Stopping Distance Calculator
How far a car travels before it stops — the distance covered while the driver reacts plus the braking distance — for any speed, reaction time and road surface.
Stopping distance has two parts.
How the stopping distance calculator works
Stopping distance has two parts. During the reaction time the car keeps going at full speed: 1.5 seconds at 100 km/h is 42 meters. Then the brakes slow it at a rate set by the tyres and the road — about 7 m/s² on dry bitumen, 4–5 wet, 1–2 on ice — over v² ÷ 2a. Braking distance grows with the square of speed, so doubling the speed quadruples it.
Formula: reaction distance = v × t; braking distance = v² ÷ (2a); total = sum
Worked examples
| Inputs | Total stopping distance | Note |
|---|---|---|
| 100 km/h, 1.5 s, dry road | 97 m | 97 m |
| 60 mph in the wet | 120 m | 120 m |
| 50 km/h on ice | 117 m | 117 m |
FAQFrequently asked questions
What reaction time should I use?
About one second for an alert driver expecting a hazard, 1.5 for a typical driver, and 2.5 or more if tired, distracted or looking at a phone. Official road-safety figures use 1.5–2.5 seconds.
Why does braking distance grow with the square of speed?
Kinetic energy is proportional to speed squared, and the brakes can dissipate it at a roughly constant rate. From 100 km/h you need four times the braking distance you need from 50.
What deceleration can a car achieve?
Modern cars with ABS reach 8–10 m/s² on dry bitumen in an emergency stop; 7 is a safe planning figure. Wet roads roughly halve it, and ice cuts it to a tenth.
Does this match the highway code tables?
Closely. The UK Highway Code’s 96 m at 70 mph (0.67 s reaction plus a 6.6 m/s² stop) and Australian guidance (1.5 s plus 7 m/s²) both come from these formulas with their own reaction times.
Where these figures come from
- ISO 4000 / ETRTO — tyre designation and dimensions — the width / aspect ratio / rim marking this page decodes
- US Environmental Protection Agency — fueleconomy.gov — the official US fuel economy database
Last checked: September 2026. Tyre dimensions follow the ISO metric marking standard; engine formulas are the standard textbook forms.