Kilometer per hour (km/h)

The kilometre per hour is a unit of speed equal to one kilometre travelled in one hour. Its symbol is km/h, and the forms kph and kmph appear informally although neither is standard. It is the speed unit almost every country in the world uses on its roads.

It is not an SI unit, since the SI prescribes the metre per second, but it is accepted for use alongside the SI and is far better suited to its purpose. Road speeds in kilometres per hour land in a comfortable range of two-digit and low three-digit whole numbers, from a thirty limit in a residential street to a hundred and thirty on a motorway, which is exactly the property a number on a sign needs.

The United States and the United Kingdom are the significant exceptions, both signing roads in miles per hour. The United Kingdom is the more unusual case, since it is otherwise largely metric: distances on road signs are in miles and yards while almost everything else sold in the country is metric, an inconsistency that has survived several attempts at reform.

Vehicle speedometers in most markets show kilometres per hour prominently with miles per hour in a smaller inner scale, a design that dates from cross-border driving in Europe and North America. Rail speeds, cycling averages and weather reports of wind speed in many countries all use the unit, and it is the default in almost every navigation application outside the two exceptions.

Road safety research has given the unit a set of figures that repeat in every campaign. Stopping distance grows with the square of speed, so a car travelling at 50 kilometres per hour needs roughly twice the braking distance of one at 35 and four times the energy has to be dissipated. The probability that a pedestrian struck by a car survives falls steeply between 30 and 50 kilometres per hour, which is why urban limits across Europe have been moving to 30. The same arithmetic explains why a small reduction in a motorway limit produces a large reduction in fuel used, since air resistance also rises with the square of speed.

One kilometre per hour equals about 0.277778 metres per second, about 0.621371 miles per hour, or about 0.539957 knots.

Kilometer per hour conversion table