Horsepower Calculator
Estimate horsepower three different ways -- from a quarter-mile elapsed time or trap speed, from torque and RPM, or by converting a dyno's wheel HP reading to crank HP for your drivetrain.
Quarter-mile horsepower (ET & trap speed)
Two independent rule-of-thumb estimates from the same run -- a big gap between them usually means a bad launch, wheelspin, or a run that wasn't on a prepped surface.
ET method
273 hp
Trap-speed method
290 hp
Torque to horsepower
Horsepower
347 hp
Wheel HP ↔ crank HP (drivetrain loss)
Estimated crank HP
353 hp
How it's calculated
Elapsed-time method: HP = weight ÷ (ET ÷ 5.825)³. This is the standard drag-strip rule of thumb (not a lab-measured constant) for estimating horsepower from a quarter-mile time and vehicle weight. For a 3,400 lb car running a 13.50-second ET: 3,400 ÷ (13.50 ÷ 5.825)³ = 273 hp.
Trap-speed method: HP = weight × (trap speed ÷ 234)³, the companion rule of thumb using trap speed instead of ET. The same 3,400 lb car trapping 103 mph: 3,400 × (103 ÷ 234)³ = 290 hp.
Torque to horsepower: HP = torque (lb-ft) × RPM ÷ 5,252 -- an exact conversion, not a rule of thumb, true at any RPM because of how the two units are defined. 350 lb-ft at 5,200 RPM: (350 × 5,200) ÷ 5,252 = 347 hp.
Wheel HP to crank HP: crank HP = wheel HP ÷ (1 − drivetrain loss), using the commonly cited rule-of-thumb loss for each layout (12% front-wheel drive, 20% all-wheel drive, 15% rear-wheel drive -- not a measurement of your specific car). A 300 whp reading on a rear-wheel-drive car: 300 ÷ (1 − 0.15) = 353 hp at the crank.
All four are widely published rule-of-thumb formulas from drag-strip and dyno practice, not lab-measured constants for any specific car -- treat every result here as an estimate to compare against, not a certified number.
Frequently Asked Questions
Why do the ET and trap-speed methods give different numbers?
Both are drag-strip rules of thumb built from real cars, not exact physics, and each is sensitive to different things -- ET is dragged down hard by a slow launch or wheelspin even if the engine is strong, while trap speed reflects power more purely once the car is already moving. A gap of 10-20 hp between the two on the same run is normal; a much bigger gap usually points to a bad launch skewing the ET number.
What weight should I enter for the quarter-mile methods?
Total weight at the moment of the run: curb weight plus driver, passengers, and a mostly-full tank. Leaving out the driver's weight is a common way people accidentally inflate their own horsepower estimate.
Why does horsepower keep changing if the torque number stays the same?
Horsepower from torque scales with RPM (HP = torque × RPM ÷ 5,252), so the same torque figure makes more horsepower at a higher RPM. This is also why torque and horsepower curves always cross at exactly 5,252 RPM on a dyno chart, whatever the engine.
Is the drivetrain-loss percentage exact for my car?
No -- it's a commonly used estimate, not a measurement of your specific drivetrain. Real loss depends on the actual transmission, differential(s), and driveline condition, and can vary noticeably between two cars with the same drivetrain layout. Treat the crank HP number as a ballpark, not a certified figure.
Is running a quarter-mile pass safe to estimate horsepower with?
Only do full-throttle runs at a sanctioned drag strip with proper safety gear, not on public roads -- the same rule that applies to any hard acceleration test.