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Braking Distance Calculator

Enter speed, reaction time, and braking deceleration. Get reaction distance, braking distance, and total stopping distance — useful for training and physics intuition, not a guarantee of real-world stops.

Calculate

Measurements convert when you switch units

km/h
s

Typical dry ~7–9 m/s²; wet often much lower

m/s²

Updates as you type

Result

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The formula

Total stopping distance is thinking/reaction distance plus braking distance. Reaction distance is speed × reaction time. Braking distance uses constant deceleration: v² ÷ (2a).

d_reaction = v × t · d_brake = v² ÷ (2a) · d_total = d_reaction + d_brake (v in m/s, a in m/s²)

Worked example

  1. 100 km/h (= 27.78 m/s), t = 1.5 s, a = 7.0 m/s²
  2. Reaction = 27.78 × 1.5 ≈ 41.7 m
  3. Braking = 27.78² ÷ (2×7) ≈ 55.1 m · Total ≈ 96.8 m

Result: ≈ 41.7 m + 55.1 m = 96.8 m

How stopping distance is calculated

We assume constant speed during reaction, then constant deceleration while braking. Real ABS stops vary with tires, surface, slope, and brake condition.

Reaction time

Typical alert drivers are often around 1–1.5 s; distraction, fatigue, and alcohol increase it. Distance grows linearly with time at a given speed.

Deceleration

Dry asphalt with good tires might approach ~7–9 m/s² (about 0.7–0.9 g). Wet roads and worn tires are much lower. Enter the a that matches your scenario.

What we do not model

No ABS modulation curves, grade, aerodynamic drag, or brake fade. Highway code charts in some countries use fixed assumptions — your inputs may differ.

Interesting facts

Speed hurts twice

Braking distance scales with speed squared. Doubling speed roughly quadruples braking distance if deceleration stays the same.

Wet roads

Water sharply cuts available friction. The same speed that stops safely in the dry can need far more room when wet.

Following distance

Rules of thumb (seconds behind the car ahead) exist because total stopping distance grows quickly with speed.

Perception vs reaction

Some models split perception and movement time; this tool uses one combined reaction time for simplicity.

Frequently asked questions

Convert speed to m/s, then use d = v² / (2a) for constant deceleration. Add v × reaction time for the thinking distance.

Try ~7 m/s² for a strong dry stop with good tires; lower for wet or poor grip. Emergency maximums vary widely.

Many official charts use fixed reaction times and assumed decelerations. Match their assumptions if you want comparable numbers.

ABS helps you steer while braking and can improve stops on mixed surfaces; peak dry distance depends more on tires and speed.

Toggle units for km/h vs mph and metres vs feet. Deceleration switches between m/s² and ft/s².

References

  1. Stopping distances — UK Highway Code Typical thinking and braking distance guidance (UK).
  2. NHTSA braking research — NHTSA Vehicle safety agency context for braking performance.