Stopping Distance Calculator — Reaction + Braking Distance
Calculate the total stopping distance for a vehicle — the sum of reaction (thinking) distance and braking distance — for any speed, reaction time and road surface.
km/h
s
Road surface
Reaction distance + braking distance
- 1
Convert speed to m/s
v = 60 km/h ÷ 3.6 = 16.667 - 2
Reaction distance
d_r = 16.667 m/s × 0.75 s = 12.5 - 3
Braking deceleration
a = μ × g = 0.75 × 9.80665 m/s² = 7.355 - 4
Braking distance
d_b = 16.667² ÷ (2 × 7.355) = 18.88From kinematics: v² = 2a·d_b, so d_b = v² / (2a). - 5
Total stopping distance
12.5 + 18.88 m = 31.4
How does this calculator work?
Total stopping distance = reaction distance + braking distance = v × t_r + v² / (2μg). At 60 km/h (16.7 m/s) with t_r = 0.75 s and μ = 0.75: reaction = 12.5 m, braking = 18.9 m, total ≈ 31 m. Doubling speed quadruples braking distance — the dominant component at higher speeds.
Formula
How this is calculated
Total stopping distance has two components. The first is the reaction (or thinking) distance: the distance the vehicle travels while the driver perceives the hazard and moves their foot to the brake, equal to d_r = v × t_r, where v is vehicle speed (m/s) and t_r is reaction time (s). An alert, sober driver typically has a reaction time of 0.7–1.0 s; distractions or impairment can push this above 2 s.
The second component is the braking distance: once the brakes are applied, the vehicle decelerates at a = μg, where μ is the tyre-road friction coefficient and g = 9.807 m/s². Using kinematics (v² = u² + 2as), the braking distance is d_b = v² / (2μg). The friction coefficient is roughly 0.75 on dry asphalt, 0.35 on wet asphalt and 0.10–0.15 on ice — meaning braking distance roughly doubles from dry to wet, and increases fivefold on ice.
The model assumes maximum braking (locked wheels or ideal ABS), a flat road and a constant friction coefficient throughout the stop. In reality, ABS may slightly increase stopping distance on wet roads; slopes and vehicle mass distribution add further variation. The formula is widely used in traffic engineering and accident reconstruction to estimate pre-crash speeds from skid-mark lengths.
Frequently asked questions
Braking distance scales with the square of speed: doubling speed quadruples the braking distance. Reaction distance only doubles. So at high speeds, braking distance dominates — a car travelling at 120 km/h has a braking distance roughly four times that at 60 km/h, with only twice the reaction distance.
Dry asphalt μ ≈ 0.70–0.80; wet asphalt μ ≈ 0.30–0.40; ice or compacted snow μ ≈ 0.10–0.15; loose gravel ≈ 0.40–0.55. ABS prevents wheel lock-up, keeping friction near its peak rather than dropping to the (lower) sliding value.
UK Highway Code figures (e.g. 23 m at 30 mph) assume a specific reaction time of approximately 0.67 s and an overall deceleration equivalent to about μ = 0.65. This calculator lets you vary both parameters — use reaction 0.67 s and μ = 0.65 to reproduce the Highway Code values.
Also known as
TG we-Calculate Editorial Team. (2026). Stopping Distance Calculator — Reaction + Braking Distance [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/stopping-distance-calculator
TG we-Calculate Editorial Team. "Stopping Distance Calculator — Reaction + Braking Distance." TG we-Calculate. 2026. https://we-calculate.com/calculator/stopping-distance-calculator.
TG we-Calculate Editorial Team, "Stopping Distance Calculator — Reaction + Braking Distance," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/stopping-distance-calculator
@misc{wecalculate_stopping_distance_calculator, title = {Stopping Distance Calculator — Reaction + Braking Distance}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/stopping-distance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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