Stopping distance calculator
Enter your speed, your reaction time and the road surface. The calculator adds the distance covered while you react to the distance needed to brake. It uses typical textbook friction values, so treat the answer as an illustration of how speed and surface matter, not as a guarantee.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose metric or US units.
- Enter the speed.
- Keep the 1.5 s reaction time or change it.
- Pick the road surface.
How it works
- Speed in m/s = km/h ÷ 3.6, or mph × 0.44704.
- Reaction distance = v × t.
- Braking distance = v² ÷ (2 μ g).
- Typical coefficients μ: dry 0.7, wet 0.4, snow 0.2, ice 0.1 (textbook values).
Worked example
At 100 km/h (27.78 m/s) with 1.5 s reaction: 41.7 m before the brakes are applied. On a dry road the braking distance is 771.6 ÷ 13.73 = 56.2 m, so about 97.9 m in total. On ice (μ 0.1) the braking distance alone is about 393 m.
Tips
- Speed matters most, because braking distance grows with its square.
- Wet and icy roads need far more room; slow down rather than rely on the numbers.
- Treat results as illustrations. Real stopping distances differ.
Frequently asked questions
What formula does it use?
Reaction distance = speed × reaction time. Braking distance = speed² ÷ (2 × μ × g), where μ is the friction coefficient and g is 9.80665 m/s². The stopping distance is the sum. Speed is converted to metres per second first.
Where do the friction values come from?
They are typical textbook round numbers, not measurements - dry 0.7, wet 0.4, snow 0.2 and ice 0.1. Real values depend on the tyres, their wear and temperature, the road surface, the brakes and whether the car has ABS, so real distances vary.
What reaction time should I use?
The default is 1.5 seconds, a figure often used in teaching examples. Tired, distracted or impaired drivers react more slowly, and a good alert driver may be quicker. Change it to see the effect.
Why does doubling the speed more than double the distance?
Reaction distance grows in proportion to speed, but braking distance grows with the square of speed. At twice the speed, braking takes four times as far.
Does it include slope or load?
No. It assumes a level road and a constant friction coefficient. Do not use it to decide a safe following distance; leave more room than the number suggests.