Engine Horsepower Calculator
Calculate engine horsepower from trap speed and vehicle weight at the drag strip.
Related calculators
About
Engine Horsepower Calculator
Two calculators sit above this, and both work backwards from a quarter-mile run rather than from anything under the bonnet. Give either one the weight of the car and a single measurement from the strip, and it estimates the power the engine had to be making to produce that result.
The elapsed time method
This one uses how long the car took to cover the quarter mile, 402.3 metres, from a standing start:
horsepower = weight ÷ (ET ÷ 5.825)3
Weight is in pounds and ET in seconds. A 5,000 pound vehicle running a 20 second quarter comes out at about 124 horsepower, which is 92,115 watts.
The 5.825 is not derived from physics. It is a fitted constant, usually credited to the motoring writer Roger Huntington, chosen because it made the equation agree with real cars and real timing slips. That is worth knowing because it explains the method's limits: it describes the population of cars it was fitted to, and something far outside that population will not fit as well.
The cube is the important part. Power scales with the cube of speed, so shaving a tenth off an ET is worth far more power at the quick end than at the slow end. Going from 14.0 to 13.9 seconds needs about 2% more power; going from 20.0 to 19.9 needs about 1.5%.
The trap-speed method
This one uses the speed the car was doing as it crossed the line, which drag strips record as the trap speed:
horsepower = weight × (speed ÷ 234)3
Weight in pounds, speed in miles per hour. The same 5,000 pound vehicle crossing at 70 mph comes out at about 134 horsepower, or 99,812 watts.
Use the speed at the line, not the average over the run. The average across a quarter mile is roughly half the trap speed, so putting the wrong one in produces an answer around eight times too small.
The 234 is fitted in the same way as 5.825, and you will see other constants in circulation. Values from 224 to 234 all appear, with the lower ones intended for slippery, low-drag cars. The constant is where the method's assumptions about aerodynamic drag are buried.
Why the two answers differ
Feed the same run into both and they rarely agree, as the example above shows: 124 against 134 from one car. That gap is information rather than an error.
Elapsed time includes everything that happened at the start. A car that spins its wheels, bogs off the line or launches badly posts a slow ET while still having the engine it always had, so the ET method reads low. Trap speed cares much less about the first sixty feet, because by the time the car reaches the line the launch is ancient history and what is left is mostly power against drag.
The practical rule: if the two disagree by a lot, the trap-speed figure is usually closer to the engine and the difference is telling you about traction. Two runs from the same car with similar trap speeds but very different ETs is a launch problem, not a power problem.
Both estimate power at the flywheel rather than at the wheels, since that is what they were fitted against, and both carry perhaps 10% to 15% of uncertainty in normal use. Treat a result as a bracket, not a number.
Getting the inputs right
Weight means the car as it ran, with fuel, the driver in it and anything else on board. Kerb weight from a brochure will understate it by a couple of hundred pounds once a driver and a half tank are added, and since the whole formula is linear in weight, a 5% error in weight is a 5% error in the answer.
Take the ET and the trap speed off the timing slip rather than from a phone app or a passenger with a stopwatch. Reaction time is not part of the ET on a proper slip, which is exactly what you want here.
Before you measure anything
Both methods need a full-throttle run to a quarter mile, and that belongs on a drag strip or at a sanctioned track day. Not on a public road. Driving above the limit is illegal, the driver's attention is on driving rather than on measuring, and the consequences of a mistake at those speeds reach past the person making it.
Anywhere legal, check the car first. Tyres correctly inflated and in good condition, brakes and fluids in order, nothing loose in the cabin, and an engine that is properly tuned and warm. Taking a vehicle to its limit finds whatever was already marginal.
What horsepower actually is
Horsepower is a rate of doing work, and it exists because James Watt needed to sell steam engines to people who owned horses. Working with pit ponies hauling coal, he estimated what a horse could sustain, then rounded the figure up to a round number: one horsepower is 33,000 foot-pounds per minute, or 550 foot-pounds per second.
Concretely, a horse lifting 330 pounds of coal 100 feet in one minute is working at one horsepower. The number is arbitrary, and picked generously, but it gave buyers a way to compare an engine against the animal it was replacing, and it stuck. Watt got the SI unit of power named after him in return.
| Unit | Equal to | Where you see it |
|---|---|---|
| Mechanical horsepower (hp) | 745.70 W, or 550 ft·lbf per second | US and UK figures, and both formulas here |
| Metric horsepower (PS, ch, pk) | 735.50 W | European brochures, about 1.4% higher for the same engine |
| Kilowatt (kW) | 1,000 W | The SI unit, and most registration documents |
The two horsepowers are close enough to be confused and far enough apart to matter. A 200 PS engine is about 197 hp, so a European figure quoted as horsepower without qualification usually flatters the car slightly.
How horsepower is really measured
On a dynamometer. The engine drives a rotor inside a housing, the dyno applies a measured load, and how hard the engine can turn that load at a given speed gives the power. Run it across the rev range and you get a curve rather than a single number, which is why a brochure quotes something like 320 hp at 6,500 rpm: the peak, and the engine speed where it happens.
Where the dyno is attached changes the answer. Gross horsepower is measured on an engine stripped of the accessories a real car needs. Net horsepower is measured with the water pump, alternator, power steering and exhaust attached, and is lower. A chassis dyno measures at the wheels, which is lower again after transmission and driveline losses. American figures switched from gross to net for 1972, which is why the same engine appears to lose a third of its power between two model years without changing at all.
Horsepower against torque
Torque is a twisting force, measured in pound-feet: ten pounds of force on a one foot wrench is ten pound-feet. Horsepower is a rate of work. Torque is what the engine can push with; horsepower is how quickly it can keep doing it.
They are tied together by engine speed:
horsepower = torque × rpm ÷ 5,252
That 5,252 is not arbitrary either. It is 33,000 divided by 2π, the conversion from a twisting force at some rpm into foot-pounds per minute. One consequence is a party trick worth knowing: on any dyno chart, the horsepower and torque curves always cross at 5,252 rpm, because that is where the two are numerically equal.
The old comparison holds. A tractor makes big torque at low rpm and is geared to push; a racing engine makes its power high in the rev range and is geared to go. Put the same peak horsepower in both and you get very different machines, because the gearing decides whether that power arrives as pulling force or as speed.
Power to weight
Power alone does not make a car quick. What matters is power divided by the mass it has to move, which is why both formulas above take weight as an input at all.
| Vehicle | Power | Weight | hp per pound |
|---|---|---|---|
| Sports car | 800 hp | 3,500 lb | 0.229 |
| Large SUV | 300 hp | 4,500 lb | 0.067 |
| Family hatchback | 150 hp | 2,900 lb | 0.052 |
The sports car in that table has roughly three and a half times the SUV's power per pound, which is most of why it feels like a different category of object. It is also why shedding weight is a legitimate route to acceleration: removing 10% of the mass does about as much for the ratio as adding 11% more power, and it is usually cheaper.
Reading your result
Take the horsepower figure as a bracket of roughly plus or minus 10%, and prefer the trap-speed answer when the two disagree. A single run at a track you have not been to before, in weather you have not run in before, is one data point rather than a measurement.
Air matters more than most people expect. A hot, humid, high-altitude day costs a naturally aspirated engine real power, which is exactly why sanctioned racing corrects timing figures for atmospheric conditions before anyone compares them.
To convert the result into other power units, the Conversion Calculator handles watts, kilowatts and horsepower directly, and the Speed Calculator covers the distance, speed and time side of a run.
Common questions
Frequently asked questions
Divide the vehicle weight in pounds by the elapsed time divided by 5.825, cubed. A 5,000 pound car running 20 seconds gives 5,000 divided by (20 / 5.825) cubed, which is about 124 horsepower.
Multiply the weight in pounds by the trap speed in mph divided by 234, cubed. The same 5,000 pound car crossing at 70 mph gives about 134 horsepower. Use the speed at the line, not the average speed over the run.
Because elapsed time includes the launch and trap speed mostly does not. A car that spins or bogs off the line posts a slow ET with the same engine, so the ET method reads low. When they disagree, the trap-speed figure is usually closer to the engine.
Within roughly 10% to 15% for an ordinary car. Both constants were fitted to real vehicles rather than derived, so anything unusual in weight, drag or traction fits the pattern less well. A dynamometer is the only way to actually measure power.
33,000 foot-pounds of work per minute, or 550 per second, which is 745.7 watts. James Watt set the figure from what a pit pony could sustain and rounded it up, so a horse lifting 330 pounds of coal 100 feet in a minute is working at one horsepower.
Mechanical horsepower is 745.70 watts and metric horsepower, written PS or ch, is 735.50 watts. PS figures are about 1.4% higher for the same engine, so a 200 PS car is about 197 hp.
Torque is twisting force, horsepower is the rate of work, and they are linked by horsepower equals torque times rpm divided by 5,252. Torque is what pushes the car; horsepower is how fast it can keep pushing. The two curves always cross at 5,252 rpm on a dyno chart.
At the flywheel, since that is what they were fitted against. A chassis dyno reads lower because it measures after transmission and driveline losses, typically 10% to 15% less on a manual car.