Mile per hour (mph)

The mile per hour is a unit of speed equal to one mile travelled in one hour. Its symbol is mph. It is the road speed unit of the United States and the United Kingdom, and of a small number of territories connected to them, and it is used almost nowhere else.

Britain's position is the odd one. The country converted its shops, its schools and its industry to metric units over the second half of the twentieth century, but left road signs alone. Distances appear in miles and yards, speed limits in miles per hour, and a plan to change them has been announced and abandoned more than once, largely because replacing every sign in the country is expensive and the political return is small.

The number sixty is what makes the unit intuitive to those who grew up with it. At sixty miles per hour a vehicle covers a mile a minute, which turns journey planning into simple mental arithmetic. Speed limits cluster around convenient multiples: thirty in towns, seventy on British motorways, sixty-five or seventy on most American interstates.

Sport and weather keep it alive beyond the road. Baseball pitch speeds, tennis serves and cricket deliveries are quoted in miles per hour in English-language coverage worldwide, and hurricane wind speeds in the Atlantic basin are reported the same way, with the Saffir-Simpson categories defined by thresholds in miles per hour.

Records set in the unit have become cultural landmarks in their own right. The four-minute mile is a pace of fifteen miles per hour, the sound barrier at sea level lies near 767, and the land speed record has stood above 700 since 1997. Everyday driving figures carry the same weight of familiarity: 30 miles per hour in a built-up area, 70 on a motorway, and the observation that a car doing 60 covers 88 feet in a second, which is the number American driving instructors use to explain following distances. Because the unit rarely appears in physics, almost all of its landmarks come from the road, the track and the runway.

One mile per hour equals about 1.609344 kilometres per hour, about 0.44704 metres per second, or about 0.868976 knots.

Mile per hour conversion table