Stopping Distance Calculator

How it is calculated

Speed is entered in km/h and first converted to metres per second: v = speed ÷ 3.6. The reaction distance is the ground covered before the brakes are applied: reaction = v × t, with t the reaction time in seconds. The braking distance is v² ÷ (2 × μ × 9.80665), where μ is the tyre-road friction coefficient. The total is the sum of the two, in metres. If you leave the reaction time empty it is taken as 1.5 s, and an empty friction coefficient as 0.7. Speed may be zero, reaction time may be zero but not negative, and friction must be above zero because with none the vehicle never stops. The model assumes level ground, constant deceleration and no braking-system delay, so it is a physics estimate rather than a legal stopping-distance table.

Common mistakes

• Entering speed in mph or m/s. The field is km/h, so 55 typed for a 55 mph limit is treated as 55 km/h, about 34 mph, and understates the distance.
• Using a dry-road friction value on wet or icy roads. Friction 0.7 and 0.3 give braking distances of 45.52 m and 106.22 m at 90 km/h, so the surface matters as much as the speed.
• Doubling the speed and expecting a doubled distance. The braking part grows with the square of speed, so 45 km/h gives 11.38 m of braking but 90 km/h gives 45.52 m, four times as much.
• Setting the reaction time to 0 to be conservative. That removes the distance travelled before braking; a driver who is tired or distracted needs a longer time, not a shorter one.
• Applying the result to a hill or a heavy vehicle with a different brake system. The tool assumes level ground and a single constant friction value.