Most of the world uses the metric system for everyday measurements — at the doctor's office, you'll see your height in centimeters and weight in kilograms, and a cookie recipe will call for grams of flour and milliliters of milk.
The United States is one notable exception, but are there others? What countries use the imperial system?
Which Countries Use the Imperial System?
Only three countries in the world use the imperial system as their official system of measurement: the U.S., Liberia and Myanmar [source: Buchholz].
Some countries, like Canada, use the metric system as their official unit system but allow the use of the imperial system in many contexts [Source: McQuillan]. In 2021, the British government announced plans to return to the imperial system as part of its exit from the European Union [source: Gross].
However, all countries have either fully adopted or legally sanctioned the International System of Units, or SI, the modern form of the metric system. That includes the U.S., Liberia and Myanmar.
Measurement in the U.S. Today
Measuring remains a mess in the States:
A football field traffics in yards while most footraces prefer meters.
Mechanics measure the power of an automobile engine in horsepower (foot-pounds per second), but express the same engine's displacement in liters.
Air pressure is denoted in all sorts of ways: pounds per square inch (or psi) for tire pressure, inches of mercury for surface atmospheric pressure and millibars for air pressure aloft.
And these are just a few examples. In the U.S. Customary System, or the inch-pound system, more than 300 different units exist to measure various physical quantities. Many of those units use the same name but have very different meanings.
On the U.S. Metric Association website, contributor Dennis Brownridge identifies at least nine different meanings for the unit we know as a "ton": short ton, displacement ton, refrigeration ton, nuclear ton, freight ton, register ton, metric ton, assay ton and ton of coal equivalent.
History of the Metric System in the U.S.
As subjects of the British Empire, North American colonists inherited and used the British Imperial System, which itself evolved from a tangled mess of medieval weights and measures. Even as France developed and refined the metric system throughout the late 1700s, England and its American colonies pressed forward with an antiquated measurement system.
It's not that colonial leaders didn't want to control the chaos. In the Constitution of the newly formed United States of America, Article I, Section 8 provided that Congress should have the power "to coin Money ... and fix the Standard of Weights and Measures."
The first practical analysis of this provision fell to George Washington's Secretary of State, Thomas Jefferson, in 1790. Jefferson endorsed a decimal system of measurement but, when presented with the basic principles of the decimal-based metric system, felt reluctant to steer his nation in that direction. He feared that the U.S. wouldn't be able to verify the metric unit of length without sending a costly delegation to France.
The evolving political situation didn't help matters of competing imperial and metric systems. Although France supported the American colonies during the Revolutionary War, it became hostile to the U.S. after Jay's Treaty was ratified in 1795.
The French viewed the treaty, which eliminated British control of posts in the Northwest Territories and provided America a limited right to trade in the West Indies, as a blossoming alliance between the U.S. and England.
France retaliated by sending privateers to target American merchant ships. By the time John Adams became president in 1797, the hostilities between the U.S. and France had grown quite intense. It's no surprise, then, that in 1798, France snubbed the U.S. when it invited dignitaries from foreign countries to travel to Paris to learn about the metric system.
Even if U.S. representatives had visited Paris in 1798 and been wowed by the metric demonstration, it's unlikely that they would have persuaded American leaders to change the country's system of weights and measures.
In 1821, after studying the various units of measurement used by the 22 states, Secretary of State John Quincy Adams determined that the U.S. Customary System was sufficiently uniform and required no changes. In addition, there was concern among American politicians that the French commitment to the metric system might falter in the aftermath of Napoleon Bonaparte's ill-fated reign during the early 19th century.
The U.S. Decision to Recognize the Metric System
Over time, however, the metric system gained traction. By the time the American Civil War ended in 1865, most of Europe had adopted the decimal-based measuring system, and the U.S. could no longer ignore it.
In 1866, an act of Congress, signed into law by President Andrew Johnson, made it "lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings or court proceedings."
This time, when France brought together the world's leading nations to discuss a new international version of the metric system, the U.S. received an invitation and sent delegates. These nations signed the Treaty of the Meter, establishing the International Bureau of Weights and Measures, an International Committee for Weights and Measures to run the bureau and the General Conference on Weights and Measures to consider and adopt changes.
The treaty also specified a lab to be maintained in Sèvres, near Paris, to house the international metric standards, such as the International Prototype Metre, and allowed for these standards to be distributed to each ratifying nation.
The U.S. received its copies of the International Prototype Metre and the International Prototype Kilogram in 1890. The Mendenhall Order of 1893 (named after T.C. Mendenhall, who served as the Superintendent of Weights and Measures at the time) stipulated that the fundamental standards for length and mass in the U.S. be based on metric units. The yard was defined as 3600/3937 meter, and the pound-mass was defined as 0.4535924277 kilogram.
In 1959, English-speaking countries agreed on new and improved conversion factors: 1 yard equals 0.9144 meter and 1 pound-mass equals 0.45359237 kilogram, exactly.
That means the U.S. has officially — and legally — recognized the metric system for over 150 years and has based the units of its standard weights and measures on metric units for over 120 years. However, recognition doesn't necessarily translate into practical use.
The Metric System in the U.S. Today
Mendenhall joined a growing number of scientists and political leaders who advocated making use of the metric system in the U.S. compulsory. When he died in 1924, however, America hadn't made the move.
That seemed about to change in 1971, when a U.S. National Bureau of Standards report titled "A Metric America" recommended that the U.S. transition to the metric system over the course of 10 years. In response, Congress enacted the Metric Conversion Act in 1975 but stripped out the 10-year deadline and made the conversion voluntary.
Although schoolchildren across America began studying SI units in earnest and a few companies embraced metrication, the rallying cry to go metric faded, as did any real movement to make the switch.
Business Impacts of Metric and Imperial Systems
In the meantime, as globalization increased, American companies found themselves competing against international interests. More and more, foreign customers buying U.S. products required that they be delivered, labeled and produced in metric units.
And when American companies went to build new factories in Europe or Asia, they faced the challenge of standardizing to U.S. measurements or the metric system — decisions with enormous financial consequences.
Recognizing these issues, Congress passed amendments to the Metric Conversion Act in 1988, designating the metric system as the "preferred system of weights and measures for United States trade and commerce" and requiring federal agencies to use "the metric system of measurement in its procurements, grants, and other business-related activities" by the end of 1992.
The amendments, however, continued to make metrication voluntary for private industry, and although they encouraged the federal government to assist small businesses interested in making the conversion, progress has been slow.
Hard Metric vs. Soft Metric
By some estimates, about 30 percent of products manufactured by American companies have gone metric [source: Smith]. The pharmaceutical industry went "hard metric," which means its products display only metric units. Beverages, on the other hand, typically show both U.S. Customary units and metric units together, making them "soft metric." Film, tools and bicycles are also sold in metric measurements.
For the most part, though, the U.S. remains the only industrialized nation that hasn't made the metric system compulsory.
Moving to Meters Means Money
Cost is one reason the U.S. has been slow to adopt the metric system. Converting technical drawings and operations manuals for complex equipment with many parts can take thousands of hours.
NASA engineers, for example, reported that converting the space shuttle's relevant drawings, software and documentation to SI units would require $370 million — about half the cost of a typical space shuttle launch [source: Marks].
Of course, cost alone can't explain the U.S.'s reluctance to go metric. Certain psychological attributes also play a significant role; individualism has always been a defining characteristic of the U.S. experience. You could easily envision bumper stickers with a variation on the old National Rifle Association (NRA) slogan: "You can have my inch-pounds when you pry them from my cold, dead hands."
The most logical explanation, however, just may be the failure of Congress to make the metric system mandatory in all 50 states, the District of Columbia and its territories. By making conversion voluntary in all major legislation since 1866, the U.S. has failed to restrict the use of traditional units in transactions that touch the daily lives of ordinary citizens.
Lots More Information
Alsdorf, Matt. "Why Hasn't the U.S. Gone Metric?" Slate. Oct. 6, 1999. (Sept. 14, 2011) http://www.slate.com/id/1003766/
Benham, Elizabeth. "Busting Myths about the Metric System." National Institute of Standards and Technology. Oct. 6, 2020. (Oct. 13, 2023). https://www.nist.gov/blogs/taking-measure/busting-myths-about-metric-system
Buchholz, Katharina. "Only Three Countries in the World (Officially) Still Use the Imperial System." Statista. Jun. 6, 2019. (Oct. 13, 2023). https://www.statista.com/chart/18300/countries-using-the-metric-or-the-imperial-system/
Gross, Jenny. "Britain Signals Intent to Revert to the Imperial System." Sept. 17, 2021. (Oct. 13, 2023). https://www.nytimes.com/2021/09/17/world/europe/imperial-measurements-pounds-ounces-return.html
International Bureau of Weights and Measures. "The International System of Units -- and the 'New SI ... '." (Sept. 14, 2011) http://www.bipm.org/en/si/
Marks, Paul. "NASA criticised for sticking to imperial units." New Scientist. June 22, 2009. (Sept. 14, 2011) http://www.newscientist.com/article/dn17350-nasa-criticised-for-sticking-to-imperial-units.html
McQuillan, Laura. "As the U.K. brings back imperial measurements, is it time for Canada to drop them?" CBC News. Jun. 2, 2022. (Oct. 13, 2023). https://www.cbc.ca/news/canada/as-the-u-k-brings-back-imperial-measurements-is-it-time-for-canada-to-drop-them-1.6472738
"Metric system." Encyclopaedia Britannica, 2011. Web. (Sept. 14, 2011) http://www.britannica.com/EBchecked/topic/378783/metric-system
National Institute of Standards and Technology. "The United States and the Metric System: A Capsule History." Oct. 4, 2006. (Sept. 14, 2011) http://ts.nist.gov/weightsandmeasures/metric/lc1136a.cfm
Nelson, Robert A. "The International System of Units: Its History and Use in Science and Industry." Via Satellite. Feb 2000. (Sept. 14, 2011) http://www.aticourses.com/international_system_units.htm
Physical Measurement Laboratory of NIST. "The NIST Reference on Constants, Units and Uncertainty: International System of Units (SI)." Oct. 2000. (Sept. 14, 2011) http://physics.nist.gov/cuu/Units/
Rowlett, Russ. "How Many? A Dictionary of Units of Measurement." Center for Mathematics and Science Education, University of North Carolina at Chapel Hill. July 11, 2005. (Sept. 14, 2011) http://www.unc.edu/~rowlett/units/index.html
Smith, David. "Metric Conversion: How Soon?" Public Roads. Summer 1995. (Sept. 14, 2011) http://www.fhwa.dot.gov/publications/publicroads/95summer/p95su14.cfm
Sobel, David. "The Kilogram Isn't What It Used to Be -- It's Lighter." Discover Magazine. March 8, 2009. (Sept. 14, 2011) http://discovermagazine.com/2009/mar/08-kilogram-isn.t-what-it-used-to-be-it.s-lighter/?searchterm=SI
Taylor, Barry N. and Ambler Thompson, eds. "The International System of Units (SI)." NIST Special Publication 330. 2008 Edition. (Sept. 14, 2011) physics.nist.gov/Pubs/SP330/sp330.pdf
U.S. Metric Association. "Metric System Information." (Sept. 14, 2011) http://lamar.colostate.edu/~hillger/
Original article: What Countries Use the Imperial System?
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