Quarter Mile Calculator
Estimates quarter-mile ET and trap speed from power and weight with the standard drag-racing formulas. Trap speed is reliable; ET depends on the launch.
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The formula
ET (s) = 6.29 × ∛(weight ÷ hp) trap (mph) = 224 × ∛(hp ÷ weight)
The cube-root forms come from the physics of constant-power acceleration — distance in a fixed time scales with the cube root of power over mass — and the constants are fitted to decades of drag-strip timeslips. They assume a decent launch on a prepared surface.
What to watch out for
Reading a timeslip against the estimate:
- Trap speed matches, ET is slow — the launch or the 60 ft time is the problem. Tyres, pressure, suspension.
- Both are slow — the car is not making the power claimed, or weighs more than you think.
- Altitude. Naturally aspirated engines lose about 3% per 300 m; the formula assumes sea level.
Frequently Asked Questions
How do I calculate quarter mile time from horsepower?
ET ≈ 6.29 × ∛(weight in lb ÷ hp). A 3,300 lb car with 268 hp is 12.3 lb/hp, cube root 2.31, so about 14.5 s at a trap speed of 224 × ∛(268 ÷ 3,300) ≈ 97 mph.
What does trap speed tell you?
How much power the car is really making, because it is nearly independent of the launch. A car that traps at the predicted speed but runs a slow ET has a traction problem, not a power problem.
How much does drivetrain loss matter?
The formulas were fitted to flywheel power on typical cars, so about 15% loss is already baked in. The adjustment here only matters if your drivetrain is well outside that — a heavy AWD automatic, say.