Horsepower calculator
Three ways to work out horsepower: from the trap speed or elapsed time on a time slip and the car’s weight, or from a torque reading and the engine speed it was taken at.
Work out horsepower
Engine power, about
620 hp
At the crank, by Hale’s formula from the trap speed.
- From trap speed, Hale620 hp
- From trap speed, Fox653 hp
These formulas were fitted to older cars. For a modern performance car or an EV they often come out a fifth or more above the rated power. An ET-based number comes out low when the tires spin or the launch is soft. A dyno at the wheels reads lower than all of these.
Engine power, about: 620 hp
As the car ran: its curb weight plus about 165 lb for the driver.
How the calculator works
From a time slip, it turns the classic quarter-mile formulas around. With weight in pounds, including the driver:
hp = weight × (trap speed ÷ 234)³ (Hale), or ÷ 230 (Fox)
hp = weight ÷ (ET ÷ 5.825)³ (Hale), or ÷ 6.269 (Fox)
Both estimate power at the engine. Trap speed is the better guide, because a poor launch slows the ET far more than the speed at the line. Jeff Lucius on the Hale and Fox formulas
From torque, it uses the definition of horsepower: 33,000 foot-pounds of work a minute.
hp = torque (lb-ft) × rpm ÷ 5,252
Trying it on real time slips
Here are Hale’s formulas run on road tests of cars whose power is published. For the everyday Civic the trap-speed formula lands close, though its ET formula reads 20% low. For the modern performance cars and EVs the trap-speed formula comes out above the rating, most by a fifth or more. Part of that is real: some engines make more than their rating, like BMW’s S58 six, which shows more on a dyno than BMW claims. So for a modern fast car, read the answer as a high estimate.
| Car | Time slip | From trap | From ET | Rated |
|---|---|---|---|---|
| BMW M4 Competition | 11.0 s @ 125 mph | 620 | 604 | 503 |
| BMW M3 Competition | 11.0 s @ 124 mph | 609 | 607 | 503 |
| BMW M2 (auto) | 11.6 s @ 124 mph | 592 | 504 | 473 |
| BMW M2 (manual) | 12.1 s @ 120 mph | 529 | 437 | 473 |
| Model Y Performance | 11.5 s @ 122 mph | 652 | 598 | 510 |
| Model Y Long Range | 12.3 s @ 115 mph | 545 | 488 | 397 |
| Cybertruck AWD | 12.4 s @ 111 mph | 726 | 705 | 600 |
| Cyberbeast | 11.0 s @ 119 mph | 930 | 1,050 | 834 |
| Model S Plaid | 9.4 s @ 151 mph | 1,341 | 1,187 | 1,020 |
| Honda Civic Sport | 16.9 s @ 86 mph | 152 | 126 | 158 |
More calculators
Tested times and mods, car by car
Questions
Is this horsepower at the engine or at the wheels?
At the engine. The formulas estimate engine power, not wheel power (Huntington measured his at the clutch). A chassis dyno measures at the wheels and reads lower: BimmerLife allows 15 percent for the drivetrain on a BMW M4.
Why is my trap-speed answer higher than my car’s rating?
The formulas were fitted to cars from the 1950s to the 2000s, and on the modern fast cars in the table on this page they come out well above the rating. Some of that is real: some engines make more than their rating, and BMW’s S58 six shows more on a dyno than BMW claims.
Where does 5,252 come from?
One horsepower is 33,000 foot-pounds of work a minute. An engine making 1 lb-ft at 1 rpm does 2π foot-pounds a minute, so horsepower is torque × rpm × 2π ÷ 33,000, which is torque × rpm ÷ 5,252.
How do I convert kW to hp?
One horsepower is 745.7 watts, so divide kilowatts by 0.7457 (or multiply by 1.341). The calculator does it for you in metric units.