Metric vs Imperial: A Short History of Two Measurement Systems

Published 2026-10-01

Most of the world measures distance in metres and weight in kilograms. A few countries still use feet and pounds. If you have ever wondered how the world ended up with two systems, the answer is a mix of revolution, empire, and habit. This guide covers where each system came from and who actually uses what today.

The old way: chaos before standards

Before either system existed, measurement was local. A foot in one town was not the same as a foot in the next. England alone had multiple conflicting gallons, and trades guarded their own units because confusion could be profitable. As trade grew, the mess became expensive, and governments started looking for standards.

The metric system: designed from scratch

The metric system was born out of the French Revolution. In the 1790s, French scientists set out to build a rational system based on nature rather than tradition. They defined the metre as one ten-millionth of the distance from the North Pole to the equator, and built everything in powers of ten: 10 millimetres in a centimetre, 100 centimetres in a metre, 1,000 metres in a kilometre.

The decimal structure was the real innovation. Converting between units is just moving a decimal point, which makes the system easy to learn and easy to calculate with. France made it compulsory in the early 1800s, and it spread through Europe and Latin America over the next century.

In 1875, seventeen countries signed the Treaty of the Metre, creating an international body to maintain the standards. The modern version of the system, the International System of Units or SI, was formalised in 1960. Today the units are defined by physical constants rather than physical objects, but the metre and kilogram you use are direct descendants of the 1790s design.

Imperial and US customary: evolved, not designed

The imperial system took the opposite path. It was not designed; it was codified. In 1824, the British Weights and Measures Act tidied up the jumble of English units into a single “imperial” system: the imperial gallon, the stone, the hundredweight, and the familiar inch, foot, yard, and mile.

Here is the wrinkle that confuses people: the United States does not actually use imperial. The US had already standardized its own “customary” units based on the older English units that existed before 1824. That is why a US gallon (about 3.785 litres) is smaller than an imperial gallon (about 4.546 litres), and why a US pint is 16 fluid ounces while an imperial pint is 20. The two systems share names but not always values, which matters for cooking and fuel.

Who uses what today

The United States is the famous holdout. US customary units dominate daily life: miles on road signs, pounds at the gym, Fahrenheit in forecasts. Less known is that the US has been legally metric for a long time. Congress legalized metric units in 1866, and the Mendenhall Order of 1893 defined the pound and yard in terms of the kilogram and metre. US customary units are now literally defined by metric ones. Science, medicine, and the military in the US largely work in metric.

Liberia and Myanmar are usually listed alongside the US as the three countries that have not adopted metric as their primary system. The reality is nuanced. Liberia, founded with strong American ties, uses US customary units in everyday life. Myanmar uses a mix of traditional Burmese units and imperial ones, though both countries have expressed intentions to move toward metric over time.

The United Kingdom is the great mixed case. Britain officially went metric in the late twentieth century, and shops, schools, and industry use metric. But road signs are still in miles, beer and milk come in pints, and people give their height in feet and inches and their weight in stone. A British person can comfortably buy 2 litres of soda and then drive 30 miles home.

Everyone else, including Canada, Australia, India, and all of continental Europe, South America, Africa, and most of Asia, uses metric for essentially everything, with small cultural leftovers like inches for TV screens.

Why the split persists

Changing units is expensive. Road signs, manufacturing tooling, building codes, and public habit all push back. The US tried a voluntary metric push in the 1970s and it faded. So the world remains bilingual in measurement, which is exactly why converters exist.

If you regularly move between systems, our converters handle weight, length, temperature, and volume instantly. For a practical example, see our guide on converting kilograms to pounds by hand, or our cooking conversions guide for the places where US and imperial units quietly differ.