Braking and Reaction Distance Calculator
Estimate reaction distance and an idealised braking distance from speed, reaction time and an assumed road friction coefficient.
How to use this calculator
- Enter speed in km/h, reaction time and an assumed friction coefficient.
- Confirm that all inputs use the stated units and the same population, period or scope.
- Change one assumption at a time when comparing scenarios.
Formula used by the braking distance calculator
dᵣ = vt; dᵦ = v²/(2μg)
- dᵣ = vt; dᵦ = v²/(2μg)
- This simplified model is not for driving decisions; real stopping distances vary with tyres, road, weather, gradient and vehicle systems.
- Inputs are validated against required ranges and outputs are rounded for readability.
Worked examples
Worked example
50 km/h, 1.5 seconds reaction and μ=0.7 on a level surface.
The calculation estimates reaction and braking components separately.
The calculation estimates reaction and braking components separately.
Understanding the result
Estimate reaction distance and an idealised braking distance from speed, reaction time and an assumed road friction coefficient.
This simplified model is not for driving decisions; real stopping distances vary with tyres, road, weather, gradient and vehicle systems.
Important notes
This simplified model is not for driving decisions; real stopping distances vary with tyres, road, weather, gradient and vehicle systems.
Use the result as a calculation aid for the inputs supplied; confirm critical specifications independently.
Frequently asked questions
Does this give a safe following distance?
No. It is a simplified physics estimate and should never be used as a safety guarantee.
Why can wet roads change the result?
Available tyre-road friction and visibility can be substantially different in real conditions.
How do I use this calculator?
Enter speed in km/h, reaction time and an assumed friction coefficient.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.