How to use the stopping distance calculator
The formula
Total stopping distance = thinking distance + braking distance.
Thinking distance (metres) = 0.3 × speed (mph) — the distance covered while the driver reacts.
Braking distance comes from the Highway Code reference table below; speeds between table entries are interpolated, and above 70 mph braking distance is scaled with the square of speed, because braking distance grows roughly with speed squared.
Reference table (UK Highway Code, typical dry-road figures)
| Speed | Thinking distance | Braking distance | Total stopping distance |
|---|---|---|---|
| 20 mph | 6 m | 6 m | 12 m |
| 30 mph | 9 m | 14 m | 23 m |
| 40 mph | 12 m | 24 m | 36 m |
| 50 mph | 15 m | 38 m | 53 m |
| 60 mph | 18 m | 55 m | 73 m |
| 70 mph | 21 m | 75 m | 96 m |
Notice the pattern road-safety instructors stress: doubling your speed from 30 to 60 mph more than triples the total stopping distance, because the braking part grows with the square of speed.
Important
These are typical figures for an alert driver, a well-maintained car, and a dry road. Rain, ice, tiredness, distractions, worn tyres, and heavier vehicles all make real distances longer. The tool is for education and planning — it is not a measurement and not an official source.
Example
At 30 mph the total stopping distance is 23 metres — 9 metres thinking + 14 metres braking. At 60 mph it is 73 metres: more than three times as far, even though the speed only doubled.
Frequently asked questions
What is the stopping distance at 30 mph?
23 metres (about 75 feet) in total: 9 metres of thinking distance while the driver reacts, plus 14 metres of braking distance. These are the UK Highway Code's typical figures for a dry road.
What is thinking distance?
The distance the vehicle travels between the driver first seeing a hazard and pressing the brake. The Highway Code works it out as roughly 0.3 metres for every 1 mph of speed, based on a reaction time of about two-thirds of a second.
How does this calculator handle speeds between the table values?
The Highway Code publishes figures at 20, 30, 40, 50, 60, and 70 mph. For speeds in between, this calculator interpolates between the two nearest published figures, and above 70 mph it scales braking distance with the square of speed. Results are always typical estimates, never exact measurements.
How much longer are stopping distances in the wet?
The Highway Code advises that stopping distances can be at least double in wet weather and up to ten times longer in snow and ice. This calculator shows dry-road figures only.
Is this an official figure I can rely on for a report?
No. These are the widely published typical reference figures used for education and road-safety awareness. Real stopping distances vary with the vehicle, tyres, road surface, gradient, weather, and driver. For official purposes, use your organisation's own guidance and methods.
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