Reaction Time & Stopping Distance Calculator
Enter your reaction time, vehicle speed and braking deceleration to find the reaction (thinking) distance, braking distance, and total stopping distance.
ms
km/h
m/s²
Reaction distance + braking distance
- 1
Speed in m/s
60 ÷ 3.6 = 16.667 - 2
Reaction distance
16.667 × 0.25 s = 4.17Distance covered at full speed before the brakes are applied. - 3
Braking distance
16.667² ÷ (2 × 7) = 19.84 - 4
Total stopping distance
4.17 + 19.84 = 24
How does this calculator work?
Stopping distance = reaction distance (v × t) + braking distance (v² / 2a). At 60 km/h with a 250 ms reaction time and 7 m/s² deceleration, the reaction distance is ~4.2 m and braking distance ~20.1 m, giving a total stopping distance of ~24 m. Doubling speed quadruples the braking portion.
Formula
How this is calculated
Stopping a vehicle involves two distinct phases. During the reaction phase — from the moment a hazard is perceived until the brakes are applied — the vehicle travels at full speed. This reaction (or "thinking") distance is simply d_reaction = v × t, where v is the speed in m/s and t is the reaction time in seconds. A typical alert driver has a reaction time of 200–250 ms; fatigue, distraction, or impairment can push it to 600 ms or more.
Once the brakes are applied, the vehicle decelerates at a roughly constant rate a (in m/s²). The braking distance is d_braking = v² / (2a), derived from the kinematic equation v² = u² − 2a·s solved for s when the final velocity is zero. This component grows with the square of speed — doubling speed quadruples braking distance. Typical emergency deceleration on a dry road with good tyres is about 7–9 m/s²; wet roads reduce this to 4–5 m/s², and icy surfaces to 1–2 m/s².
The total stopping distance is the sum of both phases. The interactive plot shows how stopping distance increases with speed for your chosen reaction time and deceleration — note the steep parabolic rise that explains why speed limits matter so much for safety.
Frequently asked questions
Research studies report typical alert driver reaction times of 200–250 ms under simple test conditions. In complex real-world traffic, reaction times of 400–600 ms are more common. Fatigue, mobile phone use, alcohol, and age all increase reaction time significantly. The UK Highway Code traditionally uses 0.67 s (670 ms) as a conservative figure.
Braking distance is proportional to v² — it follows the kinetic energy equation (½mv²) that must be dissipated by the brakes. Doubling your speed from 30 to 60 km/h quadruples the braking distance. This non-linear relationship is why the consequences of excessive speed are so severe.
On a dry road with good tyres, modern cars can achieve 7–9 m/s² (0.7–0.9 g) of deceleration. On wet roads this drops to about 4–5 m/s², and on icy or snowy surfaces to 1–2 m/s². These are typical values — tyre condition, road surface, and vehicle braking systems all affect the actual figure.
Also known as
TG we-Calculate Editorial Team. (2026). Reaction Time & Stopping Distance Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/reaction-time-calculator
TG we-Calculate Editorial Team. "Reaction Time & Stopping Distance Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/reaction-time-calculator.
TG we-Calculate Editorial Team, "Reaction Time & Stopping Distance Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/reaction-time-calculator
@misc{wecalculate_reaction_time_calculator, title = {Reaction Time & Stopping Distance Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/reaction-time-calculator}}, year = {2026}, note = {TG we-Calculate} }
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