Stopping Distance Calculator
See your thinking, braking, and total stopping distance at any speed — based on official Highway Code figures.
How does the stopping distance calculator work?
Total stopping distance has two parts: thinking distance (how far you travel before your foot reaches the brake) and braking distance (how far the car travels once the brakes are applied). This calculator estimates both from your speed, then adjusts braking distance for road conditions and thinking distance for driver state, based on the Highway Code's official Rule 126 figures.
3 worked examples
Example 1 — 30mph, dry road, alert driver
This matches the Highway Code's own published figure exactly.
9m
Braking distance:14m
Total:23m — roughly 5 car lengths
Example 2 — 70mph motorway, wet road, alert driver
The same speed most drivers consider "normal" for a motorway, but in the rain.
21m (unchanged by weather — this is reaction time, not tyre grip)
Braking distance (doubled for wet roads):75m × 2 = 150m
Total:171m — around 38 car lengths, nearly double the dry-road total of 96m
Example 3 — 30mph, dry road, distracted driver (phone)
Same speed and road as Example 1, but with a driver using a mobile phone.
9m × 2.5 = 22.5m
Braking distance:14m (unaffected by distraction, once braking starts)
Total:~36.5m — 13.5m further than the alert driver in Example 1, purely from a slower reaction to start braking at all
Example 3 shows something important: at the same speed, on the same dry road, mobile phone distraction alone adds over half the length of a bus to your stopping distance, before the car has even started braking.
Official Highway Code stopping distances
These are the Highway Code's own published figures (Rule 126) for a driver with average reaction time, dry road, good brakes and tyres:
| Speed | Thinking distance | Braking distance | Total stopping distance |
|---|---|---|---|
| 20 mph | 6m | 6m | 12m (3 car lengths) |
| 30 mph | 9m | 14m | 23m (6 car lengths) |
| 40 mph | 12m | 24m | 36m (9 car lengths) |
| 50 mph | 15m | 38m | 53m (13 car lengths) |
| 60 mph | 18m | 55m | 73m (18 car lengths) |
| 70 mph | 21m | 75m | 96m (24 car lengths) |
Notice that braking distance grows much faster than speed itself — doubling your speed roughly quadruples your braking distance, because kinetic energy increases with the square of speed.
Why distraction and tiredness matter
Thinking distance assumes an alert driver with an average reaction time of around two-thirds of a second. Anything that slows your reaction time extends thinking distance directly and proportionally — and unlike braking distance, this happens before your car has even begun to slow down.
Published research on distracted driving consistently shows reaction times can be significantly slower when using a mobile phone — commonly cited estimates suggest reaction time can more than double, in some studies comparable to driving while over the drink-drive limit. Tiredness has a smaller but still meaningful effect, and severe fatigue can impair reaction time as much as intoxication.
Using a handheld phone while driving is also a criminal offence in the UK — 6 penalty points and a £200 fine (or higher if it goes to court) — independent of any stopping-distance risk. See our Penalty Points Calculator for how CU80 offences affect your licence.
Why wet and icy roads are so dangerous
The Highway Code advises that braking distances should be treated as at least double on wet roads, and up to ten times longer on icy or snowy roads, since reduced tyre grip means far more distance is needed to shed the same amount of speed. Thinking distance itself doesn't change with weather — it's purely a function of reaction time — but the much longer braking distance means total stopping distance grows dramatically, as Example 2 above shows.
Tyre condition and stopping distance
Tread depth has a measurable, significant effect on braking distance, particularly in wet conditions, because worn tread can't clear water from under the tyre as effectively. This calculator doesn't include a tyre-condition multiplier since the effect varies too much by exact tread depth and tyre type to state a single reliable figure — but the direction is clear: less tread means longer braking distance, on top of whatever this calculator already shows for your road conditions. Check your own tyres with our Tyre Tread Depth Checker.
What this calculator can't tell you
Your actual vehicle's braking performance
Braking distance varies by vehicle weight, brake condition, tyre type and tread depth, and ABS system — this calculator uses Highway Code average figures, not your specific car's performance.
Your personal reaction time
Individual reaction time varies by age, health, and alertness even without distraction or tiredness. The Highway Code figure is a population average, not a guarantee for any individual driver.
Combined risk factors
This calculator lets you select one road condition and one driver state at a time. In reality, risk factors compound — a tired driver on a wet road with worn tyres faces a much larger combined increase than any single factor shown here suggests.
Common mistakes
Only thinking about braking distance, not thinking distance
Many drivers focus entirely on how hard they can brake and forget that a third or more of total stopping distance, even for an alert driver, happens before braking starts at all.
Assuming a "safe gap" scales linearly with speed
Because braking distance grows roughly with the square of speed, a gap that's safe at 30mph is nowhere near enough at 60mph — the total stopping distance more than triples, not just doubles.
Underestimating wet weather risk on familiar roads
Drivers often maintain their usual following distance on a route they know well, even in the rain, when the real stopping distance may have doubled.
Treating "hands-free" as equivalent to "undistracted"
Research on distracted driving generally shows the cognitive distraction of a phone conversation — not just the physical act of holding a phone — is what slows reaction time, meaning hands-free calls still carry meaningful risk even though they're not a specific driving offence in the way handheld use is.
Safer following distances
- Use the two-second rule in dry conditions — pick a fixed point ahead, and ensure at least two seconds pass between the vehicle in front passing it and you reaching it
- Double it to four seconds in wet conditions, and increase further in icy or foggy conditions
- Never use a handheld phone while driving — and treat hands-free calls with real caution too, given the cognitive distraction effect
- Don't drive tired — take a proper break on long journeys rather than pushing through fatigue
- Check your tyre tread regularly, especially before winter or a long wet-weather journey
How we calculate
Thinking distance (m) = speed (mph) × 0.3048 × reaction time multiplier — the 0.3048 factor reflects the Highway Code's approximate "1 foot per mph" thinking-distance relationship, converted to metres.
Braking distance (m) = 0.0153 × speed² (mph) × road condition multiplier — this quadratic relationship is fitted to match the Highway Code's published dry-road figures closely across the 20–70mph range.
Road condition multipliers: Dry ×1, Wet ×2, Icy/snowy ×10 (per Highway Code guidance).
Reaction time multipliers: Alert ×1, Tired ×1.5, Distracted ×2.5 — the tired and distracted figures are illustrative estimates based on published research ranges, not official Highway Code multipliers, since no single official figure exists for these.
Frequently asked questions
What is the stopping distance at 30mph?
According to the Highway Code, 23 metres (about 75 feet, or roughly 6 car lengths) on a dry road with an alert driver — 9m thinking distance plus 14m braking distance.
How much longer is stopping distance in the wet?
The Highway Code advises treating braking distance as at least double on wet roads. Thinking distance is unaffected by weather, so total stopping distance increases by roughly the braking distance portion again, not a flat doubling of the whole figure.
Does using a mobile phone really affect stopping distance?
Yes, significantly, through slower reaction time — published research suggests it can meaningfully increase the time before you even begin braking, adding real distance before the car has started to slow down at all. It's also a specific criminal offence independent of this risk.
Why does braking distance increase so much more than thinking distance as speed rises?
Thinking distance scales directly with speed (roughly 1 foot per mph), but braking distance scales with the square of speed, because it takes far more kinetic energy to stop a faster-moving vehicle. This is why braking distance dominates total stopping distance at higher speeds.
Do worn tyres increase stopping distance?
Yes, particularly in wet conditions, since worn tread can't clear water from under the tyre as effectively. Check your tyres with our Tyre Tread Depth Checker, since this calculator doesn't include a specific tyre-wear multiplier.
References & data sources
- Highway Code — Rule 126. Official UK stopping distance figures by speed, and guidance on wet and icy road multipliers, used directly for this calculator's core formula.
- THINK! Road Safety Campaign. General guidance on the risks of distraction and fatigue while driving.
- Department for Transport (DfT). UK road safety statistics and research context on reaction time and distraction.
Reaction-time multipliers for tiredness and distraction are illustrative estimates based on published research ranges, not official Highway Code figures — no single authoritative multiplier exists for these factors, unlike the dry/wet/icy road condition figures, which come directly from the Highway Code.