The Metric System Around the World

A near-global standard

The metric system, now formalised as the International System of Units (SI), is the official measurement language of most of the world. It is used in science everywhere, including in countries whose everyday life still runs on other units. Its advantages are easy to see: one base unit per quantity, prefixes that scale by powers of ten, and definitions shared through international agreement.

Still, “uses the metric system” covers a spectrum. Some countries switched completely, with road signs, shops and schools all metric. Others adopted it legally while allowing older units to continue in particular settings. Reality is closer to a gradient than a clean split.

International coordination keeps the system coherent. The Metre Convention of 1875 began with seventeen signatory countries, and the International Bureau of Weights and Measures it created continues to maintain the definitions that everyone else relies on.

How countries made the switch

Adoption usually follows a pattern. A government passes a law making the metric system the legal standard, sets a date after which new measurements must be metric, and then runs a transition period. During that period, shops display both units, schools teach the new ones, and road signs are replaced. Some countries, including Australia and Canada, carried out large conversions from the 1970s onward.

The timing matters because habits last longer than laws. People who learned feet and pounds as children often continue to describe their own height and weight in them, even where the official units are metric. The difference between the legal system and the everyday one is a recurring theme, and it explains why two people in the same metric country may quote the same quantity in different units.

Dual labelling is a common bridge during a transition. A product may show a size in both systems for a period, and the older figure is dropped from packaging only when shoppers no longer need it.

The United Kingdom: a deliberate mix

The United Kingdom adopted metric units for most trade and science, yet road signs still show distances in miles and speed limits in miles per hour. Draught beer and cider are legally served in pints, and many people give their height in feet and inches and their weight in stone and pounds. Packaged goods, by contrast, are mostly labelled in grams and litres.

This is a conscious compromise, not an oversight. For a visitor it means both systems appear on the same day, in the same shop, which makes a reliable conversion tool useful. A stone, for example, is 14 pounds, or about 6.35 kg, a unit used mainly in the UK and Ireland. A weight of 10 stone 7 pounds is 147 pounds, or about 66.7 kg, and the UK’s 70 mph motorway limit is about 112.7 km/h.

The United States: metric by law, customary by habit

The United States made metric units legal for use in 1866 and signed the Metre Convention in 1875. Congress passed the Metric Conversion Act in 1975, which declared a policy of coordinating voluntary conversion rather than forcing it. As a result, metric units are standard in science, medicine and much of manufacturing, and they appear on nutrition labels and in two-litre drink bottles, while road signs, cooking and everyday talk remain mostly in US customary units. Those units are not identical to the British imperial ones: a US gallon is 3.785 litres, while an imperial gallon is 4.546 litres.

A frequently quoted list names the United States, Liberia and Myanmar as the countries that had not officially adopted the metric system. Sources disagree on the details and the legal position has shifted over time, so it is safest to treat that list as a rough summary of attitudes rather than a precise statement of law.

Old units that survive everywhere

Even in fully metric countries, a few non-metric units remain entrenched. Aircraft altitude is given in feet in many parts of the world, and distances at sea and in the air are measured in nautical miles, with speeds in knots. Television and monitor screens are described in inches, and tyre sizes often mix inches and millimetres on a single label. Canada, which converted road signs and weather reports to metric, still commonly uses feet and inches for personal height and pounds for body weight.

These survivals persist because international industries rely on shared conventions and because changing them has real safety and cost implications. They also show that a unit is more than a number: it carries habits, training and equipment that are expensive to alter.

What this means for converting

For anyone who works, studies or travels across borders, the practical lesson is that you will meet both systems for a long time. Recipes, online shops, sports statistics and technical manuals each follow their own convention, and the unit label is the only dependable clue.

A unit converter such as this one lets you move between metric and customary units across seven categories, from length and mass to speed and time. When the stakes are high, such as medical, legal or engineering use, confirm the unit with the original source before relying on any converted figure.