acalculator

How long is the stopping distance?

Type the speed, the reaction time and how hard the vehicle brakes. The stopping distance calculator adds the distance covered while the driver reacts to the distance covered while braking. It starts with the AASHTO road design values of 2.5 s and 11.2 ft/s².

Your numbers

Units
Braking by
Stopping distance
565.7 ft

At 60 mph, the stopping distance is 565.7 ft: 220 ft of reaction and 345.7 ft of braking.

Reaction distance
220 ft
Braking distance
345.7 ft
Stopping distance (m)
172.43
Stopping distance (ft)
565.714
Time to stop (s)
10.36
Deceleration used (m/s²)
3.41376

Stopping distance: 565.7 ft. At 60 mph, the stopping distance is 565.7 ft: 220 ft of reaction and 345.7 ft of braking.

How to calculate

Stopping distance calculator: the reaction distance plus the braking distance of a vehicle from its speed, the reaction time and the braking deceleration or road friction, on a level road or a grade.

Example with the default inputs (Speed 60 mph, Reaction time (s) 2.5, Braking by Deceleration rate, Deceleration 11.2, Deceleration unit ft/s², Grade 0%): At 60 mph, the stopping distance is 565.7 ft: 220 ft of reaction and 345.7 ft of braking.

Method: Stopping distance = v × t + v² ÷ (2 × (a + g × G)), with v the speed, t the reaction time, a the braking deceleration (or μ × g), G the grade as a decimal (uphill positive) and g = 9.80665 m/s²; time to stop = t + v ÷ (a + g × G).

  • Constant speed during the reaction time and constant deceleration while braking (OpenStax §3.4).
  • The grade adds g × G to the deceleration, as in AASHTO’s braking distance on a grade (which uses g = 32.2 ft/s²).

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Speed 30 mph, Reaction time (s) 2.5, Braking by Deceleration rate, Deceleration 11.2, Deceleration unit ft/s², Grade 0% gives Stopping distance (ft) 196.428571, Reaction distance 110 ft, Braking distance 86.43 ft.Source: TxDOT Roadway Design Manual, 4.11.1 Stopping Sight Distance (2.5 s brake reaction time, 11.2 ft/s² deceleration; Table 4-23: 30 mph 110.3 + 86.4 = 196.7 ft, 60 mph 220.5 + 345.5 = 566.0 ft, from AASHTO’s SSD = 1.47Vt + 1.075V² ÷ a), https://www.txdot.gov/manuals/des/rdw/chapter-4--basic-design-criteria/4-11-sight-distance/4-11-1-stopping-sight-distance.html (retrieved 2026-10-18); FHWA, Self-Enforcing Roadways: A Guidance Report (FHWA-HRT-17-098), Chapter 3 (AASHTO 2011: SSD = 1.47Vt + 1.075V² ÷ a, t = 2.5 s; on a grade d = V² ÷ (30((a ÷ 32.2) ± G))), https://www.fhwa.dot.gov/publications/research/safety/17098/004.cfm (retrieved 2026-10-18)
  2. Speed 60 mph, Reaction time (s) 2.5, Braking by Deceleration rate, Deceleration 11.2, Deceleration unit ft/s², Grade 0% gives Stopping distance (ft) 565.714286, Time to stop (s) 10.357143.Source: TxDOT Roadway Design Manual, 4.11.1 Stopping Sight Distance (2.5 s brake reaction time, 11.2 ft/s² deceleration; Table 4-23: 30 mph 110.3 + 86.4 = 196.7 ft, 60 mph 220.5 + 345.5 = 566.0 ft, from AASHTO’s SSD = 1.47Vt + 1.075V² ÷ a), https://www.txdot.gov/manuals/des/rdw/chapter-4--basic-design-criteria/4-11-sight-distance/4-11-1-stopping-sight-distance.html (retrieved 2026-10-18)
  3. Speed 67.11 mph, Reaction time (s) 0.5, Braking by Deceleration rate, Deceleration 7, Deceleration unit m/s², Grade 0% gives Stopping distance 260.1 ft, Reaction distance 49.21 ft, Braking distance 210.9 ft.Source: OpenStax, University Physics Volume 1, §3.4 Motion with Constant Acceleration (Equation 3.14, v² = v₀² + 2a(x − x₀); Example 3.10: 30.0 m/s, 0.500 s, 7.00 m/s² dry or 5.00 m/s² wet: 79.3 m or 105 m), https://openstax.org/books/university-physics-volume-1/pages/3-4-motion-with-constant-acceleration (retrieved 2026-10-18): 15.0 m + 64.3 m = 79.3 m
  4. Speed 67.11 mph, Reaction time (s) 0.5, Braking by Deceleration rate, Deceleration 5, Deceleration unit m/s², Grade 0% gives Stopping distance 344.5 ft, Braking distance 295.3 ft.Source: OpenStax, University Physics Volume 1, §3.4 Motion with Constant Acceleration (Equation 3.14, v² = v₀² + 2a(x − x₀); Example 3.10: 30.0 m/s, 0.500 s, 7.00 m/s² dry or 5.00 m/s² wet: 79.3 m or 105 m), https://openstax.org/books/university-physics-volume-1/pages/3-4-motion-with-constant-acceleration (retrieved 2026-10-18): 15.0 m + 90.0 m = 105 m
  5. Speed 62.14 mph, Reaction time (s) 1.5, Braking by Friction coefficient, Friction coefficient (μ) 0.7, Grade 0% gives Stopping distance 321.1 ft, Deceleration used (m/s²) 6.864655.
  6. Speed 60 mph, Reaction time (s) 2.5, Braking by Deceleration rate, Deceleration 11.2, Deceleration unit ft/s², Grade -3% gives Braking distance 378.3 ft, Deceleration used (m/s²) 3.119561.Source: FHWA, Self-Enforcing Roadways: A Guidance Report (FHWA-HRT-17-098), Chapter 3 (AASHTO 2011: SSD = 1.47Vt + 1.075V² ÷ a, t = 2.5 s; on a grade d = V² ÷ (30((a ÷ 32.2) ± G))), https://www.fhwa.dot.gov/publications/research/safety/17098/004.cfm (retrieved 2026-10-18)

How it works

  • Reaction distance = v × t: the vehicle keeps its speed v during the reaction time t.
  • Braking distance = v² ÷ (2 × a_eff), from v² = v₀² + 2a(x − x₀) with the final speed 0.
  • Stopping distance = reaction distance + braking distance, and the time to stop = t + v ÷ a_eff.
  • Deceleration: a_eff = a + g × G, with a the braking deceleration, G the grade as a decimal (3% uphill is 0.03, 3% downhill is −0.03) and g = 9.80665 m/s². With a friction coefficient μ, a = μ × g.

Defaults: AASHTO road design uses t = 2.5 s and a = 11.2 ft/s² (3.41376 m/s²), deceleration that about 90% of drivers exceed (TxDOT). AASHTO writes the same physics as SSD = 1.47Vt + 1.075V² ÷ a with V in mph and a in ft/s²; its 1.47 and 1.075 are rounded (1 mph = 1.4667 ft/s exactly, and 1.4667² ÷ 2 = 1.0756), so its tables come out a little higher than this page: 196.7 ft at 30 mph against 196.4 ft here. On a grade, AASHTO uses g = 32.2 ft/s²; this page uses the exact 32.174 ft/s².

Units (exact): 1 mph = 0.44704 m/s; 1 km/h = 5/18 m/s; 1 ft/s = 0.3048 m/s; 1 ft/s² = 0.3048 m/s²; 1 g = 9.80665 m/s².

Rules

  • Speed from 0.01 to 150 m/s (0.02237 to 335.5 mph); reaction time from 0 to 10 s; deceleration from 0.001 to 400 in its unit; μ from 0.01 to 2; grade from −30% to 30%.
  • If a_eff is 0 or less (a downhill grade steeper than the braking can hold), there is no answer.

Output format

Distances show to 4 significant figures in feet (metres in Metric); the stopping distance also shows in metres and in feet to 6 significant figures, the deceleration used (m/s²) to 6, and the time to stop to 4. Typed values are read back as exact decimals and every unit size is exact, so every result is an exact fraction rounded once.

Worked examples by hand

30 mph, AASHTO values. v = 44 ft/s: reaction 44 × 2.5 = 110 ft; braking 44² ÷ (2 × 11.2) = 1,936 ÷ 22.4 = 86.43 ft; total 196.43 ft (59.87 m). TxDOT Table 4-23 lists 110.3 + 86.4 = 196.7 ft with AASHTO's rounded constants, and 200 ft for design.

60 mph, AASHTO values. v = 88 ft/s: reaction 220 ft; braking 7,744 ÷ 22.4 = 345.71 ft; total 565.71 ft; time 2.5 + 88 ÷ 11.2 = 10.357 s. Table 4-23: 566.0 ft, 570 ft for design.

OpenStax, dry concrete. v = 30.0 m/s, t = 0.500 s, a = 7.00 m/s²: 15.0 m + 900 ÷ 14 = 64.29 m, total 79.29 m.

OpenStax, wet concrete. a = 5.00 m/s²: 15.0 m + 900 ÷ 10 = 90.0 m, total 105 m.

Friction. 100 km/h (250/9 m/s), t = 1.5 s, μ = 0.7: a = 0.7 × 9.80665 = 6.864655 m/s²; reaction 41.67 m; braking (250/9)² ÷ (2 × 6.864655) = 56.20 m; total 97.87 m.

Downhill. 60 mph on a 3% downhill grade: a_eff = 3.41376 − 0.03 × 9.80665 = 3.1195605 m/s²; braking 26.8224² ÷ (2 × 3.1195605) = 115.31 m (378.3 ft). AASHTO's grade form gives 3,600 ÷ (30 × (11.2 ÷ 32.2 − 0.03)) = 377.6 ft.

Other questions people ask

How do you calculate stopping distance?

Add the reaction distance to the braking distance: d = v × t + v² ÷ (2a). At 30.0 m/s with a 0.500 s reaction and 7.00 m/s² of braking, the car travels 15.0 m while reacting and 900 ÷ 14 = 64.3 m while braking: 79.3 m in all.

What is the stopping distance at 60 mph?

With the AASHTO design values (2.5 s reaction, 11.2 ft/s² braking), 60 mph (88 ft/s) gives 88 × 2.5 = 220 ft of reaction and 88² ÷ 22.4 = 345.7 ft of braking: 565.7 ft. Highway designers round this up to 570 ft.

Why does doubling the speed more than double the stopping distance?

The braking distance grows with the square of the speed. Twice the speed gives twice the reaction distance but four times the braking distance.

What reaction time should I use?

AASHTO uses 2.5 s for road design. OpenStax uses 0.5 s in its example of a driver ready to brake.

How does a wet road change the stopping distance?

A wet road brakes less hard. OpenStax gives about 7 m/s² on dry concrete and 5 m/s² on wet concrete: from 30 m/s the braking distance grows from 64.3 m to 90.0 m. With friction, use μ = 0.7 dry and 0.5 wet.

How does a hill change the stopping distance?

Going uphill, gravity helps the brakes; going downhill, it works against them. The page adds g × grade to the deceleration: a 3% downhill grade takes 0.294 m/s² away from the braking.