Stopping Distance Calculator

How Stopping Distance Is Calculated

Total stopping distance has two parts:

Total Stopping Distance = Thinking Distance + Braking Distance

Thinking distance is how far you travel while reacting before braking begins.
Braking distance is how far you travel once the brakes are fully applied.

Thinking Distance Formula

Thinking Distance = Speed × Reaction Time

The average driver reaction time is 0.7–1.5 seconds.
Distracted or tired drivers may react in 2–3 seconds or more.

Braking Distance Formula

Braking Distance = v² ÷ (2 × g × μ)

Where:

• v = speed in m/s
• g = 9.81 m/s² (gravity)
• μ (mu) = friction coefficient between tyres and road

Road Surface Friction Coefficients

Published figures for each surface span a range, and these sit near the middle of it. The braking distance calculator uses the cautious end (0.75 dry, 0.40 wet, 0.20 snow) and a 1.5-second reaction, so it gives longer stops for the same speed. Neither is wrong. Tyres, tread depth and the asphalt itself move the real number more than the gap between them.

Typical Stopping Distances (Wet Road, μ ≈ 0.5, 1s Reaction)

Doubling your speed quadruples your braking distance.
That is the whole reason a few km/h matter so much: braking distance grows with the square of speed, while the thinking part only grows in step with it.

Why a reaction time of zero is allowed

Enter 0 and the page shows braking distance on its own, as if your foot were already on the pedal. That is the floor no driver gets below, and it is roughly what automatic emergency braking aims at, though real systems still take a few tenths of a second to decide. For a human, 1 second is a fair planning figure. The UK Highway Code builds its table on 0.67 seconds, which is an alert driver who is expecting to stop, and it is the reason its figures look short next to these.

Both metric (m/km) and imperial (ft/mph) results are shown.