Stopping distance calculator
Enter your speed to get two separate results: a physics-based stopping distance with your reaction time and road condition (dry, wet, snow, ice), and the German driving-school rules of thumb for comparison. The UK Highway Code figures are listed below.
How it is calculated
How the physics calculation works
With constant deceleration, stopping distance s = v · t + v² ÷ (2 · a): v is the speed in metres per second (mph × 0.44704, or km/h ÷ 3.6), t the reaction time in seconds and a the deceleration in m/s². During the reaction time the car keeps going at full speed (thinking distance); then it slows down evenly (braking distance).
How to calculate stopping distance
- Choose the unit (km/h or mph) and enter the speed.
- For the physics calculation, enter your reaction time and pick the road condition: dry, wet, snow, ice or your own deceleration.
- Read off thinking distance, braking distance and total stopping distance, plus the comparison for every road condition.
Example: 30 mph = 13.41 m/s, 1 s reaction time, dry road at 8 m/s². Thinking distance 13.4 m, braking distance 13.41² ÷ 16 = 11.2 m, stopping distance 24.7 m (81 ft).
Road condition values
- Dry 8 m/s²: more than 8 m/s² is quoted for a car on dry asphalt (German Wikipedia, “Bremsverzögerung”).
- Wet 4 m/s²: the Highway Code (Rule 227) says stopping distances in wet weather will be at least double those on dry roads, so the deceleration is halved.
- Snow 2.5 m/s²: the middle of the 1–4 m/s² range quoted for snow-covered roads.
- Ice 0.8 m/s²: the Highway Code (Rule 230) warns that stopping distances can be ten times greater than on dry roads.
These are typical values. Tyres, load, gradient and ABS change them considerably – use “custom deceleration” for your own figure.
UK Highway Code typical stopping distances
Rule 126 of the Highway Code publishes these typical figures (thinking + braking distance):
| Speed | Thinking | Braking | Stopping distance |
|---|---|---|---|
| 20 mph (32 km/h) | 6 m | 6 m | 12 m (40 ft) |
| 30 mph (48 km/h) | 9 m | 14 m | 23 m (75 ft) |
| 40 mph (64 km/h) | 12 m | 24 m | 36 m (118 ft) |
| 50 mph (80 km/h) | 15 m | 38 m | 53 m (175 ft) |
| 60 mph (96 km/h) | 18 m | 55 m | 73 m (240 ft) |
| 70 mph (112 km/h) | 21 m | 75 m | 96 m (315 ft) |
They correspond to a reaction time of about 0.67 s and a deceleration of roughly 6.5–6.7 m/s². Rule 126 also asks for at least a two-second gap on high-speed roads, at least doubled on wet roads and up to ten times greater on icy roads.
The German rules of thumb
German driving schools teach simple formulas with the speed in km/h: thinking distance = (v ÷ 10) × 3, braking distance = (v ÷ 10)², halved for an emergency stop. At 50 km/h: 15 m + 25 m = 40 m. These are exam rules for dry roads, not measurements – they are shown here for comparison only and are not official in the UK or US.
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Frequently asked questions
What is the stopping distance at 30 mph?
The Highway Code gives 23 metres (75 feet): 9 m thinking distance plus 14 m braking distance. With 1 s reaction time and a hard stop on a dry road (8 m/s²) the physics formula gives about 24.7 m.
How do you calculate braking distance?
Braking distance = v² ÷ (2 · a), with v in m/s and the deceleration a in m/s². At 13.41 m/s (30 mph) and 8 m/s² that is 179.8 ÷ 16 = 11.2 m.
What happens to braking distance when speed doubles?
It quadruples, because speed enters the formula squared. Thinking distance only doubles.
How much longer is the stopping distance on wet or icy roads?
The Highway Code says at least double in the wet and up to ten times on ice and snow. The comparison table shows all road conditions side by side.
What reaction time should I use?
About 1 second for an alert driver; 1.5 s is a common baseline in accident reconstruction and 2.5 s a conservative value. Tiredness, distraction and alcohol make it longer.
Sources and legal basis
- Wikipedia – Bremsweg: Fahrschul-Faustformeln (Reaktionsweg, Bremsweg, Gefahrbremsung)
- Wikipedia – Bremsverzögerung: Anhaltswerte Pkw (trockener Asphalt > 8 m/s², Schnee 1–4 m/s²)
- The Highway Code – Rule 126 (stopping distances, two-second gap)
- The Highway Code – Typical stopping distances (PDF)
- The Highway Code – Rules 227 and 230 (wet: at least double; ice/snow: up to ten times)
- Wikipedia – Braking distance (friction 0.7, perception-reaction time 1.5 s)
As of:
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