The standard quarter-mile elapsed time formula — ET = 6.269 × (weight ÷ WHP)^⅓ — was developed by drag racing engineer Roger Huntington and has been refined over decades of track data. For street-tyred cars on a prepped surface, it predicts elapsed time within 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

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 (up to ~30psi)
• Cars running on a prepped track surface
• Experienced drivers (the formula assumes an average 1.7–1.9 second 60-foot time)

Where It Breaks Down

The formula is less accurate for:

• High-traction drag radials or slicks (can lower ET by 0.3–0.8s vs the formula)
• All-wheel drive cars (AWD typically beats the formula by 0.2–0.4s)
• High-boost builds where power delivery is uneven
• Cars with significant aerodynamic downforce

For AWD cars, a more accurate constant is approximately 5.8 instead of 6.269. For slick-tyred purpose-built cars, many racers use 5.5–5.9 depending on chassis setup.

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.

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