UK stopping distance calculator
Everyone remembers that the answer is two seconds. Almost nobody can picture what that means on the road at 50 in the rain. Move the speed, pick the surface, and see the real distance in metres, feet and car lengths.
The two second rule. Pick a fixed point ahead, and if you reach it before you finish saying "only a fool breaks the two second rule" you are too close.
| Speed | Thinking | Braking | Total | Car lengths |
|---|---|---|---|---|
| 20 mph | 6.1 m | 6.1 m | 12.2 m | 3 |
| 30 mph | 9.1 m | 13.7 m | 22.9 m | 5 |
| 40 mph | 12.2 m | 24.4 m | 36.6 m | 8 |
| 50 mph | 15.2 m | 38.1 m | 53.3 m | 12 |
| 60 mph | 18.3 m | 54.9 m | 73.2 m | 16 |
| 70 mph | 21.3 m | 74.7 m | 96 m | 21 |
Figures follow Highway Code rule 126, which gives thinking distance as one foot per mph and braking distance as the speed in tens, squared, times five feet. Car lengths assume a 4.5 m family car. Real distances vary with tyres, brakes, load, gradient and how alert you are.
Thinking distance and braking distance are not the same thing
A stopping distance is two numbers added together, and they behave differently. Thinking distance is how far the car travels between the hazard appearing and your foot reaching the brake. It scales with speed and nothing else on the road: at 30 mph you cover about nine metres before you have done anything at all. Braking distance is how far the car travels once the brakes are on, and it scales with the square of speed, which is why doubling your speed roughly quadruples it.
That distinction is what the theory test is testing. Weather changes grip, so rain and ice change the braking half only. Tiredness, distraction, alcohol and drugs change your reaction, so they change the thinking half only. A candidate who has learned one total figure per speed cannot answer either kind of question; a candidate who has understood the split can answer both.
Why car lengths are the useful unit
Ninety six metres is an abstraction. Twenty one car lengths is something you can see out of the windscreen, and it is usually the moment the number lands: almost nobody leaves twenty one car lengths at 70 mph. The calculator converts every figure because the conversion is the point. The Highway Code itself illustrates rule 126 in car lengths for the same reason.
Common questions
Four seconds. The two second rule applies on a dry road; wet roads roughly double the braking distance, so the gap doubles as well. On ice the Highway Code asks for up to a twenty second gap, because ice can multiply the braking distance by ten.
Ninety six metres, or 315 feet, on a dry road. That splits into 21 metres of thinking distance and 75 metres of braking. It is about 21 car lengths. In the wet the braking half doubles, taking the total to roughly 171 metres.
No, and this is the part most candidates get wrong. Thinking distance is how far you travel while you react, so it moves with tiredness, distraction, drink or drugs, not with the weather. Rain and ice change grip, which affects braking distance only. The calculator shows the two separately for that reason.
Highway Code rule 126 is two formulas in disguise. Thinking distance in feet equals the speed in mph. Braking distance in feet equals the speed divided by ten, squared, then multiplied by five. At 60 mph that is 60 feet of thinking plus 6 squared times 5, which is 180 feet of braking, giving 240 feet in total. Every published row matches exactly.
A way of keeping a safe gap without measuring distance. Pick a fixed point ahead, such as a sign or a bridge. When the car in front passes it, start saying "only a fool breaks the two second rule". If you reach the point before you finish the sentence, you are too close. Double it in the wet.
Related: the two second rule explained, our safety margins practice questions and the free theory test practice hub.