Stopping Distance Calculator: Reaction + Braking
How far does it take to stop after you see a hazard? Enter your speed, how long you take to react and the road surface to get the full stopping distance, split into reaction distance and braking distance, plus the total time. This is the whole-stop view. If you only need the brake distance or want to add a slope, use the braking distance calculator.
Updated
Report a problem
Formula
- Speed in metres per second:
v = speed × conversion(km/h ÷ 3.6, mph × 0.44704) - Reaction distance:
d_reaction = v × t - Braking distance:
d_braking = v² ÷ (2 × μ × g)withg = 9.80665 m/s² - Total stopping distance:
d_reaction + d_braking - Total stopping time:
t + v ÷ (μ × g)
Worked example
100 km/h is 27.78 m/s. With a 1.5 s reaction time the car travels 27.78 × 1.5 = 41.67 m before braking starts. On dry asphalt (μ = 0.7) the braking distance is 27.78² ÷ (2 × 0.7 × 9.80665) = 56.20 m. The total is 97.87 m (321.1 ft) and the stop takes 1.5 + 27.78 ÷ 6.86 = 5.55 s.
At 50 km/h the same driver needs 20.83 m + 14.05 m = 34.9 m, so halving the speed cuts the total by well over half.
Use this when
You want the whole picture of a stop: following distance, safe speed near a hazard, or what a longer reaction time costs. Every extra second of reaction adds one second of travel at full speed.
Use the braking distance calculator instead when
You only need the brake distance, or you want to include a road slope. This page covers speed, reaction time and surface; the braking page covers speed, surface and gradient.
Typical values
A reaction time of about 1.5 s is a common planning value for an alert driver; tiredness or distraction can make it much longer. Friction of 0.7 (dry), 0.4 (wet), 0.2 (packed snow) and 0.1 (ice) are typical, not guaranteed.
Common mistakes
- Forgetting the reaction distance, which dominates at low speed.
- Mixing mph and km/h.
- Planning to the dry figure on a wet or icy road.
- Treating the model as exact: tires, brakes, load and slope all change real distances.
Frequently asked questions
How is stopping distance calculated?
Stopping distance is reaction distance plus braking distance. Reaction distance is speed times reaction time, and braking distance is speed squared divided by twice the friction coefficient times 9.80665. At 100 km/h, 1.5 s and dry asphalt that is 41.67 m plus 56.20 m, about 97.9 m.
Why does doubling speed more than double the distance?
Braking distance grows with the square of speed. At 50 km/h on dry asphalt it is 14.05 m, and at 100 km/h it is 56.20 m, four times as far.
What reaction time should I use?
About 1.5 seconds is a common planning value for an alert driver. Tiredness, alcohol or distraction can make it much longer, and every extra second adds one second of travel at full speed (27.8 m at 100 km/h).
How accurate are the surface friction values?
They are typical rounded textbook values (dry 0.7, wet 0.4, packed snow 0.2, ice 0.1). Real friction depends on tyres, brakes, road condition and slope, so treat the result as an estimate and leave extra room.
What is the stopping distance at 70 mph?
With a 1.5 s reaction time on dry asphalt, a car going 70 mph travels 46.94 m before braking and 71.32 m while braking. The total is 118.26 m (388 ft) and the stop takes 6.06 s. On wet asphalt the braking part rises to 124.82 m, for a total of 171.76 m.
What is the stopping distance at 50 mph?
About 69.92 m (229.4 ft) with a 1.5 s reaction on dry asphalt: 33.53 m of reaction distance plus 36.39 m of braking distance. The stop takes about 4.76 s. These are model values for an alert driver.
What is the difference between stopping distance and braking distance?
Stopping distance is reaction distance plus braking distance. At 100 km/h with a 3 s reaction, the car covers 83.33 m before the brakes go on and 56.2 m braking, a total of 139.53 m. Braking distance alone is only the 56.2 m.