Skid Mark Speed Calculator (Speed from Skid Length)
Estimate how fast a vehicle was going from the length of its skid marks with the accident-reconstruction formula S = √(30·d·f) — road surface drag factor, braking efficiency and grade, in km/h or mph.
How it works
S = √(30 × d × f × n) (US: S in mph, d in feet) · S = √(254 × d × f × n) (metric: S in km/h, d in metres) · on a grade use f × n + G, where f = drag factor of the surface, n = braking efficiency (1 = all wheels locked) and G = grade as a decimal (uphill +).
The skid-to-stop formula comes from equating the car's kinetic energy with the work done by tyre friction: ½mv² = m·g·f·d. Converting units gives the constants 30 (feet and mph) and 254 (metres and km/h) used by police accident reconstructionists. Because some speed may already have been lost before the wheels locked, and any speed left at a collision is not counted, the result is a minimum speed. Drag factors vary with tyres, temperature and pavement, so investigators measure them with a test skid when it matters.
Worked example
A car leaves 20 m of skid marks on dry asphalt (drag factor 0.70) with all four wheels locked on a level road: S = √(254 × 20 × 0.70) = √3,556 = 59.6 km/h (37.1 mph). The car was travelling at least 59.6 km/h when it started to skid.
Sources: Braking distance — energy and friction relationship · NHTSA — Crash investigation and reconstruction research
Frequently asked questions
How do you calculate speed from skid marks?
Multiply the skid length by the road's drag factor and by 254 (metres, km/h) or 30 (feet, mph), then take the square root. For 20 m on dry asphalt: √(254 × 20 × 0.7) ≈ 60 km/h.
What drag factor should I use?
Typical values are 0.65–0.80 for dry asphalt or concrete, 0.45–0.60 when wet, 0.4–0.5 on gravel, around 0.25 on packed snow and 0.1 or less on ice. Reconstructionists measure it at the scene with a test vehicle or drag sled for an exact value.
Why is the result a minimum speed?
The formula only counts the speed lost while skidding to a stop. Any braking before the wheels locked, or speed still left when the car hit something, adds to the true starting speed.
Do cars with ABS leave skid marks?
ABS prevents wheel lock, so it leaves faint, broken or curved marks instead of dark stripes. The same energy formula can still be used with the measured ABS mark length, but the deceleration is often a little higher than a locked-wheel skid.
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This calculator is for educational purposes. Double-check important results before acting on them.