How speed conversion works
To convert between any pair of speed units, the simplest approach is to use meters per second as an intermediate step, since it is the base unit of the International System. The input value is first turned into meters per second, and that total is then converted into the desired target unit. That is exactly how this calculator works behind the scenes.
Conversion factors used
1 meter per second = 3.6 kilometers per hour
1 mile per hour = 0.44704 meters per second (exact definition)
1 knot = 1 nautical mile per hour = 0.514444 meters per second
1 foot per second = 0.3048 meters per second (exact definition)
Worked example
A car going 100 km/h is traveling at roughly 62.14 mph, since 100 multiplied by 0.621371 gives that value. In meters per second, the same 100 km/h corresponds to about 27.78 m/s, since dividing the km/h value by 3.6 gives that result.
Speed at sea and in aviation: the knot
Unlike kilometers or miles per hour, the knot is a unit based on the nautical mile, used mainly in maritime navigation and aviation. A nautical mile corresponds to one minute of arc of latitude on the Earth's surface, which makes it especially useful for navigation based on geographic coordinates. A knot equals roughly 1.852 km/h, a bit faster than 1 km/h and slower than 1 mph.
Converting running pace into speed
Runners typically measure their performance in pace, meaning the time it takes to cover 1 kilometer or 1 mile, rather than in direct speed. To turn pace into speed, simply divide 3,600 seconds, one hour, by the time in seconds it takes to cover 1 kilometer. For example, a pace of 5 minutes per kilometer, or 300 seconds, results in a speed of 12 km/h. The calculator on this page also shows the equivalent pace per mile, useful for anyone tracking workouts in apps that use the imperial system.