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Taking Measure

Just a Standard Blog

Busting Myths about the Metric System

Elizabeth Gentry holding a kilogram test weight in the weights and measures training lab

Elizabeth Benham and other NIST weights and measures metrologists train weights and measures inspectors from across the country to use standard weights like this to test measurement devices like grocery store scales for accuracy.

Credit: F. Webber/NIST

This year will be the 45th anniversary of the Metric Conversion Act, which was signed on December 23, 1975, by President Gerald R. Ford. Normally, we celebrate by sharing metric education resources, but this year I want to use the occasion to dispel some common misconceptions about the U.S. relationship with the metric system.

You’ve probably heard that the United States, Liberia, and Burma (aka Myanmar) are the only countries that don’t use the metric system (International System of Units or SI). You may have even seen a map that has been incriminatingly illustrated to show how they are out of step with the rest of the world.


metric system adoption map
Countries that have not "officially" adopted the metric system (The United States, Myanmar, and Liberia) in gray.
Credit: AzaToth [Public domain], via Wikimedia Commons

It’s a compelling story and often repeated, but you might be surprised to learn that it’s simply untrue!

While it’s true that metric use is mandatory in some countries and voluntary in others, all countries have recognized and adopted the SI, including the United States.

Russ Rowlett, retired University of North Carolina at Chapel Hill professor of education and mathematics, emphasizes on his website that becoming metric is not a one-time event but a process that happens over time. Every international economy is positioned somewhere along a continuum moving toward increased SI use. There are still countries that are amending their national laws to adopt a mandatory metric policy and others pursuing voluntary metrication.

The United States was one of the original countries to sign the Treaty of the Meter in 1875, which is now celebrated annually on May 20, World Metrology Day. It’s been legal to use the metric system since 1866, and metric became the preferred system of weights and measures for U.S. trade and commerce in 1988.

metric continuum
Metric system use in the U.S. lies along a continuum where some measures are entirely in metric and others are entirely devoid of it, at least at the consumer level.
Credit: E. Benham/NIST

Did you know?

  • We use the SI every second of every day. After all, the second (s) is the SI base unit of time.

  • U.S. coins & currency are produced using metric specifications.
  • Many U.S. products, like wine and distilled spirits, have been successfully sold with only metric measures since the early 1980s.
  • Metric units are used extensively on packages to provide net quantity, nutrition, and health-related information, and for prescription drugs, over-the-counter medicine, vitamin supplement dosing, and other consumer products.
  • SI units are increasingly used on consumer product labeling in the U.S. lighting sector. Voluntary package labeling standards adopted by flashlight manufacturers help consumers make product comparisons. While shopping, consumers easily evaluate light output (lumen), peak beam intensity (candela), beam distance (meter), and impact resistance (meter).

It’s impossible to avoid using the metric system in the United States. All our measurement units, including U.S. customary units you’re familiar with (feet, pounds, gallons, Fahrenheit, etc.), are defined in terms of the SI—and mass, length, and volume have been defined in metric units since 1893! The SI’s influence is pervasive and felt even if most people don’t know it. I envision U.S. metric practice like a huge iceberg. Above the water’s surface, U.S. customary units appear to still be in full effect. In actuality, below the water’s surface, we find that all measurements are dependent on the SI, linked through an unbroken chain of traceable measurements.

metric iceberg showing SI below the surface and customary measurements above the water line.
The U.S. measurement infrastructure depends on the SI.
Credit: E. Benham/NIST

Although U.S. customary units are still seen alongside metric units on product labels and merchandise literature, it’s common for the goods themselves to be made using SI-based manufacturing processes. Why? While some businesses are concerned that consumers expect to see customary units on the package, when it comes to manufacturing processes, they are under constant pressure to stay competitive. Adopting the latest science and technology, developed using metric design practices, enables innovation. In addition, many industries extensively use international supply lines to develop, manufacture and sell their products around the world.

I’m the coordinator of NIST’s Metric Program. Because of my passion for all things metric, I encourage companies to investigate adopting metric practices whenever possible and show them how doing so can make a strategic economic impact for their organization. Changes in technology and extremely competitive domestic and global marketplaces can compel businesses with little previous experience to explore metric use. Many have found that going metric pays off, resulting in a competitive advantage.

Going metric pays off

During the recent recession, lumber companies located in the U.S. Northwest saw their U.S. customer base shrink, but their Canadian and Japanese markets, both of which use metric, expand—especially after the 2011 earthquake and tsunami. Wood-product producers made adjustments so that their production systems could flex between metric and U.S. customary measures based on what their customers needed. Because so much of the world uses metric only, more and more U.S. companies are recognizing the benefits of metric as they find new international markets for their products.

If your business is considering making the switch to metric, I would encourage you to conduct small beta tests to explore how your customers react. Research can help ensure decisions aren’t based on out-of-date information or preconceived notions. You might be pleasantly surprised by how quickly customers adapt—and how using metric benefits the bottom line.

And as always, if you need advice, be sure to give NIST a call. We’re here to help!

*Editor's note: This post was updated on October 6, 2020, to include information about units of measure for lighting, to include a new "metric continuum" graphic, and to make other minor changes. It was originally published on December 23, 2016.


About the author

Elizabeth Benham

Since becoming Metric Coordinator in 2005, Elizabeth has worked to support voluntary conversion to the International System of Units (SI), commonly known as the metric system, in the U.S. In addition...

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With climate change being such a 'hot' topic right now, in order to fully take part in international discussions, it would make sense if the USA at least used the same unit of temperature as the rest of the world!
Looking at the broader issue of measurement, I find it so strange that whilst America has advanced science and technology in so many areas but yet their political leaders have lacked the foresight to fully change to metric which would make international trade and the exchange of information so much easier for them.
Unfortunately, polititions, probably due to their education are often scientifically illiterate and don't appreciate the benifits that full metrication would bring to the USA.
Whist the people of the world may speak many languages, they all (apart from the US) measure things in the same language.

Tim, you said,

"With climate change being such a 'hot' topic right now, in order to fully take part in international discussions, it would make sense if the USA at least used the same unit of temperature as the rest of the world!"

We do. Just not in daily life. An American scientist absolutely understands what 20 degrees centigrade indicates. But the announcements on TV are done in Fahrenheit because it's a free country and that's their choice as individuals. It is certainly not illegal for a TV or radio station to start announcing the temperature in degrees centigrade. To my knowledge there's nothing illegal about announcements made in Fahrenheit in Italy, either, for the same reason.

And I think you'll find that the Americans dominate climate science just like we dominate every other form of human endeavor. Don't mistake the opinions of politicians for the facts of science which are universal, in all countries.

This article trying to explain "levels of adoption" and how "it takes time" to adopt the SI made me laugh. Of course 140+ years is not enough to sink in.

Some of the most brilliant minds and inventions came from the US, but the average Joe is not capable of adapt to SI.

Of course some countries still use a few imperial measurements, but the absolute majority of daily life things are SI, like distance (meters), weight (grams), volume (liters) or temperature (celsius).

Of course the US product packages come with both measures, otherwise it's hard to export it, since no one else will understand it.

Also, the US uses imperial to guide the size of their packaging. E.g. you sell vegetables in 2 lbs packages, which in SI is a weird total of 907g, instead of customary 1kg. Same for bottles, instead of 1 liter, 1.something which is equivalent to something else in fluid ounces. BTW, what the heck is a fluid ounce? Why would you measure things in eighths of an inch? Do you love fractions that much?

The funniest thing of all, is that the US were the first to be independent from UK (compared to Australia, Canada, etc) but you can't get rid of their weird system, which even UK ditched.

Hally blinkin' lujah! The Imperial system is utter tosh. We, the Brits, are ultimately responsible for inflicting this on the world for the longest. Difference is, in our shame, we know it's useless and have been big enough to dump it. Whereas instead of admitting they've been dumped-on and calling it British or Imperial - this to at least shift the rightful blame, Americans actually call the whole mess 'American'.

With all due respect, it's really not using the metric system when you continue teaching and measuring things with the imperial system. Defining the imperial system using the metric system isn't using the metric system. Please become practically metric.

When observing and quantifying the world around you, pounds and inches just won't do! Kilogrammes and metres are much more scalable, from chemistry to physics.

It's not a myth. When I buy gasoline in the USA I do not pay per liter, I pay per gallon. When I drive, all the signs are written in miles, not kilometers. When I get close to an exit, it says 1/2 mile, 1/4 mile, etc., not 400m or 200m. But when I am in Europe, the gasoline is priced in euros per liter and the signs are all in kilometers.

A couple points that I don't think have been brought up yet.

a) Nobody outside of the USA calls the SI (ISU) the "metric system". The term "metric system" is just a generic term for "system of units of measurement". It's totally valid to call British Imperial units a "metric system" - since "metric system" does not refer to any one system of measurement.

b) One huge benefit of the SI is that it really does make teaching the sciences easier. British Imperial units predates Newton's Laws! There would likely be a marked improvement in US educational attainment in science as a direct result of adopting the SI. And if that fails to sway people you can just tell them "Using SI means never having to worry about slug feet!"

I worked in scientific circles for many years, and worked for a while with my wife teaching her elementary students math and science.

When I taught her students measurements, I eventually found that the easiest way to get to them was to ask how many ways to measure length in our old-fashioned system, then how many ways in metric. It was shocking to them that there are many different measures of length in the old way (inch, foot, yard, rod, etc.), but only one in the metric system (meter). And nearly all the other measures are just a simple. Yes, I did have to explain that "kilo" means one thousand, "centi" means one hundredth, etc.

Unfortunately, even the teachers are hung up on the silly old-fashioned measurements, and regard the familiar as "easy", and the metric as "hard". Unless we get the educators to accept education, we are unlikely to make good headway in metrication.

The Imperial system is utter tosh. We, the Brits, are ultimately responsible for inflicting this on the world for the longest. Difference is, in our shame, we know it's useless and have been big enough to dump it. Whereas instead of admitting they've been dumped-on and calling it British or Imperial - this to at least shift the rightful blame, Americans actually call the whole mess 'American'.

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