Miles per hour (MPH) is not the metric unit for speed, as it belongs to a different system of measurement entirely. The metric system, used by most of the world, employs a coherent set of units, while MPH is a unit derived from a system with historical roots in a different tradition.
The Origin of Miles Per Hour
Miles per hour (MPH) is a unit of speed that is part of the British imperial and United States customary systems of measurement. The mile itself is a non-metric unit of distance, tracing its history back to the Roman “milia passuum,” which means “thousand paces.” The mile was standardized as equivalent to 1,760 yards or 5,280 feet. MPH came into common usage in the 18th century with the scheduling of regular stagecoaches, and later with the advent of faster travel like railroads in the 19th century. Today, MPH is predominantly used for road traffic and speed limits in the United States and the United Kingdom, along with a few smaller nations and territories.
Defining the Standard Metric Unit for Speed
The official metric system is known as the International System of Units (SI), which is the most widely adopted system of measurement globally. The SI unit for speed is the meter per second (m/s). Speed is considered a derived quantity because it is calculated from the ratio of two other base measurements: distance and time.
The meter (m) is the SI base unit for distance, and the second (s) is the SI base unit for time. Therefore, when these two base units are combined to define speed, the result is the meter per second. This unit is used extensively in scientific, engineering, and physics contexts because it simplifies calculations by avoiding conversion factors. The meter per second was established as the official SI derived unit for both speed and velocity with the creation of the International System of Units in 1960.
Why Kilometers Per Hour is Used More Often
Despite meters per second being the SI standard, the most common metric unit for measuring vehicle speeds in daily life is the kilometer per hour (km/h). Kilometers per hour is a practical unit chosen for its convenience in describing distances and durations encountered in road travel. People typically measure longer journey times in hours rather than seconds, and road distances are more conveniently expressed in kilometers than in meters.
A kilometer is equivalent to 1,000 meters, which makes it a more appropriately scaled unit for city-to-city travel and highway distances. For instance, it is far easier to visualize a speed of 100 km/h than its technical equivalent of about 27.8 m/s. Kilometers per hour relates directly to miles per hour through a simple conversion ratio; one mile per hour is approximately 1.609 kilometers per hour.