The metric system is not all that old
The metric system was formalized in France in 1795. The idea was to create a rational, base-10 measurement system that would replace the chaotic patchwork of local units that had accumulated across centuries. Different French towns used different definitions for the same words. A "foot" in one region was a different length than a "foot" in another. Commerce was difficult; communication was worse.
The meter was defined as one ten-millionth of the distance from the North Pole to the equator along a meridian through Paris. This was genuinely an attempt to anchor measurement to something universal rather than to the body of a particular king, which is where many old units came from.
Adoption spread slowly. Most of Europe had switched by the mid-1800s. The rest of the world followed throughout the 20th century, often as part of post-colonial modernization efforts.
The United States came close to switching — twice
In 1866, the US Congress passed the Metric Act, which made metric units legal for use in commerce. The US then signed the Treaty of the Metre in 1875, which established the International Bureau of Weights and Measures. American scientists had been using metric for decades, and the scientific community pushed for full conversion.
Then in 1975, Congress passed the Metric Conversion Act, which declared metric to be the preferred system for US trade and commerce and established the United States Metric Board to coordinate the voluntary transition. The key word was voluntary. There were no mandates, no deadlines, no penalties for continuing to use feet and miles. The board was abolished in 1982 under the Reagan administration, which cut it as part of broader budget reductions.
A second push came in 1988 with the Omnibus Trade and Competitiveness Act, which required federal agencies to use metric in their procurements and activities. Highways are still measured in miles. The weather is still reported in Fahrenheit. The packaging on most American food products still lists ounces before grams.
Why didn't it stick?
The usual explanation is inertia and cost. Converting road signs, retooling factories, reprinting packaging — these costs are real, and without a hard mandate they're easy to avoid. Each individual industry has an incentive to wait for everyone else to go first, and no one goes first.
There's also a cultural dimension that's often underestimated. Measurement systems are embedded in language, in recipes, in how people describe distance when giving directions. A person who has spent their whole life thinking in miles doesn't just need new road signs — they need to rebuild their intuitive sense of space. That kind of change takes a generation, and it requires children to be explicitly taught the new system from the start.
Canada switched officially in the 1970s, and younger Canadians are mostly comfortable with metric. But many Canadians over 50 still describe their height in feet and inches, weigh themselves in pounds, and think of interior home temperature in Fahrenheit. The official system changed; the informal mental models didn't, entirely.
Where imperial units still dominate in the US
Even in the United States, metric has crept in more than many people realize. Pharmaceuticals are measured in milligrams. Liters are used universally for soft drinks — nobody in America asks for a two-quart bottle of soda, but a two-liter bottle is entirely normal. Nutrition labels list both systems. Cameras and electronics follow international standards measured in metric. Aviation uses feet for altitude but nautical miles for distance, a third system entirely.
The areas where imperial is most stubbornly entrenched are everyday personal measurements — height, body weight, room temperature — and road distances. These are precisely the measurements that people use constantly and that are embedded in conversation, so they're the hardest to shift.
The practical impact today
For most people the practical impact is mostly just conversion math when crossing borders. Americans traveling abroad need to quickly develop an intuition for Celsius temperatures and kilometer distances. Everyone else visiting the US needs the reverse.
There have been professional consequences too. In 1999, NASA's Mars Climate Orbiter was lost when one engineering team used metric units and another used imperial ones. A spacecraft that cost several hundred million dollars burned up in the Martian atmosphere because of a unit mismatch. It's the most expensive unit conversion error in history, and it wasn't the only one.
For quick everyday conversions, our unit converter handles the most common metric-to-imperial conversions without any setup.
Will the US ever fully switch?
Probably not in any foreseeable timeline, absent a federal mandate with enforcement and funding for the transition. The voluntary approach has been tried twice. Scientific and engineering contexts are already effectively metric. Consumer and everyday contexts have remained imperial, and there's no strong political constituency for changing that.
The more likely outcome is what's already happening: gradual drift toward metric in technical and international contexts, while informal everyday language stays imperial indefinitely. The US will probably remain in this in-between state for decades — officially open to metric, practically still measuring butter in sticks.