Skid marks tell a story — turn their length, the road surface and the slope into the minimum speed the vehicle was travelling.
Skid Mark Speed Calculator
Work out the minimum speed a vehicle was travelling when its skid marks began — the same formula accident reconstructionists use: speed = √(30 × skid length × drag factor). Measure the mark, pick the road surface, and add the grade if the road slopes.
Skid segment 1
This is a minimum speed: it assumes the vehicle braked hard through the full measured mark and stopped at the end of it. If it was still moving when the marks end — for example, it hit something — its true speed was higher. For education and estimation only; court-grade reconstruction needs scene measurement and a qualified reconstructionist.
How the skid formula works
A locked tyre sliding to a stop spends its speed fighting friction, and that trade is exact: speed squared equals twice gravity times the drag factor times the skid length. Tidied into roadside units it becomes the formula reconstructionists carry — speed in mph = √(30 × length in feet × drag factor). The 30 quietly contains gravity and the feet-to-mph conversion; switch the tool to metres and it becomes √(254 × length × drag factor) for km/h.
Why the answer is a floor, not a verdict
The formula assumes every wheel braked hard for the full length of the marks and that the vehicle stopped at the end of them. Real crashes are messier — a car that hits something mid-skid was going faster, and anti-lock brakes leave faint marks or none at all. That is why investigators phrase the result as a minimum: the vehicle was travelling at least this fast when the marks began.
Skid mark questions, answered
Why is the answer a minimum speed, not the exact speed?
The formula assumes the vehicle used the whole skid to scrub off speed and stopped exactly where the marks end. If the marks end because the vehicle hit something, it still had speed left — so the true speed was higher than the number shown. Investigators therefore testify to it as “at least this fast.”
What is a drag factor?
It is the road’s grip expressed as a number — how much of the vehicle’s weight the surface can turn into braking force. Dry asphalt grips well (about 0.6–0.8), wet asphalt much less (0.35–0.55), and ice barely at all (0.1–0.25). Reconstructionists measure it at the scene with a drag sled or test skids; the presets here are the standard published mid-range values.
How do I measure a skid mark properly?
Measure each visible mark from where the tyre first locked to where it stopped rolling, in a straight line along the mark, and note which tyre made it. Curved (yaw) marks are a different calculation — this tool covers straight braking skids only. If the surface changes mid-skid, use the “second segment” option and measure each part separately.
Does the grade of the road really matter?
Yes. Gravity helps a vehicle stop uphill and fights it downhill. A 5% grade changes the effective drag factor by 0.05 — on a 100-foot skid on dry asphalt that is roughly the difference between 44 and 47 mph. Leave the grade at 0 for a level road.
Can this be used in court or an insurance claim?
The formula itself is the standard one used in reconstruction, but a court-grade opinion needs measured scene evidence, a verified drag factor from the actual surface, and a qualified reconstructionist — anti-lock brakes, partial braking and impact speed loss all change the picture. Treat this calculator as a well-founded estimate, not evidence.
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