Stopping Distance Calculator
Work out thinking, braking and total stopping distance at a given speed and road condition.
About the Stopping Distance Calculator
Stopping distance is thinking distance (speed × reaction time) plus braking distance (speed² ÷ 2 × deceleration). Braking distance grows with the square of speed: double the speed, four times the braking distance. The UK Highway Code's typical figures (dry road, 0.67 s reaction) are 12 m at 20 mph, 23 m at 30 mph, 53 m at 50 mph and 96 m at 70 mph; on wet roads allow at least double and on ice up to ten times.
The formula
d = v × t + v² ÷ (2a)
UK Highway Code typical stopping distances
| Speed | Thinking | Braking | Total |
|---|---|---|---|
| 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 |
Worked example
Example: Stopping distance = 64.7 m
- Inputs: Speed: 55; Speed in: mph; Reaction time: 0.74 s; Road: Dry (≈ 6.5 m/s²).
- Formula: d = v × t + v² ÷ (2a)
- Result: Stopping distance = 64.7 m
Questions people ask
What is the two-second rule?
Leave at least a two-second gap to the car in front, doubled in the wet.
Do ABS brakes stop shorter?
They mainly keep steering control; distances on loose surfaces can even be longer.