Nearly every country in the world uses the metric system. Out of roughly 200 nations, only three have not officially adopted it as their primary measurement standard: the United States, Myanmar (formerly Burma), and Liberia. Every other country, from massive economies like China and Germany to small island nations, runs on metric units for government, commerce, and daily life.
That three-country list, though, tells a simplified story. The reality is messier. The U.S. uses metric in many industries, and several officially metric countries still cling to older units in everyday situations.
How the U.S. Actually Uses Both Systems
The United States is not as metric-resistant as its reputation suggests. Congress legalized the metric system back in 1866, and in 1988 declared it the “preferred system of weights and measures for trade and commerce” through an amendment to the Metric Conversion Act of 1975. The catch: adoption remains voluntary. U.S. law permits either metric or customary units, and the decision is left to individual companies, organizations, and people.
In practice, several major sectors already operate in metric. All prescription medications in the U.S. are dosed in milligrams and milliliters. The U.S. Pharmacopeia adopted metric units as its standard almost 40 years ago, and federal health IT certification now limits electronic prescribing of oral liquid medications to milliliters only. If you’ve ever read a pill bottle, you’ve used the metric system.
American scientists work exclusively in metric. So do the military, most automobile manufacturers, and any company that exports products internationally. U.S. trading partners primarily use metric, and companies that maintain dual inventories (one set of products in customary units for domestic sales, another in metric for export) face real costs. Organizations that have fully converted generally report that the efficiency gains outweigh the switching costs, and those that can’t produce metric-compatible goods risk a competitive disadvantage in global markets.
Where customary units survive is in the texture of daily American life: miles on road signs, pounds at the grocery store, Fahrenheit on the weather forecast, feet and inches for height. These are deeply embedded cultural habits rather than technical requirements.
Countries That Mix Metric and Imperial
Several countries that officially adopted the metric system decades ago still use older units in specific, sometimes legally protected, situations. The United Kingdom is the most prominent example. British law explicitly permits imperial units for road traffic signs, distance, and speed measurement (miles and miles per hour), and for dispensing draught beer and cider in pints. A British driver sees distances in miles, orders a pint at the pub, and might describe their weight in stone, all while living in a country that is officially metric.
Canada switched to metric in the 1970s, but the blend of old and new is striking. Canadians describe their height in feet and inches and their weight in pounds, even though driver’s licenses list height in centimeters. Hospitals measure newborns in kilograms, but parents announce the birth weight to family in pounds and ounces. Cooking is a free-for-all of grams, milliliters, cups, ounces, and tablespoons, depending on the recipe and whether the cookbook was published in Canada or the U.S.
Canadian construction is largely imperial. Lumber, drywall, plywood, fasteners, pipes, and tubing are all sold in inch-based measurements. Building trades like plumbing and carpentry work in imperial. Nails are measured in inches but sold in metric-weight packages. In English-speaking provinces, commercial and residential spaces are mostly built and advertised in imperial, while French-speaking Quebec constructs in meters and advertises using both. Even Canadian farm equipment typically uses inch-system fasteners, though fluid capacities are listed in metric.
Pool temperatures often come in Fahrenheit. Most kitchen appliances are labeled in both Celsius and Fahrenheit, since many are imported from the U.S. Motors and engines are still quoted in horsepower. Most physicians chart patient height and weight in imperial, even though hospitals officially document those numbers in metric. It is a country with one official system and two practical ones.
Where Metric Is Universal, No Exceptions
Science operates entirely in metric worldwide, including in the United States. The International System of Units (SI) is the language of research, and no credible scientific journal publishes results in cups or miles. This is not a preference but a requirement for reproducibility: a measurement in grams or meters means the same thing in every lab on the planet.
Global medicine is the same. Metric dosing exists because mixed measurement systems cause dangerous errors. U.S. healthcare accreditation bodies require standardized metric designations to reduce medication administration mistakes, and official standards have progressively eliminated non-metric units like teaspoons, drams, and ounces from prescribing and labeling.
International trade is overwhelmingly metric. Product specifications, shipping dimensions, and manufacturing tolerances are communicated in SI units between countries. Companies that can’t work in metric face friction when dealing with the vast majority of the world’s economies.
Aviation: A Notable Exception
One global industry resists full metrication. International aviation measures altitude in feet, not meters, and this is standard practice worldwide. Pilots above 18,000 feet reference altitude as a “flight level” tied to a standard pressure setting measured in inches of mercury. Over busy airspace like the Atlantic Ocean, where aircraft may be separated by as little as 1,000 feet vertically, everyone must reference the same system. Switching to meters would require a coordinated global transition with enormous safety implications, so feet remain the universal aviation unit for altitude. Some countries, notably China and Russia, have used meters historically, but the global standard remains feet.
Why Three Countries Still Haven’t Switched
The U.S., Myanmar, and Liberia remain the holdouts for different reasons. Myanmar and Liberia have signaled interest in metrication but lack the infrastructure and funding for a full national conversion. The U.S. situation is unique: it already uses metric where precision and international compatibility matter most (science, medicine, military, manufacturing for export) but has never mandated a switch for consumer-facing measurements. The 1975 Metric Conversion Act deliberately left adoption voluntary, putting the decision in the hands of businesses and individuals. Without a mandate, the everyday landscape of miles, pounds, and Fahrenheit has stayed largely unchanged.
The cost of not switching is mostly invisible to ordinary Americans but real for businesses. Maintaining two measurement systems means dual inventories, dual labeling, and the occasional costly engineering mistake when conversions go wrong. Organizations involved in both domestic and international activities consistently find that working in SI is more efficient. But for a country where every road sign, recipe, and thermostat uses customary units, the inertia is enormous.