Stopping Distance Calculator
From your speed, your reaction time, and the grip of the road, get the reaction distance, the braking distance, and the total distance you need to come to a stop.
Reaction plus braking
Total stopping distance is what you cover while you react (v × t) plus what you cover once the brakes are on (v² / (2 × μ × g)) — this tool returns all three numbers at once.
What is a stopping distance calculator?
Speed and grip in, full stopping distance out
A stopping distance calculator turns four numbers — your speed, your reaction time, the tyre–road friction coefficient, and gravity — into the distance your vehicle actually needs to come to a halt. It splits that distance into two honest parts: the reaction distance, covered in the moment between seeing a hazard and pressing the brake, and the braking distance, covered while the brakes are slowing you down. Driving instructors, road-safety campaigns, and accident reconstructors all use the same physics. The headline lesson is that stopping is never instant: even a fully alert driver travels a surprising distance before the car is stationary.
Enter your speed in km/h along with the reaction time and road grip to see the reaction, braking, and total stopping distances instantly.
The speed is first converted from km/h to m/s by dividing by 3.6, then two short formulas do the rest.
distance = v × t + v² / (2 × μ × g)The reaction distance is simply speed × reaction time — at a steady speed you keep moving until you act. The braking distance comes from energy: the work the tyres do (friction force μ × m × g over a distance) has to absorb the car's kinetic energy ½ × m × v². The mass cancels out, leaving v² / (2 × μ × g). Because v is squared, doubling your speed quadruples the braking distance.
Suppose you are doing 100 km/h on dry asphalt (μ = 0.7) with a 1.5 s reaction time.
Convert the speed
100 / 3.6 = 27.777778 m/s — the speed in SI units.
Reaction distance
27.777778 × 1.5 = 41.666667 m — covered before you even touch the brake.
Braking distance
27.777778² / (2 × 0.7 × 9.81) = 56.182098 m — covered while braking.
Total stopping distance
41.666667 + 56.182098 = 97.848765 m — almost the length of a football pitch.
The split between the two distances tells the safety story. At 100 km/h the reaction distance (about 41.666667 m) is fixed by your speed and alertness alone — it does not depend on the brakes or the road at all, which is why tiredness, alcohol, and phone use are so dangerous: they stretch this part directly. The braking distance (about 56.182098 m here) is where the road surface and your tyres matter. Drop the friction coefficient from 0.7 (dry) to 0.4 (wet) and the braking distance jumps from roughly 56 m to about 98 m, while the reaction distance does not budge. Speed is the most powerful lever of all because it appears squared in the braking term: going from 50 to 100 km/h does not double the braking distance, it quadruples it. A practical takeaway is that the comfortable "two-second rule" gap exists precisely to cover the reaction distance, with the braking distance as your remaining margin. Treat the total — almost 98 m at 100 km/h on a good day — as a best case; rain, worn tyres, a loaded vehicle, or a slow reaction all make it longer.
The formula is a clean physical model, but real braking is messier.
An idealised model, not a guarantee
This model assumes constant deceleration, a level dry-or-wet road, and one representative friction coefficient throughout the stop. Real braking varies: ABS, brake fade, downhill or uphill gradients, tyre wear, vehicle load, and weight transfer all change the effective grip moment to moment. The reaction time is also an estimate — published values range from under a second for a prepared driver to well over two seconds when tired or distracted. Use the result as a physics-based ballpark for understanding and comparison, never as a precise safety distance, and always keep a generous margin on the road.