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Stopping Distance Calculator

Find how far a vehicle needs to stop.

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.

Worked example

A car at 90 km/h, 1.5 s reaction time, friction 0.7. Speed in m/s: 90 ÷ 3.6 = 25. Reaction distance: 25 × 1.5 = 37.50 m. Braking distance: 25² ÷ (2 × 0.7 × 9.80665) = 625 ÷ 13.729 = 45.52 m. Total: 37.50 + 45.52 = 83.02 m. On a slippery surface with friction 0.3 the braking distance grows to 106.22 m and the total to 143.72 m, while the reaction part stays the same.

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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.

About this calculator

This calculator adds two physically distinct phases to find total vehicle stopping distance: reaction distance = speed × reaction time (how far the vehicle travels before the driver even begins braking), and braking distance = v² / (2·μ·g), derived from the work-energy theorem where μ is the coefficient of friction between tires and road and g is gravitational acceleration. Because braking distance scales with the square of speed, doubling your speed quadruples the braking distance — a fact that surprises many drivers.

Driving instructors, driver's-ed students studying for licensing exams, road safety engineers setting speed limits and sight-distance requirements, and physics students all use this relationship.

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Comments (1)

  • Editorial team

    Compare the total with a following gap measured in time. At 100 km/h a car covers about 27.8 m every second, so a 2 second gap is roughly 56 m, well short of the 97.87 m the default inputs give for stopping. The reaction time is the most personal input here. Which reaction time do you use when you run your own numbers?

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