Stopping Distance Calculator
How far a car travels before it stops: the distance covered while you react, the braking distance for the road's grip and slope, and the total, in metres or feet.
Result…
How it's calculated
Reaction distance = speed × reaction time Braking distance = speed² ÷ (2 × g × (μ + slope)) with g = 9.80665 m/s², slope as a decimal (uphill +, downhill −) Total stopping distance = reaction distance + braking distance Time to stop = reaction time + speed ÷ (g × (μ + slope))
Stopping takes two parts. While you notice the hazard and move your foot, the car keeps going at full speed: that is the reaction distance. Then the brakes slow it at up to the friction coefficient μ times g, where g = 9.80665 m/s². From v² = 2ad, the braking distance is v² ÷ (2gμ), so it grows with the square of the speed: twice as fast takes four times as far to brake. A slope adds to the grip going uphill and takes it away going downhill, entered as a percentage grade (the usual road-design form, d = v² ÷ (2g(μ ± G))). The surface presets are typical values from J. Y. Wong's Theory of Ground Vehicles, rounded to the lower end of their ranges: dry asphalt 0.8, wet asphalt 0.5, gravel 0.6, packed snow 0.2 and ice 0.1. Speeds in mph use 1 mile = 1609.344 m, and feet use 1 ft = 0.3048 m.
Your numbers, step by step
About the Stopping Distance calculator
This stopping distance calculator estimates how far a car travels from the moment a driver sees a hazard to the moment it stops. It splits the answer into the reaction distance, covered at full speed before the brakes go on, and the braking distance, which depends on the grip between tyre and road and on whether the road slopes up or down. Enter the speed in km/h or mph and read the result in metres or feet. The road surface presets are typical example friction values that you can replace with your own. Driving instructors, learner drivers, cyclists and anyone curious about safe following gaps can use it to see why speed matters so much: the braking part grows with the square of the speed. The working is shown, and a chart plots all three distances across a range of speeds.
How to use it
- Choose “Units”: metric for km/h and metres, imperial for mph and feet.
- Enter the “Speed” and a “Reaction time” in seconds.
- Pick a “Road surface (friction)”, or choose “My own value” and enter a “Friction coefficient”.
- Enter a “Road slope” in percent if the road is not level: positive uphill, negative downhill.
- Read the total, the reaction and braking parts, and the chart by speed.
Worked example
At 80 km/h (22.22 m/s) with a 1.5 s reaction time on a wet road with friction 0.5, the reaction distance is 22.22 × 1.5 = 33.3 m. Braking takes 22.22² ÷ (2 × 9.80665 × 0.5) = 50.4 m, so the total is 83.7 m, or about 275 ft, and the car stops after about 6.0 s. At 40 km/h on the same road the total falls to 29.3 m: halving the speed cuts the braking part to a quarter (12.6 m).
Updated
Questions people ask
How do I calculate stopping distance?
Add the reaction distance (speed in m/s × reaction time) to the braking distance (speed² ÷ (2 × 9.80665 × friction coefficient)). On a slope, add the grade as a decimal to the friction going uphill and take it away going downhill.
Why does braking distance grow so fast with speed?
The car's kinetic energy grows with the square of its speed, and the brakes remove it at a steady rate. So doubling the speed makes the braking distance four times as long, while the reaction distance only doubles.
What friction coefficient should I use?
It depends on the tyres, tread depth, water and temperature. The presets use the lower end of typical ranges from J. Y. Wong's Theory of Ground Vehicles, as tabulated by HPWizard: about 0.8 on dry asphalt, 0.5 wet, 0.2 on packed snow and 0.1 on ice.
What reaction time is realistic?
It varies a lot between people and situations, and grows with tiredness, distraction, alcohol and surprise. Try a few values to see how much it changes the total, and allow more time rather than less.
Can I rely on this as my car's real stopping distance?
No. It is a physics estimate that assumes full grip from the first moment of braking. Worn tyres or brakes, heavy loads and standing water all make the real distance longer, so keep a bigger gap and follow your country's highway code. The speed calculator converts between speed, distance and time.