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Stopping Distance Calculator

Computes the reaction (thinking) distance, braking distance and total stopping distance of a vehicle from its speed, the driver's reaction time and either a road-condition friction coefficient or a given deceleration, in metres and feet.

When to use

You want to know how far a car travels before it stops from a given speed on dry, wet, snowy or icy roads, or the effect of reaction time.

Do not use when: You need the deceleration itself from speeds and times (use acceleration), or the energy dissipated in braking (use kinetic-energy).

Formula

v = speed_km_h / 3.6; a = μ × 9.80665 (or deceleration_m_s2); reaction_distance_m = v × reaction_time_s; braking_distance_m = v² / (2 a); total = reaction + braking; braking_time_s = v / a

Constant-deceleration model on level road; braking distance grows with the square of speed. Friction values are typical (dry asphalt 0.7, wet 0.4, snow 0.2, ice 0.1); real ABS-equipped cars on dry roads reach 0.8–1.0 g. Grade, tyre wear and brake fade are not modelled.

Inputs

Parameter	Type	Unit	Required	Description
`speed_km_h`	number	km/h	yes	Initial vehicle speed in km/h (mph × 1.609344). Range: > 0, ≤ 500
`reaction_time_s`	number	s	default 1.5	Perception–reaction time before the brakes act: 1.0–1.5 s alert driver, 2.5 s is the AASHTO road-design value. Range: ≥ 0, ≤ 10
`road_condition`	enum: dry \| wet \| snow \| ice		default dry	Typical tyre–road friction coefficient; overridden by friction_coefficient or deceleration_m_s2 when given.
`friction_coefficient`	number		no	Optional custom coefficient of friction (deceleration = μ × 9.80665 m/s²). Range: > 0, ≤ 1.5
`deceleration_m_s2`	number	m/s²	no	Optional braking deceleration in m/s² (e.g. 3.4 for the AASHTO design value, 8–10 for a modern car with ABS on dry road); takes precedence over friction. Range: > 0, ≤ 20

Outputs

Output	Type	Unit	Description
`speed_m_s`	number	m/s	speed_km_h / 3.6.
`deceleration_m_s2`	number	m/s²	μ × g or the given deceleration.
`reaction_distance_m`	number	m	Distance covered at constant speed during the reaction time: v × t_r.
`braking_distance_m`	number	m	v² / (2 a).
`total_stopping_distance_m`	number	m	Reaction distance + braking distance.
`reaction_distance_ft`	number	ft	In feet (÷ 0.3048).
`braking_distance_ft`	number	ft	In feet.
`total_stopping_distance_ft`	number	ft	In feet.
`braking_time_s`	number	s	v / a.
`total_time_s`	number	s	reaction_time_s + braking_time_s.

Example

100 km/h, 1.5 s reaction, dry road: {"speed_km_h":100,"reaction_time_s":1.5,"road_condition":"dry"} → {"speed_m_s":27.7778,"deceleration_m_s2":6.8647,"reaction_distance_m":41.67,"braking_distance_m":56.2,"total_stopping_distance_m":97.87,"total_stopping_distance_ft":321.1,"braking_time_s":4.046,"total_time_s":5.546}

50 km/h, 1.0 s reaction, wet road: {"speed_km_h":50,"reaction_time_s":1,"road_condition":"wet"} → {"reaction_distance_m":13.89,"braking_distance_m":24.59,"total_stopping_distance_m":38.48}

GET https://tttkmbb.com/api/v1/calculate/stopping-distance?speed_km_h=100&reaction_time_s=1.5&road_condition=dry

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/stopping-distance (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/stopping-distance · Markdown: https://tttkmbb.com/engineering/stopping-distance.md · JSON definition: https://tttkmbb.com/engineering/stopping-distance.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="stopping-distance"
OpenAPI operationId: calculate_stopping_distance
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Why does doubling the speed quadruple the braking distance?

Kinetic energy grows with v², and the brakes remove it at a roughly constant force, so the distance v²/(2a) quadruples while the reaction distance only doubles.

Which reaction time should I use?

About 1.0–1.5 s for an alert driver expecting to brake; road designers use 2.5 s to cover most drivers in unexpected situations. Distraction or fatigue adds a second or more.

Related calculators

Speed, Distance & Time Calculator — Uniform motion during the reaction phase.
Acceleration Calculator — Deceleration from speed change and time.
Kinetic Energy Calculator — Energy the brakes must dissipate.