A horsepower calculator converts engine torque and RPM into horsepower using HP = torque × RPM ÷ 5,252. 300 lb-ft at 4,000 RPM is about 228.5 hp.
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How to use the horsepower calculator
- Enter torque and choose pound-feet or newton-meters.
- Enter engine speed in RPM.
- Read the horsepower.
What is the horsepower formula?
The horsepower calculator turns engine torque and speed into horsepower. One horsepower is 550 foot-pounds of work per second, and with torque in pound-feet and speed in revolutions per minute the constants reduce to 5,252:
- Horsepower = torque (lb-ft) × RPM ÷ 5,252
- Torque (lb-ft) = horsepower × 5,252 ÷ RPM
- Kilowatts = horsepower × 0.7457
The 5,252 comes from 33,000 ÷ (2π): 33,000 foot-pounds per minute is one horsepower, and one revolution is 2π radians.
Horsepower calculator worked example
An engine making 300 lb-ft of torque at 4,000 RPM produces 300 × 4,000 ÷ 5,252 = 228.5 horsepower, or about 170 kW. At exactly 5,252 RPM, torque and horsepower are always equal, which is why the two curves cross there on every dyno chart plotted in these units.
| Torque | RPM | Horsepower | kW |
|---|---|---|---|
| 150 lb-ft | 5,000 | 142.8 | 106.5 |
| 200 lb-ft | 4,500 | 171.4 | 127.8 |
| 250 lb-ft | 5,252 | 250.0 | 186.4 |
| 300 lb-ft | 4,000 | 228.5 | 170.4 |
| 300 lb-ft | 5,252 | 300.0 | 223.7 |
| 400 lb-ft | 4,000 | 304.6 | 227.2 |
| 450 lb-ft | 5,500 | 471.2 | 351.4 |
| 600 lb-ft | 3,000 | 342.7 | 255.6 |
Units and conversions
NIST lists one mechanical horsepower (550 ft·lbf/s) as about 745.7 watts. Metric horsepower (PS) is slightly smaller, about 735.5 watts. If your torque is in newton-meters, choose N·m in the horsepower calculator; it uses kW = N·m × RPM ÷ 9,549 and then divides by 0.7457. 400 N·m at 5,000 RPM is about 209 kW, or 281 hp. The calculator also shows kilowatts for lb-ft inputs and rejects negative values.
Torque vs horsepower
Torque is the twisting force the engine applies; horsepower is how fast that work gets done. A diesel truck engine may make huge torque at low RPM, which helps it pull loads, while a sports car engine makes less torque but at much higher RPM, giving more peak horsepower. Gearing multiplies torque at the wheels but cannot add horsepower.
Tips
- Use torque and RPM measured at the same point; peak torque and peak horsepower happen at different speeds.
- Enter torque in pound-feet; the result is mechanical horsepower.
- Results are crank figures if your torque is a crank figure.
How horsepower is measured
Most published figures come from an engine dynamometer, which measures torque at the crankshaft across the rev range and then uses the 5,252 formula to calculate horsepower at each speed. A chassis dyno measures at the wheels instead, so it reads lower because the transmission, driveshaft and tires absorb power. Losses of 10% to 20% are typical, so a car rated at 300 crank horsepower might show about 250 to 270 at the wheels. Air temperature, altitude and humidity also change the result, which is why dyno sheets list a correction standard.
Everyday horsepower examples
A typical push lawn mower engine makes about 3 to 7 horsepower, a compact car about 120 to 180, a family pickup 250 to 400 and a large diesel tractor-trailer engine 400 to 600, though the truck engine makes far more torque. You can use the horsepower calculator to compare these engines at the same speed: at 2,000 RPM, 1,000 lb-ft gives 380.8 hp, while 200 lb-ft gives just 76.2 hp. Electric motors make strong torque from zero RPM, so their horsepower rises quickly with speed until the motor reaches its power limit.
The horsepower calculator is also handy in reverse. If a spec sheet lists 250 hp at 6,000 RPM, the torque at that speed is 250 × 5,252 ÷ 6,000 = 218.8 lb-ft. Comparing that with the peak torque figure tells you how much the torque falls off at high RPM.
Where the horsepower unit came from
James Watt coined the horsepower in the late 1700s to compare his steam engines with the draft horses they replaced. He estimated that a horse could do 33,000 foot-pounds of work per minute, a figure that became the standard definition used today. Real horses can briefly produce much more than one horsepower, but the unit stuck as a convenient way to rate engines and motors.
Related converters and calculators
Related calculators:
- Engine horsepower calculator: engine specs
- Torque unit converter: torque units
- Unit converter: units
- Fuel cost calculator: fuel costs
Sources
NIST Guide to the SI, Appendix B.9: Conversion factors.
Questions about this calculator
What is the horsepower formula?
HP = torque (lb-ft) × RPM ÷ 5,252.
Why 5252?
It equals 33,000 ÷ 2π.
How many watts is 1 hp?
About 745.7 W.
When do torque and horsepower match?
At 5,252 RPM.
Does it work with N·m?
Yes, choose N·m as the torque unit.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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