Skid Mark Speed Calculator
Quick answer: speed from skid marks is S = √(30 × d × f) (mph, feet) or v = √(2 × μ × g × d) (m/s, metres). A 60 ft skid on a dry road (drag factor 0.60–0.80) means the car was doing at least 33–38 mph when the marks began. It is always a minimum: the car was slowing before the marks started.
Updated · formulas checked against the sources below
Default shown: 60 ft on dry asphalt, all four wheels locked, level road. Change any value above to recalculate.
Not a substitute for an accident reconstruction expert. These are textbook physics estimates for learning and rough checks. A real reconstruction measures the road, the vehicles, the marks and the event data recorder, and can give a different answer. Do not rely on this page for an insurance claim, a court case or a police report.
How speed from skid marks is calculated
A skidding car turns its movement energy into heat at the tyres. The work done by friction over the skid (force × distance) equals the energy the car had when the marks began, so the car's mass cancels out and only three things matter: the length of the skid, the grip of the road (the drag factor, close to the friction coefficient μ) and how much of the braking left marks.
The "30" in the US form is the metric formula with the units converted (2 × 32.2 ft/s² ÷ 1.467² ≈ 29.9). BE is braking efficiency: 100% when every wheel was locked, less when only some wheels marked. On a slope, add the grade to the drag factor (uphill +, downhill −); a 5% downhill slope turns a drag factor of 0.70 into 0.65.
Worked example
A car leaves 60 ft of skid marks on dry asphalt. An officer measures a drag factor of 0.70 and all four wheels locked (BE = 100%).
In metric: 18.3 m at μ = 0.70 gives v = √(2 × 0.70 × 9.81 × 18.3) = √251 ≈ 15.9 m/s ≈ 57 km/h. If the car then hit another vehicle, the speed it still had at impact must be added with the combined speeds equation, S = √(S₁² + S₂² + …), which is why the skid result alone is a minimum.
When a car skids over two surfaces
Work out each part on its own (for example 40 ft of asphalt, then 25 ft of grass) and combine them: S = √(S₁² + S₂²). The same rule adds the speed lost in a collision once an expert has estimated it from the damage.
Typical drag factors by road surface
| Road surface | Typical drag factor | Note |
|---|---|---|
| Dry asphalt or concrete | 0.60 – 0.80 | Most common case; new, clean surfaces sit at the top of the range |
| Wet asphalt or concrete | 0.45 – 0.70 | Water film; worn or polished surfaces sit low |
| Packed snow | 0.30 – 0.55 | Compacted, not fresh, snow |
| Ice or loose snow | 0.10 – 0.25 | Lowest grip; black ice can be below 0.10 |
Source: American Prosecutors Research Institute (2003), Crash Reconstruction Basics for Prosecutors, p. 13, citing Fricke, Traffic Accident Reconstruction, Vol. 2. The guide stresses that real roads often fall outside these ranges and that the drag factor should be measured at the scene.
Skid length to speed: feet to mph
Minimum speed when the skid began, all wheels braking, level road. Each cell shows the low and high end of the drag-factor range for that surface.
| Skid length | Dry asphalt or concrete | Wet asphalt or concrete | Packed snow | Ice or loose snow |
|---|---|---|---|---|
| 20 ft | 19–22 mph | 16–20 mph | 13–18 mph | 8–12 mph |
| 40 ft | 27–31 mph | 23–29 mph | 19–26 mph | 11–17 mph |
| 60 ft | 33–38 mph | 28–35 mph | 23–31 mph | 13–21 mph |
| 80 ft | 38–44 mph | 33–41 mph | 27–36 mph | 15–24 mph |
| 100 ft | 42–49 mph | 37–46 mph | 30–41 mph | 17–27 mph |
| 150 ft | 52–60 mph | 45–56 mph | 37–50 mph | 21–33 mph |
| 200 ft | 60–69 mph | 52–65 mph | 42–57 mph | 24–39 mph |
| 300 ft | 73–85 mph | 64–79 mph | 52–70 mph | 30–47 mph |
Skid length to speed: metres to km/h
| Skid length | Dry asphalt or concrete | Wet asphalt or concrete | Packed snow | Ice or loose snow |
|---|---|---|---|---|
| 5 m | 28–32 km/h | 24–30 km/h | 20–26 km/h | 11–18 km/h |
| 10 m | 39–45 km/h | 34–42 km/h | 28–37 km/h | 16–25 km/h |
| 15 m | 48–55 km/h | 41–52 km/h | 34–46 km/h | 20–31 km/h |
| 20 m | 55–64 km/h | 48–60 km/h | 39–53 km/h | 23–36 km/h |
| 30 m | 68–78 km/h | 59–73 km/h | 48–65 km/h | 28–44 km/h |
| 40 m | 78–90 km/h | 68–84 km/h | 55–75 km/h | 32–50 km/h |
| 60 m | 96–110 km/h | 83–103 km/h | 68–92 km/h | 39–62 km/h |
| 80 m | 110–128 km/h | 96–119 km/h | 78–106 km/h | 45–71 km/h |
Assumptions and what the answer does not tell you
- It is a minimum. Tyres only mark once the wheels lock or slide, so the car was already slowing before the first mark. Any speed left at impact is not in the skid.
- Drag factor dominates. Changing 0.60 to 0.80 moves the answer by about 15%. That is why the result is shown as a range and why investigators measure the actual road.
- Braking efficiency. If only the rear wheels or one axle left marks, the braking force was lower than with all wheels locked; set braking efficiency to the share of braking the marking wheels did.
- Motorcycles. A bike often marks with the rear wheel only, and the front brake does much of the stopping. A rear-only mark therefore under-states the braking, and riders may release and re-apply the brake, leaving gaps. Treat any motorcycle result as rough.
- ABS. Most modern cars do not leave long dark skid marks, so this formula fits older cars, trucks, trailers and failed ABS best.
- Gaps and yaw. Count only the actual mark length (not the gaps), and use the longest mark of a wheel set. Curved yaw marks from a sliding turn need a different formula (critical speed), not this one.
Skid marks and speed — FAQ
Can skid marks determine speed?
What is the formula for speed from skid marks?
What drag factor should I use for a dry road?
Do cars with ABS leave skid marks?
How do you work out motorcycle speed from a skid mark?
How long are skid marks at 30, 50 and 70 mph?
Sources and method
- American Prosecutors Research Institute / National District Attorneys Association (2003): Crash Reconstruction Basics for Prosecutors (J. Kwasnoski) — speed-from-skid-marks equation S = √(30·f·d·BE), drag-factor table (p. 13, citing Fricke, Traffic Accident Reconstruction Vol. 2), combined speeds equation, minimum-speed caveat
- Rosén and Sander (2009) and other studies on our crash-speed page — what an impact speed means for a pedestrian
Method: v = √(2·g·d·(f·BE + grade)) with g = 9.80665 m/s², converted to mph (÷ 0.44704) or km/h (× 3.6). The calculator runs in your browser; nothing you type is sent anywhere. Checked 2026-11-02.