Stopping Distance Calculator
Example: 30 mph, 1.5 s reaction, dry road: 111 ft, 8.5 car lengths
How far does a car travel between spotting a hazard and standing still? Enter a speed, your reaction time, the road surface and the slope to see the thinking and braking distance, the total in car lengths, and how fast you would still be moving when you reach a hazard ahead.
Calculation results updated
Your stopping distance in car lengths
Each 🚗 = one car length (13 ft)
Thinking and braking distance by speed
Same speed, different road surface
Total Stopping Distance
In Car Lengths
Thinking Distance
Braking Distance
Time to Stop
Hazard Ahead
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How to use this calculator
- Choose mph and feet or km/h and metres.
- Enter your speed and your reaction time. Measure your own with the ruler-drop reaction time calculator if you like.
- Pick the road surface and enter any uphill or downhill slope.
- Enter how far ahead a hazard appears to see whether you stop in time or how fast you would still be going.
- Compare speeds in the chart and the Speed Comparison table.
How We Calculate This
Thinking distance = v × t, where v is speed and t is reaction time. Braking distance = v² / (2 × (a + g × G)), where a is the surface deceleration, g is 9.81 m/s² and G is the road grade as a decimal (positive uphill). Dry firm braking uses a = 6.5 m/s², fitted to the Highway Code typical braking distances. Wet uses half that and icy one tenth, following Highway Code Rules 227 and 230. The AASHTO design option uses 3.4 m/s², the deceleration US engineers use for stopping sight distance. Car lengths use 4 m, the length implied by the Highway Code figure of 96 m or 24 car lengths at 70 mph. Hazard impact speed is √(v² − 2 × (a + g × G) × (d − thinking distance)) when the hazard lies inside the stopping distance.
Methodology last reviewed: September 2026. How SparkCalc works
Sources: GOV.UK: The Highway Code, Typical stopping distances (Rule 126) · GOV.UK: The Highway Code, Driving in adverse weather conditions (Rules 227 and 230) · Washington State DOT Design Manual, Chapter 1260 Sight Distance (AASHTO stopping sight distance) · Minnesota DOT: Sight Distance and Vertical Alignments (AASHTO formula and 2.5 s, 11.2 ft/s² parameters) · Green, M. (2000). "How Long Does It Take to Stop?" Methodological Analysis of Driver Perception-Brake Times. Transportation Human Factors 2(3)
Speed matters more than reaction time
Thinking distance grows in step with speed, while braking distance grows with its square. At low speed most of the stopping distance is reaction; at motorway speed braking dominates. That is why a small speed reduction near schools or in rain shortens the stop more than any reflex could.
Why the hazard result matters
A car that cannot stop in time does not reach the hazard at a crawl. Because braking sheds speed slowly at first in distance terms, a car that would need a few more metres to stop can still arrive at a surprising speed. Compare the hazard result at two speeds a few mph apart to see the effect.
Slopes and ice
Gravity adds to braking uphill and works against it downhill. On ice, where grip is very low, a steep downhill slope can leave the brakes unable to stop the car at all. The calculator reports that case instead of a distance.
Key terms
- Thinking distance
- The distance travelled at full speed between seeing a hazard and the brakes starting to work.
- Braking distance
- The distance travelled from the start of braking until the car stops.
- Deceleration
- How quickly the car loses speed under braking, in metres per second squared. Grip and brakes set its limit.
- Perception-reaction time
- The time a driver needs to notice a hazard, decide to brake and move a foot to the pedal.
Frequently Asked Questions
How is stopping distance calculated?
Stopping distance is thinking distance plus braking distance. Thinking distance is speed multiplied by reaction time, because the car keeps moving at full speed until the brakes engage. Braking distance is speed squared divided by twice the deceleration, so it grows with the square of speed.
Why does doubling my speed more than double the stopping distance?
Thinking distance doubles with speed, but braking distance quadruples, because the energy the brakes must remove rises with the square of speed. At 60 mph the braking part is four times as long as at 30 mph on the same road.
What reaction time should I use?
Research by Marc Green found about 0.7 seconds when a driver knows exactly when and where a signal will appear, about 1.25 seconds for common unexpected events such as brake lights ahead, and about 1.5 seconds for a surprise such as something moving into the road. US road design uses 2.5 seconds to cover slower drivers.
Why are the default figures close to the Highway Code table?
The dry-road setting uses 6.5 m/s² of deceleration, which reproduces the Highway Code typical braking distances of 14 m at 30 mph and 75 m at 70 mph. The Highway Code uses a quick thinking time of about 0.67 seconds, so enter 0.67 to match its overall figures such as 23 m at 30 mph.
How much do rain and ice change stopping distance?
The Highway Code says stopping distances are at least double on wet roads and can be ten times greater on ice. This calculator applies those factors to braking deceleration, so the thinking distance stays the same while the braking part grows.
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This calculator gives educational estimates. Real stopping distances depend on tyres, brakes, load, road texture, weather and the driver, and can be much longer. Do not use it for accident reconstruction or to judge a safe following distance; leave more room than any calculation suggests.
References
- GOV.UK: The Highway Code, Typical stopping distances (Rule 126)
- GOV.UK: The Highway Code, Driving in adverse weather conditions (Rules 227 and 230)
- Washington State DOT Design Manual, Chapter 1260 Sight Distance (AASHTO stopping sight distance)
- Minnesota DOT: Sight Distance and Vertical Alignments (AASHTO formula and 2.5 s, 11.2 ft/s² parameters)
- Green, M. (2000). "How Long Does It Take to Stop?" Methodological Analysis of Driver Perception-Brake Times. Transportation Human Factors 2(3)