Stopping Distance Calculator
Estimate reaction and braking distance by speed.
Typical ~1.5 s
Dry ~0.7, wet ~0.4, ice ~0.1
Results update as you type.
How it works
Stopping distance is how far a vehicle travels from the moment a hazard appears until it comes to a complete stop, and this calculator estimates it. You enter your speed, your reaction time, and the road's friction coefficient, and it returns the reaction distance, braking distance, and total stopping distance in both feet and meters. Total stopping distance has two parts. The reaction distance is how far you travel during the time it takes to perceive the hazard and hit the brakes — at highway speed you cover a surprising distance in even a second or two. The braking distance is how far the car travels while actually decelerating, and it depends on the square of your speed divided by the road grip: doubling your speed roughly quadruples the braking distance, which is why high speeds are so dangerous. The friction coefficient captures road conditions — around 0.7 for dry pavement, 0.4 for wet, and as low as 0.1 for ice — so slippery roads dramatically extend stopping distance. A typical reaction time is around 1.5 seconds but rises with distraction, fatigue, or age. This calculator is useful for understanding safe following distances and the real danger of speeding, especially in poor conditions. Enter your speed, reaction time, and friction to estimate the stopping distance.
Total = (speed × reaction time) + speed² ÷ (2 × μ × g)
Total stopping distance is the reaction distance — how far you travel during your reaction time — plus the braking distance, which depends on the square of speed divided by twice the friction coefficient times gravity. Because braking distance grows with the square of speed, higher speeds dramatically lengthen the stop.
Source:Vehicle dynamics
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter speed.
- 2
Adjust the options
Adjust reaction time and road friction coefficient if they apply — sensible defaults are already set.
- 3
Read your result
Total stopping distance updates instantly as you type — no button to press.
Frequently asked questions
What is total stopping distance?
Total stopping distance is the sum of reaction distance and braking distance — the full distance a vehicle travels from when a hazard appears until it stops. Reaction distance is covered while you perceive and respond, and braking distance while the brakes slow the car. Both grow with speed, so faster driving means a much longer total stop.
Why does braking distance grow with the square of speed?
Braking distance depends on kinetic energy, which is proportional to the square of speed. The brakes must dissipate all that energy, so doubling your speed quadruples the energy and roughly quadruples the braking distance. This is why a modest increase in speed disproportionately lengthens how far you need to stop, making speeding especially hazardous.
How does road condition affect stopping distance?
Road grip, expressed as the friction coefficient, strongly affects braking distance. Dry pavement offers good grip around 0.7, wet roads about 0.4, and ice as low as 0.1. Lower friction means the tires can't decelerate the car as quickly, greatly extending the braking distance, so stopping on ice can take many times the distance it does on dry pavement.
What is a typical reaction time?
An alert driver's reaction time is often around 1.5 seconds, covering perceiving the hazard, deciding to brake, and moving the foot to the pedal. It increases with distraction, fatigue, alcohol, or age, sometimes to 2 seconds or more. Even at 1.5 seconds, a car at highway speed travels well over 100 feet before braking even begins.
How does this relate to safe following distance?
Safe following distance should be at least your total stopping distance, so you can stop before reaching a vehicle that brakes ahead of you. The common three-second rule approximates this at typical speeds and conditions. In rain, snow, or ice, increase your following distance substantially, since stopping distance can multiply as road friction drops.