Stopping distance calculator

Enter speed, reaction time, and deceleration. The calculator adds s_reaction = v t_r and s_brake = v²/(2a). 14 m/s, about 50 km/h, at 0.8 s and 7 m/s² is about 25 m to a full stop.

Result

Speed v, Reaction time t_r (s), Deceleration a (positive) and μ (optional → a = μg). The result shows up here.

How it works

Thinking distance + braking distance = stopping distance.

Stopping distance is a sum: you still travel at constant v during t_r, then you brake s_brake = v²/(2a). At 14 m/s, t_r = 0.8 s, and a = 7 m/s²: s_reaction = 11.2 m, s_brake = 14 m, total 25.2 m. 50 km/h is 13.9 m/s, not 50 in an m/s field. At 13.9 m/s, t_r = 0, and a = 6 m/s² you keep s_brake alone ≈ 16.1 m; the same v and a at t_r = 1 s add 13.9 m.

Fields: v, t_r (often 0.8 s), positive a, or μ (then a = μ g). Result in meters. 50 in an m/s field is 180 km/h. A comma and a period are the same t_r: 0,8 and 0.8. At 20 m/s, t_r = 0.8 s, and μ = 0.4 you have a = 0.4 × 9.81 ≈ 3.92 m/s², s_brake ≈ 51 m, total about 67 m.

v must be a number. a = 0 does not stop you: you divide by 2a. If both μ and a are present, follow the field labels; do not mix them blindly. Typed 0 m/s gives 0 m. v is squared in s_brake, so twice the speed is four times the braking distance, plus a longer reaction stretch.

Decelerated motion without t_r is on the neighbouring page: there from the moment you hit the pedal. Here it stays the sum of reaction and braking. t_brake = v/a is braking time only, not t_r. At 14 m/s and 7 m/s², t_brake = 2 s.

36 m/s, about 80 mph, at t_r = 0.8 s and a = 7 m/s² is s_reaction = 28.8 m and s_brake ≈ 92.6 m, total about 121 m. License tables may use different a and t_r; this is the classroom formula.

Type 14, 0.8, and 7, click Calculate, and match about 25 m. Then 13.9, 0, and 6, then 13.9, 1, and 6. The header symbol does not hit the pedal. Thinking distance is v times reaction time.

Braking with no t_r: decelerated motion.

How to use

Type v in m/s, for example 14. 50 km/h is 13.9 m/s, not 50. 50 in an m/s field is 180 km/h.
Type t_r in seconds, for example 0.8. Zero means the pedal goes down at once, as on decelerated motion.
Type deceleration a, for example 7, or μ. At μ = 0.4 the calculator uses a = μ g ≈ 3.92 m/s². Do not mix μ with a blindly.
Click Calculate. 14 m/s, 0.8 s, and 7 m/s² give 11.2 m of reaction, 14 m of braking, and 25.2 m together. 13.9 m/s, t_r = 1 s, and a = 6 give about 30 m.
With no reaction time, open decelerated motion. Energy ½mv² is on the Ek page. Here it stays the sum v t_r + v²/(2a).

Formula

s_react = v·t_r

s_brake = v²/(2a)

s_total = s_react + s_brake

t_brake = v/a; optional a = μ g

t_r, braking s, and a at 14 m/s

Total stopping distance here is thinking plus braking. 14 m/s, t_r = 0.8 s, and a = 7 m/s² give 11.2 m + 14 m = 25.2 m. 50 in an m/s field is not 50 km/h.

t_r: Reaction time. You still roll at constant v. At 14 m/s and 0.8 s that is 11.2 m. Zero t_r leaves braking distance alone.
v: Speed in m/s on the label. 50 km/h = 13.9 m/s. 50 in the field is 180 km/h.
a: Positive deceleration. Or type μ, then a = μ g. At μ = 0.4 and 20 m/s, braking s ≈ 51 m.
μ: Optional coefficient. Dimensionless. Do not mix μ with a in the same run blindly.
Thinking distance: v times t_r, the first slice of the stop. At 14 m/s and 0.8 s it is 11.2 m, then 14 m of braking.

Real-life examples

Example 1

v = 13.9 m/s, a = 6, t_r = 0: like decelerated.

Example 2

Same v and a, t_r = 1 → +13.9 m.

Example 3

v = 36 m/s (~80 mph), a = 7, t_r = 0.8.

Example 4

v = 20 m/s, t_r = 0.8, μ = 0.4.

Example 5

v = 8 m/s, a = 3, t_r = 0.6.

Example 6

v = 25 m/s, t_r = 1, μ = 0.7.

Example 7

v = 8.3 m/s, a = 5, t_r = 0.8.

Example 8

v = 22 m/s, a = 9, t_r = 0.5.

Example 9

v = 15 m/s, t_r = 1, μ = 0.1.

Ways to use this calculator

You estimate stopping distance at 14 m/s and 0.8 s reaction.
You compare t_r = 0 with t_r = 0.8 s.

Frequently asked questions

How many meters at 14 m/s, 0.8 s, and a = 7 m/s²?

s_reaction = 11.2 m. s_brake = 14 m. Total 25.2 m. t_brake = 14/7 = 2 s, and that is not t_r.

Is 50 in the field 50 km/h?

Not if the label is m/s. 50 km/h = 13.9 m/s. 50 m/s is 180 km/h. 36 m/s is about 80 mph.

Which units do I type?

v in m/s on the label, t_r in seconds, a in m/s². Result in meters. μ is dimensionless: a = μ g.

How is t_r different from t_brake?

t_r is time before you hit the pedal, still at constant v. t_brake = v/a is braking time only. At 14 m/s and 7 m/s², t_brake = 2 s.

Where is s_brake with no reaction?

On the decelerated-motion page. Here the sum includes t_r. At 13.9 m/s and a = 6 m/s², s_brake alone is about 16.1 m.

Does μ replace a?

If you type μ, a = μ g. At 0.4 that is about 3.92 m/s². Do not mix μ with a blindly; follow the labels.

Is this a driving-test table?

This is the classroom formula v t_r + v²/(2a). License tables may use different a and t_r. Here 14 m/s, 0.8 s, and 7 m/s².

How much at 36 m/s, 0.8 s, and a = 7 m/s²?

s_reaction = 28.8 m. s_brake ≈ 92.6 m. Total about 121 m. 36 m/s is about 80 mph.

What if a = 0?

You do not stop. You divide by 2a. v = 0 gives 0 m. v must be a number before 25.2 m can appear.

Does a comma in 0.8 work?

Yes. 0,8 and 0.8 are the same t_r. The calculator does not require a period.

Knowledge sources

The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.

Page updated in 2026.