Stopping distance rises with the square of speed, so going a little faster adds a lot of distance, especially on wet roads.
How to use the Stopping Distance Calculator
- Speed
- Reaction time
- Road condition
Results update as you type, and every field accepts the units shown in its menu.
How it works
Speed times reaction time gives the reaction distance. Braking distance uses the friction coefficient between tyres and road.
Inputs explained
| Field | What to enter |
|---|---|
| Speed | Units: km/h, mph, m/s |
| Reaction time | In s |
| Road condition | Choose one: Dry asphalt (μ ≈ 0.7), Wet asphalt (0.4), Gravel (0.35), Snow (0.2) … |
What each result means
- Reaction distance = [Speed] × [Reaction time]
- Braking distance = [Speed]^2 ÷ (2×[Road condition]×g)
- Total stopping distance = [Reaction distance] + [Braking distance]
Example values
How the main result changes when Speed changes and everything else stays at the example values above:
| Speed (km/h) | Total stopping distance (m) |
|---|---|
| 25 | 13.93 |
| 37.5 | 23.53 |
| 50 | 34.88 |
| 62.5 | 48 |
| 75 | 62.86 |
| 100 | 97.87 |
Formula
Reaction distance = v × tBraking distance = v² ÷ (2 μ g)Worked example
Inputs — Speed: 50 km/h; Reaction time: 1.5 s; Road condition: Dry asphalt (μ ≈ 0.7).
Results — Reaction distance = 20.83 m; Braking distance = 14.05 m; Total stopping distance = 34.88 m.
Frequently asked questions
How much does speed matter?
Doubling speed roughly quadruples braking distance.
What reaction time should I use?
1–1.5 s is typical for an alert driver; fatigue and phones increase it.
What formula does the Stopping Distance Calculator use?
Reaction distance = v × t.