Kilometers per Hour to Miles per Hour Converter

What are Kilometers per Hour and Miles per Hour?

Kilometers per hour (km/h or kph) and miles per hour (mph) are the two most widely used units for measuring speed around the world. While they serve the same purpose, they belong to different measurement systems and are used in different regions.

The History of Kilometers per Hour

The kilometer has its roots in France, where the French Academy of Sciences developed a new measurement system in 1793. Initially, the French used "millaire" (equal to 10,000 meters) before the term "kilometer" (1,000 meters) became standardized.

The term "kilometers per hour" began to appear in English literature in the 1860s. It wasn't until 1975 that a European Union directive required the use of the "km/h" abbreviation on all speedometers in cars sold within the EU.

The History of Miles per Hour

The mile has a much longer history, dating back to the Romans. The Roman mile ("mille passus") from 29 BC was defined as the length of 1,000 paces, where a pace was the distance of the left foot hitting the ground twice.

The Old English Mile from medieval times was approximately 2.1 kilometers. The modern international mile (1,609.344 meters exactly) was established in 1959 by agreement between the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa.

"Miles per hour" first became common with the advent of regular stage coaches. During the early 19th century, as railroads began operating at higher speeds, there were concerns that passengers might suffer harm at speeds exceeding 20 mph!

Global Usage

Today, km/h is used as the standard speed measurement in most countries around the world that have adopted the metric system. This includes all of continental Europe, most of Asia, Africa, Australia, and Latin America.

Miles per hour remains the standard in the United States and the United Kingdom, though the UK has largely converted to the metric system in other areas.

Scientific Definition and SI Units

In the International System of Units (SI), the standard unit for speed is meters per second (m/s). One kilometer per hour equals 0.277778 m/s, while one mile per hour equals 0.44704 m/s.

The official SI symbol for kilometers per hour is "km/h", though non-SI abbreviations such as "kph", "kmph", and "km/hr" are also commonly used in various contexts.

Conversion Relationship

The exact conversion between kilometers per hour and miles per hour is defined by the relationship between the kilometer and the mile:

• 1 mile = 1.609344 kilometers (exactly)
• Therefore, 1 mph = 1.609344 km/h
• And conversely, 1 km/h = 0.621371 mph

Common Reference Speeds