Braking Distance Calculator: Surface and Slope
How far does a car travel while the brakes are on? Enter the speed, the road surface and the slope to get the braking distance from full grip, with reaction time off by default. This is the brake-only part of a stop. If you want the whole stop, including the distance covered before you react and the stopping time, use the stopping distance calculator instead.
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Formula
v = speed in m/s (km/h / 3.6, mph x 0.44704)
a = g (mu cos(theta) + sin(theta)) theta = atan(slope / 100), uphill positive
braking distance = v^2 / (2 a) (flat road: v^2 / (2 mu g))
Constants: g = 9.80665 m/s2. Typical friction coefficients: dry 0.7, wet 0.4, snow 0.2, ice 0.1. The model assumes constant maximum braking with no drag, no brake fade and no change of grip with speed.
Worked example
100 km/h on dry asphalt, flat road:
- v = 100 / 3.6 = 27.78 m/s, so v squared = 771.6
- a = 0.7 x 9.80665 = 6.86 m/s2
- Braking distance: 771.6 / 13.73 = 56.2 m (184.4 ft)
Same speed on wet asphalt is 98.4 m, on snow 196.7 m and on ice 393.4 m. A 5% uphill shortens the dry stop to 52.5 m and a 5% downhill lengthens it to 60.6 m.
Use this when
You want the distance the brakes need: comparing surfaces, checking the effect of a slope, or seeing how speed squares the distance. Add a reaction time only if you want a quick total.
Use the stopping distance calculator instead when
You want the complete stop: reaction distance plus braking distance plus the time it takes, for a driver who needs a moment to react. That page has no slope input; this one has no stopping time.
Typical values
Friction of about 0.7 on dry asphalt, 0.4 wet, 0.2 packed snow and 0.1 ice are typical textbook figures, not guarantees.
Common mistakes
- Treating braking distance as the whole stop. It leaves out the distance travelled while you react.
- Using km/h in the formula without converting to m/s.
- Assuming grip is the same at every speed or on every wet road.
- Entering a downhill slope so steep that friction cannot hold the vehicle: the calculator reports an error.
Frequently asked questions
What is the braking distance formula?
d = v squared / (2 x mu x g), with g = 9.80665 m/s2. At 100 km/h (27.78 m/s) on dry asphalt with mu = 0.7, d = 771.6 / 13.73 = 56.2 m.
How does the road surface change braking distance?
Distance is inversely proportional to the friction coefficient. With mu 0.7, 0.4, 0.2 and 0.1 for dry, wet, snow and ice, a stop that takes 56.2 m on dry asphalt takes 98.4 m wet, 196.7 m on snow and 393.4 m on ice from 100 km/h.
How does a slope change the distance?
Deceleration becomes g x (mu x cos(angle) + sin(angle)), with the angle positive uphill. A 5% uphill dry road shortens 56.2 m to 52.5 m, and a 5% downhill lengthens it to 60.6 m. If a downhill is steep enough that friction cannot stop the vehicle, the calculator reports an error.
Is this the same as stopping distance?
No. Stopping distance is the reaction distance (speed x reaction time) plus the braking distance. At 100 km/h, a 1.5 s reaction adds 41.7 m, so the total is 97.9 m. Real distances also depend on tyres, brakes, load and ABS, so treat these numbers as estimates and drive to the conditions.
What is the braking distance at 60 mph?
On dry asphalt (friction 0.7) a vehicle needs about 52.4 m (171.9 ft) to stop from 60 mph once the brakes are applied. On wet asphalt (friction 0.4) that grows to 91.7 m (300.9 ft). These are model values, so real stops vary with tires and brakes.
Does vehicle weight change braking distance?
Not in this model. Mass cancels out because grip force and inertia both scale with weight, so a heavy and a light vehicle stop in the same distance for the same friction. In practice brake size, tire load and fade make heavier vehicles take longer.
How do I estimate braking distance from speed quickly?
Use d = v squared / (2 x 0.7 x 9.80665) for dry asphalt, with v in m/s. At 50 km/h that gives 14.05 m, and at 120 km/h it gives 80.93 m. Doubling the speed always makes the distance four times longer.