Speed Units Around the World

Most countries use kilometers per hour (km/h) for road speeds. The United States, United Kingdom, and a handful of territories (Liberia, Myanmar, some Caribbean islands) use miles per hour (mph). The US and UK choice reflects historical ties to the imperial measurement system, with the infrastructure (road signs, speedometers sold in those markets) having entrenched the system over many decades.

Rental cars sold in multiple markets often include both scales on the speedometer. Digital displays on modern vehicles can switch between units. If you're driving in a country with different speed limits, it's worth noting the conversion: the European urban limit of 50 km/h is 31 mph; the motorway limit of 130 km/h is 81 mph.

All commercial aviation and maritime navigation worldwide uses knots as the primary speed unit. The International Civil Aviation Organization (ICAO) standardizes units for aviation; knots are the global standard regardless of national measurement preferences. A pilot flying from London to Tokyo uses the same speed units as one flying from New York to Los Angeles.

Altitude in aviation is measured in feet globally (not meters), which reflects the historical dominance of US and UK aviation in establishing international standards after World War II.

Science uses SI units: meters per second (m/s) for speed and acceleration. Ballistics, fluid dynamics, and physics calculations all use m/s. Light speed is expressed in m/s (299,792,458 m/s exactly), as is the speed of sound (approximately 343 m/s at sea level).

Space agencies typically work in km/s or m/s. The famous 'Mars Climate Orbiter' failure in 1999 partly involved a unit mismatch between metric and imperial thrust values (a broader category than just speed, but illustrating the cost of unit inconsistency in engineering).