Ask an American how tall they are, and you will get an answer in feet and inches. Ask how much they weigh, and the answer comes in pounds. Ask the temperature, and it is Fahrenheit. Ask how far away the next town is, and it is miles. Then ask the same person how much soda is in the bottle in their hand, and they will tell you two liters. Ask a nurse for a dose of medicine, and it is milligrams. Ask a scientist, a soldier, or an auto mechanic, and the answers come in metric. America lives in two measurement systems at once, and it has done so for more than two centuries.
The United States is one of only three countries in the world that has not fully adopted the metric system for everyday life, alongside Myanmar and Liberia. Nearly every other nation on Earth measures distance in kilometers, weight in kilograms, and temperature in Celsius. America, by contrast, still runs its daily life on inches, pounds, gallons, and degrees Fahrenheit. This is the story of how that happened, why every attempt to change it has stalled, and why the answer is more complicated than most Americans realize.
One of Only Three Countries
The claim that America is one of three non-metric countries is repeated so often that it has become a piece of trivia, and it is broadly true in everyday terms, with an important nuance. The United States, Myanmar, and Liberia are the only countries that have not made the metric system their primary system for daily life. Every other country has officially adopted it. But the National Institute of Standards and Technology, the federal agency in charge of weights and measures, calls the simple version of this claim a misconception, because every country, including the United States, has recognized and adopted the International System of Units, known as SI, in some form.
That nuance matters, because America is not actually a stranger to metric. Under American law, the metric system is the preferred system of weights and measures for trade and commerce, a status it has held since Congress amended the Metric Conversion Act in 1988. The customary units Americans use every day are themselves defined in metric terms: one inch is defined as exactly 2.54 centimeters, and one pound is defined as exactly 453.59237 grams. In a literal sense, when you measure something in feet and pounds, you are measuring in meters and kilograms with a conversion factor applied. The system Americans think of as their own is built on top of the metric system, as the history of metrication in the United States shows.
So the real question is not whether America uses metric at all. It does, constantly, in science, medicine, the military, and manufacturing. The real question is why metric never took over daily life: the road signs, the weather reports, the recipes, the bathroom scales, and the speedometers. The answer is a story of pirates, politics, a well-meaning law with no teeth, and a country that never found a reason to pay the price of changing.
Pirates, Jefferson, and the Metric System That Never Arrived
The story starts at the birth of the republic. The new United States had inherited its weights and measures from Britain, and the system was a mess. Different states used different definitions, which made trade difficult, and the Constitution gave Congress the power to fix a standard. George Washington called for uniformity in weights and measures in his first address to Congress in 1790. Thomas Jefferson, then Secretary of State, took up the cause. In 1790, he asked Congress to adopt a decimal system of weights and measures similar to the one the French were developing, and a congressional committee recommended his proposals. Congress, busy with other matters, put off taking a vote.
Meanwhile, France was inventing the metric system, a rational, decimal system based on the natural world. Jefferson admired it and wrote to French scientists about bringing it to America. The French agreed to help, and in January 1794 they sent a scientist named Joseph Dombey across the Atlantic carrying official metric standards: a copper meter rod and a copper kilogram. Dombey never made it. A storm drove his ship off course to the Caribbean, where it was captured by British privateers. Dombey was imprisoned and died in captivity. The kilogram somehow ended up in the hands of American surveyor Andrew Ellicott, whose family kept it until it was given to the National Institute of Standards and Technology in 1952.
A Century of Half-Measures
The pirate story is a wonderful tale, and it is largely true, but historians are careful not to blame the whole thing on one shipwreck. America had many chances to go metric after Dombey, and it took none of them. The first real step came in 1866, when Congress passed the Metric Act, making it lawful to use metric weights and measures anywhere in the country and ensuring that contracts written in metric units could not be thrown out of court. That law removed a legal barrier, but it created no obligation. Nobody was required to change anything, so almost nobody did.
In 1875, the United States became one of the original signatories of the Treaty of the Meter, the international agreement that created the modern metric system. In 1893, the Mendenhall Order declared the international meter and kilogram to be the fundamental references for all American measurements, so that customary units were officially defined in metric terms. None of these steps changed daily life.
By the middle of the twentieth century, the rest of the world had moved on. Most nations adopted the International System of Units when it was defined in 1960, changing road signs, retraining workers, and teaching metric in schools. Even the United Kingdom, which had lagged for years, mostly embraced the system. America watched, debated, and did nothing. The country was the largest economy in the world, its industries were built around customary units, and there was no crisis forcing a change. Inertia, which is the most powerful force in the history of measurement, did the rest.
The 1975 Law That Changed Almost Nothing
The most serious American attempt to go metric came in 1975, when Congress passed the Metric Conversion Act. The law acknowledged, in unusually blunt language, that the United States was the only industrially developed nation that had not established a national policy of converting to the metric system. It created the United States Metric Board to coordinate a voluntary shift toward metric and to educate the public. President Gerald Ford signed it into law. The key word was voluntary. The law called for increasing use of metric, but it set no deadlines, imposed no penalties, and gave no agency the power to compel anyone to switch.
Without a mandate, the effort faded. The Metric Board ran public education campaigns and worked with industries that were willing to convert, but businesses that would have borne the cost of retooling lobbied against change, and the public never warmed to the idea. President Ronald Reagan, looking to cut federal spending, eliminated the board’s funding, and the board was formally dissolved in 1982. Congress tried again in 1988, amending the 1975 law through the Omnibus Trade and Competitiveness Act to declare metric the preferred system for trade and commerce and to require federal agencies to adopt it wherever economically feasible. In 1991, Executive Order 12770 directed executive branch departments to use metric in procurement and grants. Both measures carved out broad exceptions, and neither touched the daily life of ordinary Americans. Road signs stayed in miles. Weather reports stayed in Fahrenheit. Recipes stayed in cups.
The Spacecraft That Proved the Point
If there is one story that shows why the two-system life is dangerous, it is the loss of NASA’s Mars Climate Orbiter in 1999. The spacecraft launched on December 11, 1998, on a mission to study the climate of Mars, and it traveled for nine months to reach the planet. On September 23, 1999, it fired its main engine to enter orbit around Mars, passed behind the planet as planned, and was never heard from again. The cause was not a hardware failure or a bad engine. It was a unit mismatch.
According to NASA’s own investigation, ground software built by Lockheed Martin reported the spacecraft’s small thruster firings in pound-force-seconds, an imperial unit, while the navigation software at NASA’s Jet Propulsion Laboratory expected newton-seconds, the metric unit. One pound-force-second equals about 4.45 newton-seconds, so the navigation software treated each thruster firing as much smaller than it really was. Over nine months, the small errors added up. The orbiter was supposed to pass about 226 kilometers above the Martian surface. Instead, it came down to roughly 57 kilometers, deep inside the atmosphere, and was destroyed. A spacecraft that had flown hundreds of millions of miles was lost because two teams were speaking different measurement languages.
Metric Is Already Hiding in Plain Sight
Here is the twist that surprises most Americans: the metric system is already everywhere in their lives. They just do not notice it. American science runs almost entirely on metric, as does American medicine, where drug doses are measured in milligrams and milliliters and hospital patients are weighed in kilograms. The United States military uses metric almost exclusively. NASA works almost entirely in metric. American cars are now built largely to metric standards, and engine sizes are given in liters. Soda comes in liter bottles. Nutrition labels list grams and milligrams. Most packaged consumer goods carry both customary and metric units on the label, as required by federal law.
The pattern is consistent: wherever precision, safety, or international trade matters, America uses metric. Wherever habit and familiarity matter, it uses customary units. Air traffic control uses Celsius. Camera lenses are measured in millimeters. The Transportation Security Administration’s liquid limit, 3.4 fluid ounces, is really a metric limit of 100 milliliters in disguise. Even the second, the unit of time, is an SI unit that America has fully adopted. The country did not reject metric. It adopted it selectively, in the places where the cost of confusion was highest, and left the rest alone.
This split is why the debate feels so strange to outsiders. To a visitor from Europe, America looks like a country that refuses to use metric. To an American scientist or pharmacist, America looks like a country that already uses metric and just will not admit it at the grocery store. Both views are true, because America is not one measurement culture. It is two, living side by side, separated by context rather than by law.
Why the Switch Never Happened
The simplest explanation for why America never switched is cost. Converting a country the size of the United States would mean replacing every road sign, retooling factories, redesigning tools, reprinting packaging, rewriting textbooks, and retraining millions of workers. Other countries did it by government order, over a fixed timetable. America never made that choice. Every American attempt at conversion was voluntary, and voluntary conversion has a fatal flaw: the people who pay the cost are not the people who get the benefit. A factory owner who retools today pays today, while the benefit of a common system arrives years later and mostly helps someone else.
Culture did the rest. Measurement is not just math. It is how people think about their bodies, their homes, and their world. Americans know what 70 degrees feels like and what a pound of ground beef looks like. They know how far a mile is because they have walked it and driven it. Asking them to think in Celsius and kilometers is not asking them to learn a new fact. It is asking them to relearn their intuition, and most people will not do that unless they have to. Politicians understood this, which is why no president or Congress has ever been willing to force the change. There were no votes in metric, and there were plenty of angry letters from voters who did not want their road signs changed.
There is also a quieter reason, and it is the one Americans least like to admit. For most of the twentieth century, America did not need to switch. It was the largest economy in the world, and the world came to America on American terms. Exporters who needed metric learned metric, and the rest of the country carried on as before. Today, that calculation is changing as global supply chains and international science make metric fluency more valuable, but the daily incentives still point the other way. Until there is a crisis, a mandate, or a generation that grows up thinking in metric first, the most likely future is the present: a country that measures its medicine in milligrams and its highways in miles, and sees no contradiction in doing both.
FAQ
Is the United States the only country that does not use the metric system?
No. The United States, Myanmar, and Liberia are the three countries that have not fully adopted metric for everyday life. But metric is legal here and widely used in American science, medicine, and the military.
When did the United States try to switch to metric?
The most serious attempt was the Metric Conversion Act of 1975, signed by President Gerald Ford, which created a United States Metric Board to coordinate a voluntary shift. Because the law was voluntary, with no deadlines or penalties, the effort faded, and the board was dissolved in 1982.
What happened to NASA’s Mars Climate Orbiter?
The spacecraft was lost on September 23, 1999, because ground software reported thruster data in imperial pound-force-seconds while NASA’s navigation software expected metric newton-seconds. The error sent the orbiter too low into the Martian atmosphere, where it was destroyed.
Where is the metric system already used in America?
Metric is used in science, medicine, the military, and NASA, in car manufacturing and engine sizes, in soda bottles measured in liters, on nutrition labels, and on most packaged goods, which carry both customary and metric units by law.
Will America ever switch to the metric system?
A full switch is unlikely without a government mandate, because conversion would be expensive and unpopular, and there is little political or public demand for it. The most likely future is continued dual use: metric where precision and trade matter, and customary units in daily life.