Physics · Kinematics

Braking and stopping distance calculator

Separate distance travelled during reaction from distance under constant deceleration. This is an ideal physics calculation, not a guaranteed real stopping distance or a driving rule.

Created Oct 3, 2026v1.0

By Thibaut Grzelak, Mechanical Analysis Engineer

Kinematics

Two consecutive segments show reaction then braking, in proportion to their calculated distances.

Quantity to calculate

Initial speed

Speed before reaction and braking, ≥ 0.

Reaction time

Time at unchanged speed before braking, ≥ 0.

Deceleration magnitude

Constant positive braking deceleration.

Results

Stopping distance

—

Reaction distance plus braking distance in the ideal model.

Reaction distance

—

Distance travelled before braking begins.

Braking distance

—

Distance during constant deceleration to rest.

Braking time

—

Excludes reaction time.

Relation and numerical substitutiond_s = v t_r + v² / (2a)

—

Constant initial speed during reaction, then constant positive deceleration on a straight path until rest.

01Formulas and symbols

Formulas used

d_rd_r = v × t_rDistance travelled before braking begins.

d_bd_b = v² / (2 × a)Distance during constant deceleration to rest.

d_sd_s = d_r + d_bReaction distance plus braking distance in the ideal model.

t_bt_b = v / aExcludes reaction time.

02Assumptions and limits

Scope of validity

Constant initial speed during reaction, then constant positive deceleration on a straight path until rest.

03Validation example

Reference numerical case

72 km/h = 20 m/s, tr = 1 s, a = 5 m/s²: dr = 20 m, db = 40 m and ds = 60 m. Braking alone lasts 4 s.

04References

Technical references

University Physics 1, §3.4 Constant acceleration — OpenStax

FAQ

What happens if speed doubles?

At fixed reaction time and deceleration, reaction distance doubles and braking distance quadruples.

Is the displayed time the total stopping time?

The secondary time is braking only, v/a. Add the reaction time to obtain the ideal total time.

05Model and conventions

Separate distance travelled during reaction from distance under constant deceleration. This is an ideal physics calculation, not a guaranteed real stopping distance or a driving rule.

06Limits of this model

Slope, tyres, road surface, ABS, weather, adhesion and driver response can strongly change real stopping distances. No regulatory driving recommendation is provided.

07Common mistakes

Convert km/h to m/s before squaring speed. Enter a positive deceleration magnitude, not a negative signed acceleration.