Multiply kilometers per hour by 0.621371 to get miles per hour. For example, 100 km/h equals 62.14 mph, which is the typical highway speed limit in many countries. To go the other way, multiply mph by 1.60934.

A knot is a unit of speed equal to one nautical mile per hour, or approximately 1.852 km/h (1.151 mph). It is used in maritime and aviation navigation because nautical miles correspond to angular measurements on Earth's surface, simplifying chart calculations and position reporting.

The speed of sound in dry air at 20°C is approximately 343 m/s, 1,235 km/h, 767 mph, or 667 knots. This value changes with air temperature, humidity, and altitude — it is slower in cold, thin air, which is why supersonic aircraft experience different sonic boom conditions at altitude.

The speed of light in a vacuum is exactly 299,792,458 meters per second (about 300,000 km/s or 186,282 miles per second). It is a universal physical constant and forms the basis of the definition of the meter in the SI unit system.

Mach number is the ratio of an object's speed to the local speed of sound. Mach 1 at sea level is roughly 340 m/s (1,225 km/h or 761 mph), but it decreases with altitude and temperature. Aircraft exceeding Mach 1 are called supersonic; those exceeding Mach 5 are hypersonic.