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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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The wife and I were traveling through NY and Canada the day Canada made the change, I'm of the opinion that the common man (American) is just to hardheaded to change everyday life to METRIC?? (not invented here)
Of course, you realize U.S. customary units were not invented here, either.
In fact, they are the British (pre-Imperial) units in use at the time of the Revolutionary War. The US gallon and bushel were defined by British Parliament circa 1700. We incorporated none of the improvements of the 1824 Imperial Act. Not quite the "freedom units" some Americans like to imagine.
What is most interesting is that the origin of USC is Roman. The Roman influence in Europe established the ancestors of USC and imperial. The British before the Norman invasion used a decimal system with the wand as the fundamental length unit equal to about 1007 mm. Some British traditionalists oppose the metric system on the false belief it was forced on them by EU regulations when it fact the British actually invented it and left their mark heavily upon it. After the Norman invasion, the Norman yard replaced the British wand (metre) to the point that the wand in its last days as the yard and the hand. See metrology.

This is one of the most educated and concise history of legacy units I've seen - It's appreciated. Yes, about 95% of the old imperial units were actually from the era when England was under imperial Roman rule. Oddly enough, even the Americans can lay claim to development of the metric system early on as both Franklin and Jefferson were Francophiles and history states that they actually assisted the French in it's development (America's metric currency system is a direct result). It is long overdue for a nation that regards itself as progressive and intelligent to be carrying along the baggage of antiquated and useless units of measure.

Although none of the modern measurement systems were invented in the U.S., our role as an original signatory to the Meter Convention established our commitment to improving measurements that’s guided the journey towards a coherent system based on fundamental constants, rather than barleycorns, Roman soldiers paces, or His Royal Highness’ body parts. The old British system was a hodgepodge of units, assembled from a variety of cultures and sources. U.S. colonists brought it here; but subsequent changes over time created additional differences between the British and U.S. Customary systems. But that’s another blog article! NIST HB 44 Appendix B (2.3)…) explains how the U.S. Customary System differs from the British System. NIST SP 447 Weights and Measures Standards of the United States, A Brief History (…) offers a more detailed explanation.
The US failed to metricate in the '70s mostly due to a misapplied sense of arrogance. Thinking one is better than everybody else can cause one to make bad and irrational decisions. Decisions that can come back to haunt you generations later or when your industries that would like to change can't find local talent with a proper metric functionality and are forced to close shops in the US and go elsewhere where the metric system is first nature or the only nature among the populations. Like any profitable business, metrication should have been achieved by strong leadership, a well devised plan, extensive education, and a system of punishment for any resistance. All of this could have accomplished a completed metrication by now with no one looking back. The NIST and Congress both failed the nation. The NIST could have applied more pressure on the congress and the congress should have set the standard as their right per the constitution and even if initially unpopular in the best interest of the nation for the present and future generations. But it isn't too late. The NIST really needs to rise up and present the congress with a timely workable plan and push until the job is done. It will be a sacrifice, but nothing good comes from stagnation and going in reverse. Pay now or pay later. The choice is yours to make.

Fantastically well put and I could NOT agree more with your assessment. Thanks for finding the words I seem unable to express.

Unfortunately, it will be extremely difficult to do, especially now. The US is already failing at getting large swaths of its population to understand: wearing masks prevents transmission of the corona virus, shooting unarmed citizens is wrong, and climate change is real caused by greenhouse gases.

The best way to do this is to pass a federal law banning the selling, advertising and the manufacture of all products using the imperial system, even if metric is specified along side it. Old stock products can still use the imperial system. The law should go into effect immediately starting with the federal government and its suppliers with a 2 year grace period for all private and public businesses. The penalty for using imperial would be an invalidation of any implied, verbal or written transaction or agreement with terms favoring the buyer. If consumers, for instance, are sold products in imperial, they are automatically entitled to keeping the product along with a full refund.

I must ask - how on God's green earth do you think Americans somehow invented our goofy method of measure we cling to so dearly? A large portion of USCS are throwbacks from the Roman empire, the remaining are edict of a king (um, the 'foot' and the 12 inch 'ruler'). If we were being honest and fair, we would've abandoned imperial (English) units for French (our staunchest ally in the Revolutionary War) just out of principal on or about 1790, as intended. Now, SI is not bound to any one nation. The kg and the meter (metre) have all been redefined using constants from nature. It is truly a system for all mankind and it's time we adopt it.

I like the "metric continuum" but I think the lightbulb might be misplaced on it. It is designed to operate on a certain voltage (volts), draw a certain amount of power (watts), produce a certain amount of light (lumens) at a color temperature (kelvin) and do so for a certain lifetime (hours). The common screw base (E26) is designated by its diameter in millimeters. Only the bulb diameter is in eights of an inch (A19 for common household bulb). My remarks largely apply to other bulb styles as well. I would move it up two notches, certainly at least one. In fact, neither the electricity used nor the light produced can be measured in Customary as Customary has no such units.
Calling me a Luddite does not help your campaign. The "customary" units go beyond just measurements. Many words and phrases a ingrained in our lexicon and used in many areas of life. One small example: ymmv, or your mileage may vary. The phrase "your kilometers/hour may vary" just doesn't cut it.

I don't know. I'm perfectly capable of using litres per 100km and I can figure out in my head how many more km's I can travel before running out of gas. It just took a little bit of flexing the brain muscles to acclimate to SI - but I'm in a small minority of Americans who seem to enjoy doing just that. On one hand, I'm glad I became fluent in SI on the other hand, being forced to use road signs in miles, feet and barleycorns makes me gag just a little bit knowing full well they'll never change. We are outliers here and we're rejecting what the rest of the world has acknowledged that SI is simply, unequivocally a superior way of measuring our world.

So you're staying in the 11th century because a modern, rational, universal system based on decimals isn't pithy enough? I'm sorry, but that is simply incomprehensibly short sighted.

The map is accurate visualization of the commonly accepted senses of units by country. For example, those people in the three countries do not easily understand how long a 1-m stick is in general. Although SI is used in very limited areas by scientists and engineers, it does not mean the unit is accepted in the country. There are only two type of people; who tell their weights in kg, and those who use their traditional unit. There is no gray area in between them. The metrification continuum is a total myth. Go outside your country and see what other people do, because that is the only way to find out how absurd this article is.

Although the “metric map” has been elevated to a pop culture meme on the internet, it’s based on out-of-date information. This metric myth is rooted in an early map published in the 1971 U.S. Metric Study that depicted countries “uncommitted” to mandatory metrication. To see the historic map, visit the NIST U.S. Metrication FAQ website. Over the years, the term “uncommitted countries” has morphed into “non-metric” countries, a depiction that’s been used to criticize voluntary metrication by the United States.

If i ever start a business I will look into using the metric system. It look like i will make some money since it is no way around using the metric system everyday.

A nation of pig headed, recalcitrant citizens who for the most part seem blissfully unaware that any part of the globe exists but themselves. I feel terrible for kids in schools today. Especially those who may be inspired to join the scientific world. They claim they're teaching SI in American schools but the disconnected, chaotic 'real world' pressure of kings feet, barleycorns and Roman soldiers makes them soon forget what they've learned. Retaining our impractical, insular method of measure here in the US is a clear detriment to the future of our kids and their offspring. It is a foolish, quaint mish-mosh of nonsense. I'm in my 50's and still cannot conceptualize a "floz" vs an "oz", 12th, 8ths, 16th, 32nds of "inches", teaspoons, tblsp's, etc. It's absolutely confounding when you step back and objectively analyze it. I just use it because I am forced by American society to do so. I'm not overstating when I suggest that my own personal failure in the American education system was, at least in part, due to trying to grasp obtuse concepts like "American units" and relate them to each other. It's just not possible.

Both Myanmar and Liberia have made a decision to adopt the SI Metric System. The USA is literally alone as the only country on the planet Earth not making a serious effort to go metric.

The metric system is very good because when you are cooking you can use it to distrubute your ingredeints

Resistance to change is part of every human's DNA. It is part of what makes humans human. Some are more resistant than others. There isn't a right or wrong about the balance. There are several well documented reasons why people resist change.
(1) Complacency such as inertia or maintaining status quo
(2) Resignation such as anxiety the change is not an improvement and is viewed as a personal failure, or
(3) Cynicism or expectation the change result is not an improvement and viewed as someone else failing

Most resistance to change is rooted in Complacency. And most intentional progress is derived from organized efforts to overcome the complacent.

To the good people at NIST - don't get discouraged by those expecting failure. Work through complacency, gun your change engines, and get your KPH to a high rate of travel! :-)

Its 2020 and we still have huge differences in what should be a common understanding of measurement systems. Invisible borders is what halts our progress into the future.

One important field of activities extending worldwide - aviation, was not mentioned. No problem making the conversion here either. I have been flying (lawful) my glider in the US since 1996. All instruments are S.I. - Altimeter (m), ASI (kilometers/hour), variometer (M/s), etc. My flights are all VMC (Visual Meteorological Conditions) and I can't maintain a level as the IMC (Instrument Met. Conditions) like the big guys. This is adjustable and can be done. It will take time and money (many instruments) and reeducation, but it is doable. Time to start, like everything else..
By the way, I am very proud of my two 750Km (454.545mi) flights.

Time is not measured metrically. I have spent 49 years (omg almost 50 years now) as a metrology technician. 99 percent of my work still to this day is done in pounds, ounces, foot pounds, inch pounds, gallons quarts, etc. only electrical units of measure have been adopted as standards in use for my customers. Until the day we rework the day into 100 (some measurement unit) we will never be able to convert Time to a metric unit. This old man still prefers to use non metric units.

I cannot visualize metric units and must convert them to the measuring system that I grew up with before I know what the given measurements actually mean. I know that metric measures are useful and more precise, but they are of no use to me.

I was a child during the push to switch over to Metric and I can say one of the biggest mistakes was why the emphasis on converting back and forth! Fortunately, my time card in the military Metric is widely used and once I realized that if it is in Metric there's no need to convert! Also, I did a lot of traveling, abroad so the Metric System is natural to me. I use it whenever possible. My GPS I use Metric, phone temp Celsius, even my car most of the settings are set on Metric! I'm actually embarrassed that we are so slow with the changing, most people abroad look at us like we're idiots. At least I know the younger generations are being taught Metric properly,and most agree we should be using it. It's past time, and it's mainly the baby boomer generation that refuse to adapt, to change.

Nice try but the vast majority of Americans have no clue on metric. For example, they don't even know how far a kilometer is compared with a mile.

Interestingly, the British still officially use miles for road signs. But the younger generations think almost exclusively in metric, since that's what the state schools and NHS teach/use. Stones and pounds for a person's weight, and feet and inches for a person's height, are only used in everyday parlance among older generations.

Although Canada adopted SI on April 1, 1975 (to some hatdcore Canadians this was indeed a joke) we still use Imperial measures widely, in fact exclusively in construction and plumbing. In western Canada temperatures are still announced in F and C in medias. Elders tend to state distances in miles although our maps and traffic signs are exclusively metric. SUMMARY: Canada still uses, albeit unofficially, Imperial measures.

Yes, the myth that US does not use metric system is really stupid.
But this is still true that only US, Liberia and Myanmar in the whole world use only or mostly imperial system. And the fact that US was one of the original countries to sign the Metre Convention, only makes it look worse.

Yanno, the real question I have is why does anyone care? If it is such an inconvenience, feel free to live somewhere else. Literally, almost anywhere else... Using the current customs of the rest of the world as some sort of guiding principal is as logically empty as admonishing a holdout for refusing to use leeches for headaches in the dark ages. Lets be honest, even many of the SI units are fundamentally meaningless. How many times has the Meter changed? The gram? Lets look no further than a 2019 article from Physics World:

"Metrologists and policy-makers from 60 countries around the world have unanimously agreed to change the definition of four units of measurement. At a meeting today at the General Conference on Weights and Measures (CGPM) in Versailles, France, delegates voted to redefine the International System of Units (SI), changing the world’s definition of the kilogram, the ampere, the kelvin and the mole. The changes will now come into force on 20 May 2019."

The point is, enough is enough. The condescending stream of preaching about the United State's refusal to adopt the metric system has grown dull. You want to know why the US doesn't use the metric system? Its arbitrary, just like the Imperial system. The metric system has at least given us a framework for how to base an entire measurement system off of a pointless, arbitrary creation like the meter. Now lets build one that is actually universal. Dare I say one that is based off of something substantial, apparently universal, and easily attainable, like the hydrogen atom? No? Then perhaps the electron? At least it would be something fundamental to the cosmos, as opposed to some arbitrary construct which has had to be redefined half a dozen times in the last 200 years. Make a meaningful system with true foresight and longevity, we will gladly change. Continue to argue for swapping out one arbitrary system for another simply because "everyone is doing it" and its easier to multiply? Moving on...

Oh, and since base 12 mathematics are apparently too hard for anyone outside of the U.S. these days, feel free to make the new system base 10. I would hate to make anyone resort to using elementary school multiplication after all.

No, the lenght didn't change, only the definitions and protocoles were improved for additional rigour.
Seriously, in the era of micro-processors and nanotechnologies, do you really think it's still consistent to use primitive measures as the finger of someone, its foot or a random stone? There is a lenght unit now carrefully defined and based on the most precise way humanity was able for the metre (speed of light) and one for the mass based on water at sea level of a cubic tenth of that lenght, the whole metric system is named by comprehensive terms and logically dividable with the base the world decided to use (whether 12 might have been better) without having to do multiple conversions and used everywhere.
Long story short, imperial is not compatible even with itself. If you're happy with that, good for you but I and many others want better than mediocrity for our country!

The US is already metric; and will eventually become fully metric. The problem is that it is not happening fast enough. NIST needs to push harder. The new generations are not recalcitrant like previous generations, and will indeed accept the new system. We just need to educate.

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