Horsepower Calculator
Work out horsepower from torque and rpm, from force and speed, from volts and amps, or straight from kilowatts. Every mode also shows the power in kW and PS, the torque at your rpm, and a quarter-mile estimate so the number means something.
Mechanical horsepower is 33,000 ft·lb/min = 550 ft·lb/s = 745.7 W, so hp = torque in ft·lb × rpm ÷ 5,252 — at exactly 5,252 rpm the torque and horsepower numbers are the same. 1 ft·lb = 1.356 N·m, and 1 kW = 1.341 hp.
Formula and calculation
Worked example
A 300 ft·lb engine spinning at 3,000 rpm makes 171.36 hp — 127.79 kW, or 173.74 PS. Pull 3,000 rpm on the same engine at only 150 ft·lb and the calculator would read half the power: twice the speed at the same torque is twice the horsepower, which is why power peaks where torque starts falling off.
Torque, rpm and the 5,252 crossover
Horsepower is not a second engine output sitting next to torque — it is torque times speed. Because one horsepower is 33,000 ft·lb/min and one revolution sweeps 2π radians, the two numbers are equal at 33,000 ÷ 2π ≈ 5,252 rpm. That is why every dyno chart shows the curves crossing there: below 5,252 rpm the torque number is the bigger one, above it horsepower is. A big slow-turning engine can make enormous torque and modest hp, while a small engine spinning to 8,000 rpm makes the same hp from much less torque.
Why an ET estimate is only an estimate
The 5.825 and 234 constants were fitted to real drag-strip passes, not derived from physics. They assume the weight is right (car with driver), the launch hooks up, gearing is sensible and drivetrain losses are already off the power figure. Change the tires, the air density or the launch and a real car drifts a few tenths either way — more on a cold track. Use the numbers to compare setups on paper, and treat the strip's timing ticket as the truth.
What the results mean
- Horsepower
- The power in mechanical horsepower (745.7 W): computed from torque and rpm, force and speed, volts and amps or kW, or the engine power you entered for the quarter mile.
- Kilowatts
- The same power in kilowatts (1 hp = 0.7457 kW). Enter a kW figure in the Convert kW tab to go the other way — one mode reads kW in, every mode reads it out.
- Metric horsepower
- Power in metric horsepower (PS), the European rating: 1 hp = 1.0139 PS, because PS is 735.5 W against 745.7 W.
- Torque at 3,000 rpm
- The torque at your rpm that this power corresponds to (hp × 5,252 ÷ rpm), in ft·lb with the N·m equivalent in the subtitle — the number a dyno chart plots.
- Quarter-mile ET
- Estimated quarter-mile time for the race weight you set, or for a 3,200 lb reference in the other modes — an empirical drag-racing estimate, good to a few tenths.
- Trap speed
- Estimated speed at the finish line of a quarter-mile run (mph), from the same rule of thumb as the ET and just as approximate.
- How this compares to people
- Your power set against typical human outputs — cruising cyclist, pro racer, pro sprint — so a big hp number has a human scale beside it.
- Substituting your numbers
- The arithmetic with your numbers substituted, so the result can be checked by hand.
Frequently asked questions
How do I convert horsepower to kilowatts?
Multiply by 0.7457: 1 hp = 745.7 W, so 100 hp is 74.57 kW, and 1 kW = 1.341 hp. Metric horsepower (PS) is a smaller unit, 735.5 W, so 100 hp is 101.4 PS. The Convert kW tab does the kW-to-hp direction for you; every other mode prints hp and kW side by side.
Why is 5,252 used in the horsepower formula?
One horsepower is 33,000 ft·lb per minute, and one crank revolution is 2π radians of rotation. Dividing 33,000 by 2π gives 5,252.11, so hp = torque in ft·lb × rpm ÷ 5,252. It is also why the torque and horsepower numbers are equal at 5,252 rpm on a dyno chart.
How accurate is the quarter-mile ET estimate?
It is the classic drag-racing rule of thumb rather than physics: ET = 5.825 × (weight ÷ hp)^(1/3) seconds, with trap speed from 234 × (hp ÷ weight)^(1/3) mph. Real passes scatter by a few tenths of a second with traction, tires, gearing and air density, and the weight must include the driver, so treat it as a comparison tool.
How much power can a person produce?
Barely trained, an adult sustains about 100 W (0.13 hp) on a bike; a fit recreational rider about 200 W; a pro about 400 W for an hour, with a five-second sprint near 1,200 W (1.6 hp). For scale, a legal EU e-bike motor is limited to 250 W and a US one to 750 W — this calculator's reference table lists them all.
What is the difference between hp, PS and bhp?
bhp is brake horsepower — power measured at the engine, as on a dyno, with no drivetrain losses. PS is metric horsepower, 735.5 W against 745.7 W for mechanical hp, so the same engine reads about 1.4% higher in PS. Electrical horsepower is defined as exactly 746 W. The definitions table shows your figure in each unit.