Drag racers have long estimated power from time slips. The trap-speed formula is usually more reliable because it depends less on the launch and traction; the elapsed-time formula is sensitive to wheelspin and reaction at the start. Both estimate power at the wheels converted back to an approximate flywheel figure.
How to use the Horsepower Calculator
- Race weight (car + driver)
- Quarter-mile elapsed time
- Trap speed
- Every result field is editable: type into Horsepower from elapsed time and Quarter-mile elapsed time plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the race weight of the car with driver, then the quarter-mile elapsed time and/or the trap speed.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Race weight (car + driver) | Units: lb, kg, g, oz |
| Quarter-mile elapsed time | In s |
| Trap speed | Units: mph, km/h, m/s, knots |
What each result means
- Horsepower from elapsed time = ([Race weight (car + driver)]÷0.45359237)÷([Quarter-mile elapsed time]÷5.825)^3
- Horsepower from trap speed = ([Race weight (car + driver)]÷0.45359237)×([Trap speed]÷0.44704÷234)^3
- Trap-speed power in kW = [Horsepower from trap speed]×0.74569987158227
Example values
How the main result changes when Quarter-mile elapsed time changes and everything else stays at the example values above:
| Quarter-mile elapsed time (s) | Horsepower from elapsed time (hp) |
|---|---|
| 6.75 | 2,249 |
| 10.13 | 666 |
| 13.5 | 281 |
| 16.88 | 144 |
| 20.25 | 83 |
| 27 | 35 |
Formula
From elapsed time: hp = weight (lb) ÷ (ET ÷ 5.825)³From trap speed: hp = weight (lb) × (trap speed mph ÷ 234)³Worked example
Inputs — Race weight (car + driver): 3,500 lb; Quarter-mile elapsed time: 13.5 s; Trap speed: 105 mph.
Results — Horsepower from elapsed time = 281 hp; Horsepower from trap speed = 316 hp; Trap-speed power in kW = 236 kW.
Frequently asked questions
Which formula is more accurate?
The trap-speed formula, because a poor launch hurts elapsed time much more than trap speed.
What ET does 400 hp give?
A 3,500 lb car with 400 hp should run about 12.4 s in the quarter mile.
Is this wheel or crank horsepower?
The formulas approximate flywheel (crank) horsepower for a typical drivetrain.
What formula does the Horsepower Calculator use?
From elapsed time: hp = weight (lb) ÷ (ET ÷ 5.825)³.