Following Distance Calculator (3-Second Rule)
Work out a safe following distance from your speed with the 3-second rule — in metres or feet and car lengths — with extra time for rain, snow or towing, and compare it with the stopping distance.
How it works
following distance = speed × time gap · speed in m/s = km/h ÷ 3.6, in ft/s = mph × 1.467 · time gap = base gap × condition factor (dry ×1, wet ×2, snow or ice ×3) + 1 s when towing, in a heavy vehicle or on a motorcycle · comparison stopping distance = v × 1.5 s + v² ÷ (2 × 0.7 × g)
Driver handbooks express a safe following distance as time rather than length because the space you need grows with speed: pick a fixed point, and when the vehicle ahead passes it you should be able to count at least three seconds before you reach it. That gap covers the time to notice the brake lights and react. Most handbooks say to double the gap on wet roads and to leave much more on snow or ice, and to add time when towing or riding a motorcycle.
Worked example
At 100 km/h in good conditions with a 3-second gap: 100 ÷ 3.6 = 27.8 m per second, so the following distance is 27.8 × 3 = 83 m — about 19 car lengths. For comparison, stopping from 100 km/h on a dry road with a 1.5 s reaction takes about 98 m.
Sources: Two-second rule and three-second following distance · NHTSA — Safe driving tips
Frequently asked questions
What is the 3-second rule?
When the vehicle ahead passes a fixed object such as a sign, count "one thousand one, one thousand two, one thousand three". If you reach the object before you finish, you are too close. Three seconds is the minimum most handbooks recommend in good conditions.
How many metres should I keep behind at 100 km/h?
Three seconds at 100 km/h is about 83 m, and two seconds is about 56 m. On a wet road double it to roughly 170 m.
Is the 2-second rule still valid?
Some countries, such as the UK, teach a two-second minimum on dry roads. Many US states and safety organisations now recommend three or four seconds because of distraction and heavier traffic.
Why use seconds instead of car lengths?
Seconds scale automatically with speed and are easy to measure on the road. Counting car lengths is hard to judge and the space you need grows with speed.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.