How far does a vehicle travel before it stops? Compute braking distance from the physics formula d = v²/(2·μ·g), add reaction distance for total stopping distance, invert the formula to get the max safe speed for a given braking room, or compare dry vs wet vs icy surfaces across speeds.
Example: at 100 km/h on dry asphalt (μ = 0.7) the braking distance is about 56.20 m; with a 1.5 s reaction time the total stopping distance is 97.868 m (321.09 ft).
Ideal physics model (constant friction, level road, no air drag) for estimation and learning, not driving advice. If you need crash energy (E = 1/2 m v²) instead of braking distance, see our impact energy calculator.
Inverts d = v²/(2·μ·g). The distance here is braking-only distance (excludes reaction).
Braking distance is d = v squared divided by (2 times mu times g), where v is speed in m/s, mu is the tire-road friction coefficient and g = 9.80665 m/s squared. Example: at 100 km/h (v = 100/3.6 = 27.7778 m/s) on a dry road (mu = 0.7), d = (100/3.6) squared / (2 times 0.7 times 9.80665) = 56.2013 m, about 56.20 m.
Because speed is squared in the formula, doubling v multiplies the distance by 2 squared = 4. On a dry road (mu = 0.7): at 50 km/h the braking distance is about 14.0503 m, but at 100 km/h it is 56.20 m, exactly 4 times longer. The same squaring applies to the total stopping distance.
Reaction distance is the distance travelled before you start braking: speed times reaction time (d = v times t). At 100 km/h (27.7778 m/s) with a typical 1.5 s reaction time it adds 41.6667 m. Adding the 56.20 m of braking on dry road gives a total stopping distance of 97.868 m (321.09 ft).
Typical values: dry asphalt 0.7, wet asphalt 0.4, icy road 0.1. Wet roads roughly double the braking distance versus dry, and ice roughly multiplies it by seven. At 60 mph the braking distance is about 52.40 m on dry asphalt, 91.70 m on wet asphalt, and 366.81 m on ice.
Invert the formula: v = square root of (2 times mu times g times d). For example, if you have 100 ft (30.48 m) of braking room on wet asphalt (mu = 0.4), v = sqrt(2 times 0.4 times 9.80665 times 30.48) = about 55.67 km/h (34.59 mph). Use the inverse mode of the calculator for any distance and surface.
No. This is an ideal physics model (constant friction, level road, no air drag) for learning and estimation only. Real stopping distances are affected by tyre condition, brakes, road slope, ABS, load and weather. Never use this tool to choose a safe speed on real roads.
Stopping distance answers how far you travel while braking, which depends on speed squared and friction but not on mass. Impact energy (see our impact energy calculator) answers how much energy is dissipated in a crash, which is 1/2 m v squared and depends on mass as well. Use stopping distance for braking room, impact energy for crash severity.
Related: Impact Energy Calculator (crash energy, E = 1/2 m v²). This page models braking distance only: it does not compute impact force or energy.
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