How to use this calculator
Choose a speed unit and enter the speed before the driver reacts. Enter the delay from perceiving a need to stop until braking begins, then a positive constant deceleration for the braking phase. The result separates travel during that delay from travel while slowing to rest. Set reaction time to zero to examine braking distance alone.
The model holds speed constant during reaction, then applies the entered deceleration instantly until rest. It does not model brake-force buildup, changes in grip, ABS behavior, brake fade, curves, road grade or a vehicle-specific braking system. Do not use the result to set a minimum following gap, a safe driving speed or a road-design standard. Real stopping performance requires conditions and vehicle data that these inputs cannot establish.
The formula
A worked example
For a hypothetical vehicle traveling at 72 km/h (20 m/s), a 1.5-second delay covers 30 m. Braking at a constant 5 m/s² then takes 4 seconds and 40 m. Total distance is 70 m (229.66 ft), and total time is 5.5 seconds. These are calculated values for the entered assumptions, not a road test or a stopping guarantee.
How speed changes the two distance components
Original hypothetical scenarios using 1.5 s reaction time and 5 m/s² constant deceleration. No vehicle or surface performance is implied.
| Speed (km/h) | Reaction (m) | Braking (m) | Total (m) |
|---|---|---|---|
| 36 | 15 | 10 | 25 |
| 72 | 30 | 40 | 70 |
| 108 | 45 | 90 | 135 |
Common questions
What is the difference between braking distance and stopping distance?
Braking distance starts when the assumed deceleration begins. Total stopping distance also includes the distance traveled before that point during perception and reaction. A published braking test may exclude the driver delay, so check its measurement definition before comparing it with the total result.
Why does doubling speed more than double the result?
At unchanged reaction time, reaction distance doubles. At unchanged deceleration, braking distance grows with speed squared and becomes four times as large. In the table, increasing speed from 36 to 72 km/h changes reaction distance from 15 to 30 m and braking distance from 10 to 40 m.
What deceleration should I enter for rain, ice or worn tires?
There is no surface preset here. A label such as wet road cannot establish the deceleration a particular vehicle will achieve. Enter a supported measurement or clearly labeled scenario assumption. Do not use the illustrative default as a claim about available grip.
Is deceleration in g the same as tire friction coefficient?
No. The input is the net rate at which speed decreases, expressed relative to standard gravity. Equating it with a friction coefficient requires additional assumptions about forces and the road. This calculator does not infer tire friction or add a grade correction.
Why does vehicle mass not appear?
Deceleration is already supplied as an input. Once that rate is specified, the motion equations use speed and time directly. This does not imply that vehicle loading has no effect on real braking; the calculator does not predict deceleration from mass, brakes or tires.
Can I use a 60–0 mph stopping-test distance?
For a braking-only distance d from speed v, an equivalent constant deceleration is v²/(2d), using meters and seconds. For example, 20 m/s to rest in 40 m corresponds to 5 m/s². This reproduces that distance in the model, but does not recover a real variable deceleration curve or predict another road condition.
What does zero reaction time mean?
It removes the pre-braking travel so you can isolate the braking calculation. It is a mathematical scenario, not a statement that a driver or braking system can respond instantaneously. When initial speed is zero, all distances and stopping times are zero.
Sources & calculation method
FHWA research report, Chapter 5: stopping sight distance describes the reaction-plus-braking relationship. This implementation uses the underlying constant-deceleration motion equations with exact speed conversions, rather than rounded km/h coefficients. It does not adopt the report’s participant results as automobile performance data.
FHWA: driver, vehicle and roadway characteristics explains why reaction, vehicle performance and road conditions affect stopping. These references provide context; this calculator is not a highway-design or compliance tool. Its examples are original arithmetic.
For tire dimensions, use the Tire Size Calculator; size alone does not establish braking grip. The Rolling Resistance Calculator estimates a different road-load force, not braking friction. The Hill-Climbing Power Calculator treats the power needed to climb a grade.
References accessed September 27, 2026. Neither source endorses this site or independently reviews this calculator.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.