Donaldson R F Braking & Stopping Distance Calculator
See how far a vehicle travels before it stops — while the driver reacts and while the brakes work — on dry, wet, snowy or icy roads.
How stopping distance is calculated
Stopping distance has two parts. During the reaction time (about 1–1.5 seconds for an alert driver, longer when tired, distracted or impaired) the vehicle keeps its full speed: reaction distance = speed × reaction time. Then the brakes work against friction: braking distance = v² ÷ (2 × g × (μ + slope)), where v is the speed in m/s, g = 9.81 m/s², μ is the tyre-to-road friction coefficient and the slope is the road grade as a fraction (positive uphill helps, negative downhill hurts). Because speed is squared, doubling your speed makes the braking distance four times longer. The friction values are typical ones; real braking depends on tyres, brakes, load, road condition and anti-lock systems, and heavy vehicles need more distance. This tool is for education and comparison — leave more space than you think you need and slow down in bad weather.
Highway Code typical stopping distances
For comparison, The Highway Code (Rule 126, GOV.UK) gives these typical stopping distances for an average car in good conditions with an alert driver. They are a guide, not a guarantee: wet roads roughly double the braking distance.
| Speed | Thinking distance | Braking distance | Overall |
|---|---|---|---|
| 20 mph | 6 m (20 ft) | 6 m (20 ft) | 12 m (40 ft) |
| 30 mph | 9 m (30 ft) | 14 m (45 ft) | 23 m (75 ft) |
| 40 mph | 12 m (40 ft) | 24 m (80 ft) | 36 m (118 ft) |
| 50 mph | 15 m (50 ft) | 38 m (125 ft) | 53 m (175 ft) |
| 60 mph | 18 m (60 ft) | 55 m (180 ft) | 73 m (240 ft) |
| 70 mph | 21 m (70 ft) | 75 m (245 ft) | 96 m (315 ft) |
Source: The Highway Code, rules 103–158 (GOV.UK), Rule 126 "Stopping distances".