Braking Distance Formula: Stopping and Reaction Distance
The rules of thumb for reaction, braking and stopping distance with a table, the physics for wet and icy roads and what German road rules require.
Car & Mobility2 linked calculatorsReading time: 6 min.
How far does a car keep going after a hazard appears? The answer is made up of two distances: the distance before braking starts and the distance while braking. Two rules of thumb let you estimate it in your head.
Key takeaways
- Stopping distance = reaction distance + braking distance.
- Rule of thumb for reaction distance: speed ÷ 10 × 3; braking distance: (speed ÷ 10)² – half of that for emergency braking.
- Double the speed means four times the braking distance, because speed enters as a square.
- On wet or icy roads, braking distance becomes several times longer.
- German road rules require you to be able to stop within the distance you can see and to keep enough distance.
Reaction, braking and stopping distance
- Reaction distance: the distance the car travels until your foot is on the brake – during this time the car continues unbraked.
- Braking distance: the distance from the start of braking to standstill.
- Stopping distance: both together, i.e. the whole distance from noticing the hazard to standing still.
The rules of thumb
Two approximations are enough for a quick estimate. They are not legal rules but rough formulas with speed in km/h and the result in metres:
- Reaction distance = (speed ÷ 10) × 3
- Braking distance (normal braking) = (speed ÷ 10)²
- Braking distance (emergency braking) = (speed ÷ 10)² ÷ 2
| Speed | Reaction distance | Braking distance | Stopping distance | Stopping distance, emergency braking |
|---|---|---|---|---|
| 30 km/h | 9 m | 9 m | 18 m | 13.5 m |
| 50 km/h | 15 m | 25 m | 40 m | 27.5 m |
| 70 km/h | 21 m | 49 m | 70 m | 45.5 m |
| 100 km/h | 30 m | 100 m | 130 m | 80 m |
| 130 km/h | 39 m | 169 m | 208 m | 123.5 m |
Because speed is squared, braking distance grows much faster than speed: at 50 km/h it is 25 m, at 100 km/h already 100 m – four times as much.
Calculate braking and stopping distance
Enter a speed – reaction, braking and stopping distance by rule of thumb and by physics with road conditions.
The physics: the road surface matters
The physics is more precise: reaction distance = speed in m/s × reaction time, braking distance = v² ÷ (2 × μ × g). Here v is the speed in m/s (km/h ÷ 3.6), g = 9.81 m/s² is gravitational acceleration and μ is the friction coefficient between tyre and road. The friction coefficients in the table are the braking distance calculator's assumptions for typical road conditions.
| Road surface | Friction coefficient μ (assumption) | Stopping distance |
|---|---|---|
| dry | 0.8 | 76.9 m |
| wet | 0.5 | 106.4 m |
| snow | 0.2 | 224.4 m |
| ice | 0.1 | 421.1 m |
How the table is calculated
100 km/h, 1 s reaction time. Tyres, brakes, load and gradient change the real value – the table shows the order of magnitude, not a guarantee.
What German road rules require
Driving within your sight: you may only drive fast enough to stop within the distance you can see; on roads so narrow that oncoming traffic could be endangered, within half of it. If fog, snow or rain limit visibility to less than 50 m, the maximum is 50 km/h (Section 3(1) StVO).
Following distance: as a rule it must be large enough to stop even if the vehicle in front brakes suddenly (Section 4(1) StVO). Above 80 km/h, the German fines catalogue measures the distance against half the speedometer reading in metres – at 100 km/h, that is 50 m.
Calculate overtaking distance
How much road an overtaking manoeuvre needs and how much visibility is required.
Frequently asked questions
What is the formula for braking distance?
How do I calculate the stopping distance at 50 km/h?
Why is braking distance four times as long at double speed?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- Section 3 StVO – speed (accessed September 30, 2026)
- Section 4 StVO – following distance (accessed September 30, 2026)
- BKatV table 2 – following distance (accessed September 30, 2026)
Full-Stack Developer & Founder of HEADON.pro
Full-stack developer and founder of HEADON.pro. Developer of Rechnerzentrale.