Stopping Distance Calculator
Calculate reaction distance, braking distance and total stopping distance from speed, reaction time and road conditions.
A physics estimate for a car braking hard on a level road. Real stopping distances depend on tyres, brakes, load, slope and the driver, and can be much longer.
About this tool
Enter your speed, your reaction time and the road surface — dry or wet asphalt, a good or poor winter road, or ice — and see how far the car travels before you react, how far it travels while braking and the total stopping distance. Braking distance = v² ÷ (2 × μ × g), with friction coefficients from 0.8 on dry asphalt to 0.1 on extremely slippery roads.
Frequently asked questions
Stopping distance = reaction distance + braking distance. Reaction distance = speed in m/s × reaction time. Braking distance = v² ÷ (2 × μ × 9.81), where μ is the friction coefficient. At 80 km/h (22.22 m/s), 1 second and dry asphalt (μ 0.8): 22.2 m + 31.5 m = 53.7 m.
Braking distance grows with the square of the speed, because the car’s kinetic energy does. On dry asphalt, braking from 50 km/h takes about 12.3 m, from 100 km/h about 49.2 m. The reaction distance only doubles, so the total goes from 26.2 m to 76.9 m with a 1-second reaction time.
Typical values from the Norwegian Public Roads Administration: dry asphalt 0.8, wet asphalt 0.5, good winter road 0.35, poor winter road 0.2 and extremely slippery (ice) 0.1. On ice the braking distance is eight times as long as on dry asphalt. Worn tyres, summer tyres in winter and downhill slopes make it longer still.
The UK Highway Code gives typical stopping distances of 23 m at 30 mph and 96 m at 70 mph. With a 1-second reaction time on dry asphalt, this calculator gives 93.7 m at 70 mph. A tired or distracted driver can take well over a second to react, which adds about 31 m at 70 mph per extra second.
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