Quarter-Mile Horsepower Calculator
Estimated engine power of a car from a quarter-mile run, using both the trap speed and the elapsed time, as drag racers have done for decades.
Results
The dashed line marks the value you entered.
What this tool does
Drag racers use two rules of thumb to back out engine power from a timeslip. The trap speed method is the more trustworthy, because the speed at the end of the run depends mainly on power and weight; the elapsed time depends heavily on launch, traction and gear changes, so a poor start makes the car look weaker than it is. The constants were fitted to rear-wheel-drive cars with manual gearboxes, and the result approximates flywheel power; four-wheel-drive cars and automatic gearboxes tend to read differently.
Formula
hp ≈ weight[lb] × (trap speed[mph] ÷ 234)³ · hp ≈ weight[lb] ÷ (ET[s] ÷ 5.825)³
Variables
| Symbol | Meaning | Unit |
|---|---|---|
w | Vehicle weight with driver | kg |
tr | Trap speed at the end of the 1/4 mile | km/h |
et | Quarter-mile time (ET) | s |
HT | Power from the trap speed | hp |
KT | Power from the trap speed in kW | kW |
HE | Power from the elapsed time | hp |
Worked example
- Vehicle weight with driver1500 kg
- Trap speed at the end of the 1/4 mile160 km/h
- Quarter-mile time (ET)13.5 s
- Power from the trap speed254 hp
- Power from the trap speed in kW189 kW
- Power from the elapsed time266 hp
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.