase motor drawing 15 A at 230 V with 85 percent efficiency and a 0.8 power factor delivers 230 × 15 × 0.85 × 0.8 ÷ 746 = 3.14 hp. A three phase motor drawing 50 A at 480 V, 92 percent efficient with a 0.85 power factor, delivers about 43.6 hp. Reversing the formula gives current: a 5 hp single phase motor at 230 V with the same efficiency and power factor draws about 23.85 A.
How do you estimate horsepower from a quarter mile time?
Use Patrick Hale's empirical formulas with the race weight in pounds, including the driver. From trap speed, hp = weight × (mph ÷ 234)³. From elapsed time, hp = weight × (5.825 ÷ ET)³. A 3,500 lb car with a 105 mph trap speed gives 3,500 × (105 ÷ 234)³ = 316.2 hp, and a 13.0 second ET gives 314.9 hp. The result estimates net engine horsepower, not wheel horsepower. Trap speed depends mostly on power and weight, while ET also depends on traction and the launch, so the trap speed estimate is usually the steadier of the two.
Sources & References
- National Institute of Standards and Technology. NIST Guide to the SI (Special Publication 811), Appendix B.9: Factors for Units Listed by Kind of Quantity (horsepower 550 ft·lbf/s, metric, electric and boiler horsepower, ft·lbf/s and Btu per hour in watts). nist.gov
- Encyclopaedia Britannica. Horsepower (Watt's 33,000 foot pounds per minute, metric horsepower, brake, shaft and indicated horsepower). britannica.com
- U.S. Department of Energy. Determining Electric Motor Load and Efficiency, fact sheet DOE/GO 10097 517, 1997 (three phase input power equation, NEMA efficiency definition). energy.gov
- SAE International. J1349: Engine Power Test Code, Spark Ignition and Compression Ignition, As Installed Net Power and Torque Rating, revised November 2025. doi.org
- Lucius, J. Calculators for 1/4 Mile ET and MPH vs. HP and Weight (history of the Huntington and Fox formulas and Patrick Hale's 234 and 5.825 constants). stealth316.com
- Fox, G. T. On the Physics of Drag Racing, American Journal of Physics 41, 311 to 313, 1973. doi.org
Results are estimates for planning and education. Motor nameplate data and dynamometer tests take priority. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.