Driving / Distance
Stopping Distance Calculator: Reaction, Braking and Grade
Estimate stopping distance in metres and feet from speed, reaction time, road friction and grade. Compare speeds in a chart and table, alongside the UK Highway Code reference distances.
Stopping Distance Calculator: Reaction, Braking and Grade: Adds reaction distance to constant-deceleration braking distance, including gravity on a slope. Surface presets are illustrative research ranges. This is an educational estimate; real distances vary and it is not a following-distance recommendation. Runs 100% locally in your browser with zero server file uploads.
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Educational estimate only. Real stopping distances vary with attention, tyres, brakes, load and road conditions. Do not use this result to choose a following distance.
41.7 m reaction + 49.2 m braking = 90.8 m total
| km/h | mph | Reaction (m) | Braking (m) | Total (m) | Total (ft) |
|---|---|---|---|---|---|
| 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| 20.0 | 12.4 | 8.3 | 2.0 | 10.3 | 33.8 |
| 40.0 | 24.9 | 16.7 | 7.9 | 24.5 | 80.5 |
| 60.0 | 37.3 | 25.0 | 17.7 | 42.7 | 140.1 |
| 80.0 | 49.7 | 33.3 | 31.5 | 64.8 | 212.6 |
| 100.0 | 62.1 | 41.7 | 49.2 | 90.8 | 298.0 |
| 120.0 | 74.6 | 50.0 | 70.8 | 120.8 | 396.4 |
Formula and assumptions
v = km/h ÷ 3.6; θ = atan(grade ÷ 100); reaction = v × t; braking = v² ÷ [2g(μ cos θ + sin θ)], with g = 9.80665 m/s². Friction and grade are constant; aerodynamic drag and brake fade are omitted.
The editable 1.5 s default follows Manual for Streets, section 7.5.7, for lower-speed street design. It is not a universal reaction time; AASHTO stopping-sight-distance design uses 2.5 s. Department for Transport: Manual for Streets
Typical example ranges come from MSU/CHPP report MSU7 (2020), Table 3-5. Wet here means bare wet pavement, snow means packed snow, and ice means wet black ice. Actual grip can fall outside these ranges. Road-friction research
FHWA: stopping sight distance| mph | Thinking (m) | Braking (m) | Total (m) |
|---|---|---|---|
| 20 | 6 | 6 | 12 |
| 30 | 9 | 14 | 23 |
| 40 | 12 | 24 | 36 |
| 50 | 15 | 38 | 53 |
| 60 | 18 | 55 | 73 |
| 70 | 21 | 75 | 96 |
This fixed reference uses different reaction and braking assumptions and does not update with your inputs. GOV.UK: Highway Code, rule 126
Reaction time and braking model
Department for Transport, Manual for Streets, section 7.5.7: 1.5-second perception–reaction time for the street-design context. https://www.gov.uk/government/publications/manual-for-streets
FHWA describes AASHTO's 2.5-second stopping-sight-distance allowance. https://highways.fhwa.dot.gov/safety/speed-management/guidelines-use-variable-speed-limit-systems-wet-weather/chapter-2-driver
With v in m/s, t in seconds, θ = atan(grade/100) and g = 9.80665 m/s²: reaction = vt, braking = v²/[2g(μ cos θ + sin θ)]. Grade is rise/run and distances are along the road. The model assumes constant grip and omits aerodynamic drag and brake fade.
Typical surface examples
MSU/CHPP report MSU7 (2020), Table 3-5: dry bare 0.8–1.0, wet bare 0.7–0.8, packed snow 0.2–0.3, wet black ice 0.05–0.10. These are typical examples, not guaranteed grip or universal limits. https://chpp.egr.msu.edu/wp-content/uploads/2021/10/21.-CHPP-Report-MSU7-2020.pdf
UK Highway Code reference
Rule 126 typical totals at 20, 30, 40, 50, 60 and 70 mph are 12, 23, 36, 53, 73 and 96 metres. https://www.gov.uk/guidance/the-highway-code/general-rules-techniques-and-advice-for-all-drivers-and-riders-103-to-158
Official reference chart: https://assets.publishing.service.gov.uk/media/65f97642aa9b760011fbda4e/the-highway-code-stopping-distances.pdf . Contains public sector information licensed under the Open Government Licence v3.0.
How to use it
- Enter speed and perception–reaction time.
- Choose a surface preset, edit friction if needed, and enter grade with uphill positive.
- Read reaction plus braking distance, compare speeds, and check the assumptions.
Privacy & limitations
All calculations run in your browser. Your inputs are not uploaded.
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Frequently asked questions
Why start with 1.5 seconds?
The Department for Transport's Manual for Streets uses 1.5 seconds for lower-speed street design. Other design contexts use longer times: AASHTO stopping sight distance uses 2.5 seconds. Change the input to explore the effect.
What does a negative grade mean?
It means downhill. Gravity reduces net braking deceleration. If friction cannot overcome the downhill component, this constant-friction model cannot give a stopping distance.
Why does the Highway Code table differ?
It is a fixed reference with different reaction and braking assumptions. It does not change when you edit this calculator. Weather, tyres and attention can substantially change real stopping distances.
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