Tag: ET formula

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.

Build	Weight (lb)	WHP	Predicted ET	Trap Speed
Stock V8 Mustang	3,800	380	13.5s	109 mph
Naturally Aspirated Build	3,400	500	11.9s	124 mph
Turbo Build (street weight)	3,200	700	10.4s	141 mph
Purpose-built drag car	2,600	900	8.9s	164 mph

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)

Frequently Asked Questions

Is wheel horsepower or crank horsepower used in the ET formula?

Wheel horsepower (WHP) — measured at the rear wheels on a dyno. Crank figures are not used because drivetrain losses vary too much between cars (manual gearboxes and automatics lose meaningfully different amounts), so a crank-HP figure would need a correction factor that isn’t consistent across vehicles.

Why is my actual ET slower than the formula predicts?

The most common reasons are: poor 60-foot time (launch/traction issue), high altitude (air density), hot ambient temperature, or an overestimated dyno figure. The formula assumes a well-driven car on a prepped surface at sea level.

How accurate is the ET formula for turbocharged cars?

For street-driven turbo cars on radial tyres, accuracy is typically within a few tenths of a second. High-boost builds that make power in a narrow RPM band can deviate further because the formula assumes relatively linear power delivery.