Stopping Distance Calculator

How it works

1. 1Enter your speed, driver reaction time, and braking deceleration — the tool converts them to SI units before computing.
2. 2Reaction distance is v·t: the distance travelled while the driver perceives and responds to a hazard.
3. 3Braking distance is v² ÷ (2a): the distance from the moment braking begins until the vehicle stops. The two are summed for the total.

Use cases

• Road-safety education — illustrating how doubling speed roughly quadruples braking distance.
• Driver training — showing why reaction time and road-surface condition both matter.
• Engineering — estimating safe following distances and stopping zones for vehicles or machinery.

Frequently asked questions

What is the formula for stopping distance?

Total stopping distance = reaction distance + braking distance. Reaction distance = v × t, where v is speed (m/s) and t is reaction time (s). Braking distance = v² ÷ (2a), where a is deceleration (m/s²). So d = v·t + v² ÷ (2a).

What is a typical reaction time and deceleration for a car?

A conservative real-world reaction time is about 1.5 s. Typical dry-road deceleration for a modern car with good tyres is around 7–9 m/s². Wet roads can reduce this to 3–5 m/s², lengthening the braking distance.

Can you show a worked example?

At 50 km/h (≈ 13.89 m/s) with a 1.5 s reaction time and 7 m/s² braking: reaction distance ≈ 13.89 × 1.5 ≈ 20.8 m; braking distance ≈ 13.89² ÷ 14 ≈ 13.8 m; total ≈ 34.6 m. At 100 km/h the total jumps to about 110 m.