Braking Distance Calculator

About this tool

Stopping distance is two distances added together, and the half that tends to be forgotten is the first one. While you are moving your foot from the accelerator to the brake, the car keeps going at full speed. At 100 km/h that is about 28 metres every second, so a reaction time of 1.5 seconds already costs 41.7 metres before the brakes have done anything at all. The braking itself, on dry asphalt, takes another 49.2 metres. Splitting the total into its two parts is the difference between leaving 50 metres and leaving 90.

The braking half follows a simple physical law: it goes with the square of speed and inversely with the deceleration the tyres can generate. Doubling the speed quadruples the braking distance, which is why the gap that feels generous in town is nowhere near enough on a motorway. The deceleration available depends on how much grip the surface offers — a dry asphalt road is worth about eight times as much deceleration as sheet ice.

A slope changes that arithmetic in a way most calculators ignore. Going downhill, gravity has a component pulling the car along, and it works against the brakes rather than for them. The usable deceleration becomes the friction term minus the gravity term, and if the grade is steep enough relative to the available grip, that difference reaches zero: the car will not stop at all, however hard you press the pedal. Reporting that honestly is more useful than printing a very large number.

Reading the deceleration figure as a fraction of g

On a level road the deceleration shown, in units of g, is simply the friction coefficient: dry asphalt reads 0.80 g and ice 0.10 g. Only the grade makes the two differ, because on a slope gravity takes its own share.

Deceleration is reported both in metres per second squared and as a multiple of g, the acceleration of a falling body. On flat ground that second figure is a direct readout of grip: a coefficient of 0.8 computes to exactly 0.80 g and a coefficient of 0.1 to 0.10 g, because the vehicle mass cancels out of the equation entirely.

A slope breaks that clean identity. A 6% descent drops the dry-asphalt figure from 0.80 g to 0.74 g, and the same grade uphill lifts it to 0.86 g — the brakes are unchanged, but gravity is now either fighting them or lending a hand.

Because the mass cancels, two vehicles on the same surface stop in the same distance no matter how heavy they are, which cuts against the instinct that a heavier car grips better. Framing the result in g is also what lets it be compared against the grip a tyre can actually deliver on a public road.

Why a positive grade number means downhill

The grade field takes percent, and positive means downhill: the car faces a descent. That is the convention because a positive slope helps the car instead of the brakes, so it adds distance. A 6% downhill at 100 km/h costs about four extra metres.

The sign convention trips people up, so it is worth stating plainly: a positive grade is a descent. The tool turns the percentage into an angle, subtracts the share of gravity that runs along the road, and that reduces the deceleration the brakes can deliver. Typing 6 therefore lengthens the stop; typing −6 shortens it.

On a good surface the effect is modest. At 100 km/h on dry asphalt a level road stops in 90.8 metres; a 6% downhill pushes that to 94.9 metres, about four metres more, while the same grade uphill pulls it back to 87.5 metres. The spread between the two directions is roughly seven metres.

To make that cost visible, the tool also computes the same stop on a level road and reports the difference. That baseline is what turns the grade from an abstract percentage into a figure you can weigh against a wet road or a steeper descent.

Why the friction figures are only a starting point

The seven coefficients are typical peak values, and real grip swings by more than 30% with tyres, temperature, wear and standing water. The distances assume the tyres hold that peak throughout, which a locked wheel or a cycling ABS never quite does.

The coefficients behind the surface menu are a fixed reference table, not a measurement of your car on your road. They are drawn from road-engineering figures, and the same surface can shift by a third or more depending on the tyre, the temperature, how worn the tread is and how much water sits on the tarmac.

The formula also assumes the tyre keeps its peak grip for the whole stop, which is optimistic. A locked wheel slides at a lower coefficient, and an anti-lock system cycles around the peak rather than resting on it, so the fraction actually achieved is usually a little below the peak value quoted.

That is why the panel labels its output a reference figure rather than a prediction, and why the low-grip warning spells the caveat out. Read the number as a floor for planning and leave more room than it suggests on ice or packed snow.

Reaction distance is often longer than braking distance

At 60 km/h with a 1.5 s reaction time, reaction distance is 25 m. Braking distance at the same speed with μ = 0.7 is about 20 m — so more than half of the stopping distance is spent before the pedal goes down.

Where each number comes from: reaction distance is speed × reaction time, 16.7 m/s × 1.5 s = 25 m, pure multiplication. Braking distance is v² ÷ (2μg) = 16.7² ÷ (2 × 0.7 × 9.81) ≈ 20.2 m. The first grows linearly with speed, the second quadratically — so reaction dominates at low speed, braking takes over as speed rises, and the two cross at some point in between.

Why the ratio feels wrong: while driving, braking is the active act and all attention is on it, whereas nothing at all happens during the reaction second — it feels like it does not exist. Yet at 60 km/h, the most common town speed, it really is the larger half. That also explains a pattern many drivers recognise: the brakes feel fine, yet the same kind of junction keeps producing near-misses. The difference is not the brakes, it is how late the hazard was seen.

The real range of reaction times is wider than people assume: rested, focused and expecting it, roughly 0.7–1.0 s; an unexpected event such as the car ahead braking hard, around 1.5 s; tired, at night, or glancing at a phone, 2–2.5 s. Redo 60 km/h at 2.5 s: 41.7 m of reaction plus 20.2 m of braking is 62 m total — seventeen metres more than the alert case, about four car lengths. That is why following distance is measured in seconds rather than metres.