Horsepower Calculator
Calculate engine or motor horsepower four ways: from torque and RPM (with lb-ft or N·m), from a power figure in watts, or from a quarter-mile run using either elapsed time or trap speed. Switch between mechanical, metric (PS) and electrical horsepower, reverse-solve for torque at any RPM, and get the full power breakdown in every common unit.
Formula
Worked example
A 350 lb-ft engine at 5252 rpm: (350 x 5252) / 5252 = 350 hp = 261 kW = 354.8 PS. Same engine at 6000 rpm: (350 x 6000) / 5252 = 399.8 hp. Reverse: what torque makes 400 hp at 6000 rpm? (400 x 5252) / 6000 = 350.1 lb-ft. Quarter mile: a 3500 lb car doing a 13.5-second ET: 3500 / (13.5/5.825)^3 = 212 hp.
How horsepower is calculated from torque and RPM
Horsepower measures the rate at which an engine does work. Torque measures the twisting force a shaft can apply. To turn a rotating force into a power figure you multiply torque by how fast the shaft spins. With torque in pound-feet and speed in revolutions per minute, the constant 5252 converts the units into mechanical horsepower. That constant derives from the 33,000 foot-pounds per minute in one horsepower divided by 2 x pi radians per revolution, so it is fixed and never changes. If your torque reading is in N·m (Newton-metres, common on European dynos), multiply by 0.737562 to convert to lb-ft first, or use the N·m option in this calculator to skip that step.
The four horsepower types and how to convert between them
Mechanical (imperial) horsepower is 745.7 watts and is the automotive standard in the US and UK. Metric horsepower, written PS in German or CV in French and Spanish, is 735.5 watts, about 1.4% smaller. European car specifications almost always use PS. Electrical horsepower is exactly 746 watts, defined by the IEEE, and appears on US motor nameplates. Boiler horsepower (about 9810 watts) is a steam-era unit rarely seen outside industrial boiler ratings. When comparing vehicles across markets, always check which standard the figure uses, because a German car rated at 300 PS is only 296 mechanical HP, a meaningful difference at high power levels.
Quarter-mile methods: estimating power from a drag run
If you have clocked a quarter-mile run you can estimate flywheel power without a dyno. The elapsed-time (ET) method uses the formula HP = weight / (ET / 5.825)^3, where weight is the total vehicle weight in pounds and ET is the time in seconds from start to finish. The trap-speed method uses HP = weight x (speed / 234)^3, where speed is the mph at the finish line. Trap speed tends to reflect peak power more reliably because it captures the car at its fastest point, while ET is influenced more by launch technique, grip, and gearing. Both assume a well-driven run in standard conditions; account for real-world variation by treating the result as an estimate within roughly 5 to 10 percent.
Why torque and horsepower cross at 5252 RPM
Plot torque and horsepower against engine speed on the same dyno chart and the two lines always intersect at exactly 5252 RPM. This falls straight out of the formula: at 5252 RPM the constant in the equation equals one, so torque in lb-ft and horsepower are numerically equal. Below that speed the torque number is larger; above it, horsepower pulls ahead. Knowing this helps you read a dyno graph and understand why peak power usually arrives at a higher RPM than peak torque. The reverse mode on this calculator uses the same relationship to find the torque required to produce a given horsepower at any RPM target.
Horsepower types and power unit equivalents
| From | To | Factor / Value |
|---|---|---|
| 1 mechanical HP | watts | 745.7 W |
| 1 metric HP (PS/CV) | watts | 735.5 W |
| 1 electrical HP | watts | 746.0 W |
| 1 kW | mechanical HP | 1.341 hp |
| 1 kW | metric HP (PS) | 1.360 PS |
| 1 mechanical HP | BTU per hour | 2545 BTU/h |
| 1 mechanical HP | foot-pounds/second | 550 ft-lb/s |
| Torque = HP at | 5252 RPM | (always) |
| 1 lb-ft | N·m | 1.35582 N·m |
| 1 N·m | lb-ft | 0.73756 lb-ft |
All conversions from one mechanical horsepower as the reference unit.
Frequently asked questions
Why is the constant 5252 used in the horsepower formula?
It comes from the definition of horsepower: 33,000 foot-pounds of work per minute. Dividing 33,000 by 2 x pi (the radians in one revolution) gives 5252.11, the constant that converts torque in lb-ft and speed in RPM into mechanical horsepower. It is a fixed conversion constant that never changes.
What is the difference between mechanical HP, metric HP (PS) and electrical HP?
Mechanical (imperial) horsepower is 745.7 watts and is the automotive standard in the US and UK. Metric horsepower (PS or CV) is 735.5 watts, about 1.4% smaller, and is used in European car specifications. Electrical horsepower is exactly 746 watts, defined by the IEEE, and appears on US electric motor nameplates. Always check which type a specification refers to before comparing figures across markets.
How does the quarter-mile horsepower estimate work?
Two methods exist. The elapsed-time (ET) method uses HP = weight / (ET / 5.825)^3, where weight is in pounds and ET is in seconds. The trap-speed method uses HP = weight x (speed / 234)^3, where speed is in mph at the finish line. Both are estimates; drivetrain losses, launch quality, and track conditions all affect accuracy. The trap-speed method is generally considered more reflective of peak engine power.
Can I calculate torque if I only know horsepower and RPM?
Yes. Rearranging the formula gives Torque (lb-ft) = (HP x 5252) / RPM. Select the "Reverse: HP + RPM to torque" mode in this calculator and enter your horsepower and RPM figures to get the torque in both lb-ft and N·m instantly.
Can I get horsepower from torque alone, without RPM?
No. Horsepower is the rate of doing work, so you also need the rotational speed. The same torque produces more horsepower at higher RPM. A torque figure alone tells you how hard the engine can twist, not how much power it produces.
How accurate is this calculator for real engines?
The torque-and-RPM method gives exact results for a given measurement point, matching what a chassis dyno or engine dyno reports. The quarter-mile methods are empirical estimates, typically within 5 to 10 percent of dyno figures for well-driven runs. Watts-to-HP conversions are mathematically exact for the chosen HP standard.