Stopping Distance Calculator
Works out reaction, braking and total stopping distance at any speed and surface. The squared relationship with speed is the point most worth taking away.
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The formula
reaction distance = speed × reaction time braking distance = speed² ÷ (2 × g × (μ ± gradient)) total = reaction + braking
During the reaction time the car does not slow at all, so that part is just speed times time. Braking converts kinetic energy, which goes with speed squared, into friction work — so the braking distance quadruples when the speed doubles. The gradient adds to or subtracts from the friction the tyres can use.
What to watch out for
What the figures assume and where real cars differ:
- Maximum braking from the start. Real drivers build up pedal pressure; ABS and brake assist help, but the figure here is the best case.
- Friction is the tyre and the road together. Worn tyres on wet asphalt are nearer 0.3 than 0.5. The surface options are typical values, not guarantees.
- Reaction time is the hidden half. At 100 km/h a 1.5 s reaction is 42 m — nearly as far as the braking itself on a dry road.
Official highway-code figures are built on similar assumptions with a conservative reaction time, which is why they look long.
Frequently Asked Questions
What is the stopping distance at 100 km/h?
About 91 m on dry asphalt with a 1.5 s reaction time — 42 m reacting and 49 m braking. On wet asphalt it is about 119 m, and on ice over 400 m.
Why does braking distance go up with the square of speed?
Because the brakes have to absorb the car’s kinetic energy, which is proportional to speed squared. Twice the speed is four times the energy and four times the braking distance.
What is the difference between braking distance and stopping distance?
Braking distance is from the moment the brakes act to standstill. Stopping distance adds the distance covered during the driver’s reaction time, before the brakes do anything — often half the total on a dry road.