The simplest formula for calculating the braking distance is ((v/10)^2)/2 where v is the car's speed in km/h. In words, the formula is: speed divided by ten – to the power of two – divided by two.
This is how you calculate the braking distance at 30 km/h using the formula: ((30/10)^2)/2 = 4.5.
You do not need to know the formula to pass the theory test. But you need to know the approximate braking distances at different speeds.
If the braking occurs in other than ideal conditions, the braking distance can be many times greater. For example, if the road is wet or if car's brakes or tyres are in poor condition.