Stopping distance calculator: reaction and braking distance

How far does a car travel before it stops?

When something appears in the road, your car keeps moving while you react and then while the brakes slow it down. This calculator works out both parts from your speed, the road surface and your reaction time. It shows thinking, braking and total stopping distance plus a safe following distance, with the working, a drawing to scale and a table from 20 to 80 mph, in feet or metres.

How to use it

1. Type your "Speed" and choose mph or km/h.
2. Set the "Reaction time (s)": "1 s: alert driver" or "2 s: tired or distracted".
3. Pick the "Road surface": "Dry", "Wet", "Snow", "Ice", or "Custom μ" to enter your own friction coefficient.
4. If you like, add a "Gradient (%)" downhill or uphill and change the "Following gap (s)".
5. Read the results, the "Working" and the "Rules taught to learner drivers". "Copy link to this calculation" shares your settings.

Thinking, braking and stopping distance

Thinking distance is how far you travel at full speed before your foot reaches the brake: speed in metres or feet per second × reaction time. 60 mph is exactly 88 feet per second (26.82 m/s), so one second of reaction covers 88 ft.

Braking distance uses the physics formula v² ÷ (2 × μ × g), where μ is the grip between tyres and road and g = 9.81 m/s². On a dry road (μ = 0.8) at 60 mph: 26.82² ÷ (2 × 0.8 × 9.81) ≈ 45.84 m, about 150.38 ft.

Stopping distance is the sum: 88 + 150.38 ≈ 238.38 ft (72.66 m) at 60 mph on a dry road. At 70 mph on a wet road (μ = 0.5) it grows to about 430 ft, or 131.11 m.

What makes the distance longer

• Speed. Thinking distance grows in step with speed, but braking distance grows with the square of speed. Double the speed and you need four times the braking distance: 37.6 ft at 30 mph, 150.38 ft at 60 mph on a dry road.
• Road surface. The μ values are typical figures, not measurements: around 0.8 dry, 0.5 wet, 0.2 on snow and 0.1 on ice. On ice, braking takes about eight times as far as on a dry road.
• Hills. Going downhill, gravity pushes the car forward. With a 5% downhill gradient at 60 mph, braking on a dry road goes from 150.38 ft to about 160.41 ft.
• Tiredness, alcohol and phones. They all slow your reactions, sometimes a lot. With 2 seconds instead of 1, the stopping distance at 60 mph jumps from 238.38 ft to 326.38 ft.

Following distance and the rules you learn for your test

Many US state driver manuals teach the three-second rule: pick a fixed point, and when the car ahead passes it, you should count at least three seconds before you reach it. At 60 mph that is 264 ft. Leave more in bad weather.

In the UK, the Highway Code asks for at least a two-second gap on dry roads, at least doubled on wet roads and up to ten times greater on icy roads. It also publishes a table of typical stopping distances, from 12 m at 20 mph to 96 m at 70 mph; enter 20, 30, 40, 50, 60 or 70 mph and the tool shows the published figure, which uses its own assumptions.

The tool also shows the rule of thumb taught in German driving schools and the French (v ÷ 10)² shortcut, both worked out with your speed in km/h.

Every figure here is an estimate: slow down for the conditions and keep your distance.

Frequently asked questions

What is the stopping distance at 30 mph?

With a 1 second reaction and a dry road, about 81.6 ft (24.87 m): 44 ft of thinking distance and 37.6 ft of braking. The Highway Code gives 23 m for 30 mph.

Why does braking distance grow so fast?

The energy to absorb depends on the square of the speed: from 30 to 60 mph, thinking distance doubles and braking distance quadruples.

How many car lengths should I leave?

Counting seconds is easier and works at any speed. Three seconds at 60 mph is 264 ft, roughly 18 car lengths of 15 ft.

Is this my car's exact stopping distance?

No. These are estimates based on typical grip. Tyres, brakes, load and the road all matter, and real distances can be longer.