Stopping distance and brake energy calculator

Stopping distance has two parts: the distance covered while the driver reacts, and the distance covered while the brakes slow the vehicle. The second grows with the square of speed, so driving 20% faster needs about 44% more braking distance. These calculators estimate stopping distance on different surfaces and slopes, the energy the brakes must absorb, and the deceleration achieved in a measured stop. They are physics estimates for understanding and education, not a guarantee of how any real vehicle will stop.

Stopping distance

Adds reaction distance and braking distance for a given speed, road surface and slope. The friction coefficient represents tyre grip; worn tyres, poor brakes, a heavy load or ABS behaviour on loose surfaces can make real distances considerably longer.

SpeedSpeed when the hazard appears.Reaction timesAbout 1–1.5 s for an alert driver; 2 s or more if tired or distracted.Tyre–road friction coefficientμAbout 0.7–0.8 dry asphalt, 0.4–0.5 wet, 0.2 snow, 0.1 ice.Road gradient%Positive uphill, negative downhill.

Results

Reaction distance—Distance travelled before the brakes are applied.

Braking distance—Distance travelled while braking.

Total stopping distance—Reaction plus braking distance — the full distance to a standstill.

Formula

reaction = v × t
braking = v² / (2 × g × (μ + grade/100))

Brake energy and disc temperature rise

Calculates the kinetic energy the brakes must turn into heat when slowing from one speed to another, and how much that would warm the brake discs if they absorbed it all. It shows why repeated hard stops or long descents overheat brakes.

Vehicle massMass including load.Starting speedSpeed when braking starts.Final speed0 for a full stop.Total brake disc massAll discs combined; about 5–10 kg each on a car.

Results

Energy to absorb—kJKinetic energy removed by the brakes.

Disc temperature rise—°CIf the cast-iron discs absorbed all the heat (specific heat 460 J/kg·K).

Formula

E = ½ × m × (v₁² − v₂²)
ΔT = E / (m_disc × 460)

Deceleration from a measured stop

From a speed and the distance it took to stop once braking began, calculates the average deceleration achieved. Around 0.8–1.0 g on dry roads is typical of a modern car with good tyres.

Speed at start of brakingSpeed when the brakes were applied.Braking distanceDistance from brake application to standstill.

Results

Average deceleration—Switch to g to compare with tyre grip.

Formula

a = v² / (2 × d)

Worked example: stopping from 100 km/h

100 km/h is 27.8 m/s. With a 1.5 s reaction the car travels 41.7 m before braking.
On dry asphalt (μ = 0.7) braking takes 27.8² / (2 × 9.81 × 0.7) = 56.2 m.
Total stopping distance: about 98 m. On a wet road (μ = 0.45) it grows to about 129 m.
The brakes must absorb 579 kJ — enough to warm 32 kg of discs by about 39 °C in one stop.

Approximate stopping distances, 1.5 s reaction, level road

Speed	Dry (μ 0.7)	Wet (μ 0.45)	Snow (μ 0.2)
50 km/h	about 35 m	about 43 m	about 70 m
80 km/h	about 69 m	about 89 m	about 159 m
100 km/h	about 98 m	about 129 m	about 238 m
130 km/h	about 149 m	about 202 m	about 387 m

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About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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