How to calculate thinking and braking distance

Updated July 19, 2017

Stopping distance is the total distance it takes for a driver to bring a car to a complete stop. This includes the thinking distance and braking distance. The thinking distance is split into perception distance, the time it takes to perceive a problem, and reaction distance, the time it takes to react to the problem. Being more aware of stopping distance allows a driver to adjust the amount of space between their car and the car in front of them for any speed. Although these calculations provide a rough estimate of the stopping distance, road conditions and driver state can greatly alter the stopping distance.

Identify the speed of the car in miles per hour.

Divide the speed by 60 to convert to miles per minute. Divide again by 60 to convert to miles per second.

Multiply the speed by 5,280 to convert to feet per second.

Multiply the speed by the average perception time of 0.75 seconds. This value is the thinking distance or distance travelled when a problem is perceived before a reaction.

Multiply the speed by the average reaction time of 0.75 seconds. This value is the time taken to react to a situation, such as applying brakes or turning the wheel.

Divide the speed in miles per hour by the deceleration amount. Deceleration for a braking car is roughly 20mph per second. This value is the total stopping time or number of seconds the car will take to come to a complete stop.

Multiply the total stopping time by the speed in feet per second. Divide the value by two to account for the deceleration over the stopping distance. This value is the total braking distance or approximate distance it will take the car to reduce its speed to zero.

Add the thinking distance to the braking distance to calculate the total stopping distance.


Divide the stopping distance by 15 feet (average car length) to determine the number of car lengths. This value is a rough estimate of the space you should keep between cars when travelling.


Age, health condition and level of distraction can change the reaction time of the driver. Condition of the car, brakes and road can alter the deceleration rate of the vehicle.

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Nicholas Johnson is a Web programmer who has been moonlighting as a freelance writer since 2008, primarily in the realm of technical research documents and school curricula. He has also worked as an English teacher at an elementary school in South Korea. Johnson holds a Bachelor of Science in Web architecture from the University of Advancing Technology.