mph to km/h Converter

The factor and the shortcut

Speed conversion uses the same factor as distance, because the time unit is identical on both sides. Anything true of miles and kilometres is true here.

The practical shortcut is that five miles is almost exactly eight kilometres, an error of 0.6%. So multiply by 8 and divide by 5 to go from mph to km/h, or reverse it coming back. 60 mph becomes 96 km/h against a true 96.56; 100 km/h becomes 62.5 mph against a true 62.14. Close enough to drive on, never close enough for a speeding fine to care about.

The speed limits worth memorising

Most countries cluster their limits at similar real-world speeds, which makes the pairs easy to learn. A 30 mph urban limit is 48 km/h, close to the 50 used across Europe. A 60 mph rural limit is 97 km/h, close to a European 100. A 70 mph motorway limit is 113 km/h, close to a European 110 or 120.

The systematic point is that metric countries round to convenient tens and imperial countries round to convenient tens of their own, so the limits are never exact translations. A British driver in France at an indicated 130 is doing 81 mph, which is meaningfully faster than the 70 they are used to and is legal there.

The direction error that matters

Getting the direction wrong here is unusually consequential. Reading a 100 km/h sign as 100 mph means driving at 161 km/h — a sixty percent overshoot, in serious licence-losing territory. The reverse error, reading a 70 mph sign as 70 km/h, means crawling dangerously slowly on a motorway.

The safe rule is that the metric number is always the larger one for the same physical speed. If a conversion made your number smaller when going from mph to km/h, it is inverted. The swap button above exists because this is the single most common mistake in this pair.

Speed, stopping distance, and why the difference is not linear

A detail worth knowing when adjusting to a foreign limit: braking distance rises with the square of speed, not in proportion to it. Going from 50 km/h to 60 km/h is a 20% speed increase but roughly a 44% increase in braking distance once the vehicle is actually braking.

So a driver moving from a 70 mph habit to a 130 km/h motorway is not adding 11% to their speed in a way that adds 11% to their risk. The conversion above tells you the number on the speedometer; the physics of what that number means changes faster than the number does.

Frequently asked questions