A Short History of the Measurement Units We Use Today

Every unit in a converter once had a story. The inch, foot and mile began as human-scale guesses, and the meter and kilogram began as an attempt to base measurement on nature. Understanding where they came from explains why the conversion factors look so odd.

Body-based units

Early civilizations used the human body as a ruler. The cubit was the length from elbow to fingertip, the foot was a foot, the inch was the width of a thumb and the yard was roughly the distance from nose to fingertip. The mile came from the Roman mille passus, a thousand paces. Because bodies vary, kings and towns fixed their own standards, which is why dozens of different “feet” existed across Europe.

Standardizing the imperial system

England gradually standardized its units, culminating in the Weights and Measures Act of 1824, which set the imperial yard and the imperial pound. The United States kept the older English units it had inherited, so differences still exist today: the US gallon is based on an old wine gallon of 231 cubic inches, while the imperial gallon was redefined in 1824 as the volume of ten pounds of water.

The birth of the metric system

During the French Revolution, scientists proposed a rational system based on nature and on the number ten. The meter was defined as one ten-millionth of the distance from the North Pole to the equator, and the kilogram as the mass of a liter of water. The system spread across Europe in the 19th century, and in 1875 seventeen nations signed the Metre Convention to maintain common standards.

The inch is defined by the meter

In 1959 the United States, the UK and other English-speaking countries agreed to define the inch as exactly 25.4 millimeters, and the pound as exactly 0.45359237 kilograms. This means that even American customary units are now defined in terms of metric ones. That is why our conversion factors for inches and pounds are exact.

Modern definitions

• Second: since 1967, defined by the radiation frequency of cesium-133 atoms, which makes atomic clocks accurate to a second in millions of years.
• Meter: since 1983, defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
• Kilogram: until 2019 it was defined by a platinum-iridium cylinder kept in Paris. On 20 May 2019 it was redefined using the Planck constant, so that it no longer depends on a physical object.
• Kelvin: also redefined in 2019, using the Boltzmann constant.

Why there are still two systems

Almost every country has adopted the metric system officially, but the United States did not make it mandatory, and in everyday life Americans still use feet, pounds and gallons. The UK uses a mixture: metric for most products, but miles on roads and pints for beer and milk. Because of this, conversions between systems remain a daily need, and you can find the common ones in our distance, weight and volume converters.

For a practical overview of both systems, read our metric vs imperial guide.