The standard quarter-mile elapsed time formula — ET = 6.269 × (weight ÷ WHP)^⅓ — traces back to drag racing writer Roger Huntington’s empirical work in the 1950s (his original formula used a constant of 6.290), later refined into the 6.269 version by physics professor Geoffrey Fox at Santa Clara University.¹ For street-tyred cars on a prepped surface, it predicts elapsed time within roughly 0.1–0.2 seconds. Use our ET calculator to run the numbers for your build.

The Formula Explained

The three variables are:

• Weight — total weight of car plus driver at launch (lb)
• WHP — wheel horsepower (measured at the rear wheels on a dyno)
• 6.269 — the empirical constant derived from track data

For trap speed, the companion formula is: Trap Speed = 234 × (WHP ÷ weight)^⅓ (mph). These two numbers together let you cross-check your ET. If your actual trap speed is higher than predicted but your ET is slower, you’re losing time in the 60-foot (reaction + launch).

Sample Calculations

Recomputed directly from the formula above — always verify any published ET/weight/power table yourself rather than trusting it at face value.

Where the Formula Is Accurate

The ET formula works best for:

• Rear-wheel-drive cars on DOT-approved radial tyres
• Naturally aspirated or mildly boosted engines
• Cars running on a prepped track surface
• Experienced drivers with a consistent, well-practiced launch

Where It Breaks Down

The formula is less accurate for:

• High-traction drag radials or slicks — these consistently beat the base formula’s assumptions
• All-wheel drive cars — AWD launch traction typically outperforms what the formula assumes for a RWD car
• High-boost builds where power delivery is uneven
• Cars with significant aerodynamic downforce

Some racers use a lower constant (roughly 5.5–5.9 rather than 6.269) to model slick-tyred or AWD cars more accurately — treat this as a rough community rule of thumb to calibrate against your own timeslips, not a validated universal substitute.

Using the ET Formula for Build Planning

The real value of the ET formula is working backwards. Want to run 10.0? Set ET = 10.0, plug in your target weight, and solve for the WHP you need. This lets you plan your build around a performance target rather than guess. Use the DragPlus ET calculator to model multiple build scenarios side by side.

For a deeper look at the underlying physics — including how power-to-weight ratio, aerodynamic drag, and launch dynamics interact — see How to Predict Your Quarter-Mile ET: The Physics, Formula & Real-World Results.

Sources

• Stealth 316 — Formulas for 1/4 Mile ET & MPH vs HP & Weight (Huntington/Fox formula history)

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