What it is
The Speed Converter moves between metres per second, kilometres per hour, miles per hour, knots and mach.
Driving across a border
The practical case is a speed limit sign in the wrong unit. 100 km/h is about 62 mph, and 60 mph is about 97 km/h, which is why limits so often look like rounded equivalents of each other: 30 mph and 50 km/h, 70 mph and 110 km/h.
Getting this wrong in either direction is expensive. Reading 70 as km/h in a country that meant mph puts you dangerously slow on a motorway; the reverse puts you well over the limit.
The knot, and why it is not just stubbornness
A knot is one nautical mile per hour, about 1.852 km/h. It looks like a leftover, but there is a reason it survives in shipping, aviation and weather: a nautical mile is one minute of latitude. A vessel doing 60 knots covers one degree of latitude in an hour, which makes navigation by chart arithmetic instead of conversion.
Mach is not a fixed speed at all. It is a ratio against the speed of sound, which falls as air gets colder. Mach 1 is around 1,225 km/h at sea level and closer to 1,060 at cruising altitude, so an aircraft can accelerate into Mach 1 by climbing.
Pace against speed
Runners think in minutes per kilometre, which is the inverse of speed and behaves the opposite way: a smaller number is faster.
To convert, divide 60 by the pace. Five minutes per kilometre is 12 km/h. Six minutes is 10 km/h. Four minutes is 15 km/h, which is roughly elite marathon pace.
How to use it
Set both units and type in either box; the conversion runs both ways, in your browser.