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Here's the real reason the U.S. doesn't use the metric system

The United States is one of just three countries that "prefers" imperial measurements over the international metric standard. The motivation why has less to do with official policy than you might think.

Here's the real reason the U.S. doesn't use the metric system _ Weighing News
A park ranger takes a photo of a thermometer at California’s Death Valley National Park in 2021. Metric was officially declared America’s preferred system of measurement in 1975, but the U.S. public has been slow to embrace it. PHOTOGRAPH BY ROGER KISBY, REDUX

What do Libya, Myanmar, and the United States have in common? Proponents of the metric system claim all three nations have a shameful similarity: They use imperial measurements—feet, pounds, and miles—instead of meters, grams, and kilometers.  

The truth, however, is more complicated: Though imperial units are commonly used in the U.S., the metric system is actually the nation’s preferred system of measurement.

So why don’t Americans use it? Here’s how the metric system evolved—and why it’s still struggling to gain a foothold in daily life.

Hiding in plain sight

First, the facts: “It’s been legal to use the metric system in the United States since 1866,” says Elizabeth Benham,  program leader of the federal metric program at the National Institute of Standards and Technology, the federal agency in charge of standard weights and measures.

In fact, government policy since the 1970s has designated the metric system—also known as SI, or the International System of Units—the nation’s preferred system of measurement for trade and commerce. But since the U.S. has adopted a voluntary approach, nudging industries and individuals toward using SI instead of making its use a blanket policy, adoption is turning out to be a multi-century endeavor.

Here's the real reason the U.S. doesn't use the metric system _ Weighing News
A traffic sign in Quebec, Canada, photographed near the U.S. border in 1970, reads "THINK METRIC, 1 Mile = 1.6 Kilometers." The sign reminds travelers from the United States that metric measurements are used in Canada. PHOTOGRAPH BY OWEN FRANKEN, CORBIS/GETTY IMAGES

The meter’s chaotic origins

The challenge is reflected in the measurement system’s often chaotic history—and its origins in the French Revolution. In the late 18th century, Enlightenment-era Frenchmen saw a tantalizing opportunity in the political upheaval that gripped their nation. Up until then, France had been home to a dizzying array of weights and measures, with up to 250,000 units of measurement in use in France alone, and other nations and even regions within nations had their own ways of quantifying the world around them. This was a measurable nightmare for scientists, who dreamed of an international standard based on some universal, unchanging constant.

Tasked with determining a new system, the French Academy of Sciences decided that the measurement underpinning it should consist of the length of one ten-millionth of the distance from the North Pole to the Equator as measured from Paris, France. That measure—known as a meter and painstakingly documented by a group of scientific luminaries—became the basis of all metric units of weight and measurement that followed. Even measures of volume were based on the meter—a milliliter, for example, is the volume of a cubic centimeter of water.

A slow switch

The new system was readily adopted by the French state, but the public has always proven much slower to follow suit. Implementation lagged, even as the new system electrified scientists worldwide, using the meter to help define things like electricity and magnetism. And the concept began to spread: By 1866, the idea was adopted by the U.S., which passed a law that year allowing the use of metric measurements in commerce.

Slowly, the metric system spread throughout the nation, first thanks to brass standards issued to the states to help them standardize their weights and measurements, then thanks to the Treaty of the Meter, an 1875 agreement signed by the U.S. and other powerful nations like Germany, Russia, and France. This treaty created an international governing body for weights and measures, paving the way for the U.S. to make the switch on a wider scale.

But American adoption still lagged, even as scientists kept improving the system and applying it to more and more fields of inquiry. By 1960, the metric system had been expanded and modernized to cover aspects of all of Earth, from voltage to speed, heat capacity to radiance. That year, the International System of Units, or SI, was defined and adopted worldwide.

Most other nations dutifully adopted SI, changing road signs and packaging and teaching the metric system in schools. Even the United Kingdom, which had lagged for years, mostly embraced the system in an effort to keep pace with other European Union nations. (Since the U.K. left the EU, metric opponents there have argued the nation should stop using metric units, a controversial proposition that has yet to be adopted.)

Despite international adoption and increasing federal policy encouraging the use of metric units, the U.S. continued to drag its feet. Resistance was fueled in part by industrialists who argued the system was too complicated and expensive to implement, legislators suspicious of “foreign” influence, and controversies over whether wide-scale federal adoption might infringe on states’ rights.

The end result was confusion. Though the U.S. officially declared SI the nation’s preferred system through the 1975 Metric Conversion Act, even federal agencies were slow to adopt metric in industry, education, commerce, and daily life. One example is road signs: Though federal officials attempted to turn a new interstate in Arizona into an SI poster child in the wake of the Metric Conversion Act, even giving it kilometer markers instead of mileposts, transportation officials never extended metric-only signage to the remainder of the federal highway system.

Instead, says Benham, both systems are still widely used nationwide. “We’re working in a risky hybrid measurement environment,” she notes. Dual labels are common, and often metric measurements hide in plain sight on rulers, road signs, and tools. This can lead to costly miscalculations and public confusion.

“Large organizations that had the resources and technical staff to facilitate [metric] saw right away that it would give them a strategic advantage,” Benham explains. “They adopted best practices and moved on.” But smaller businesses and individuals need help making the switch, as do educators poised to inform future generations of metric-native learners.

Making the change to metric

Nonetheless, Benham still believes voluntary metrification in the U.S. is possible—and encourages individuals to look for the metric measurements that already surround them. “I use the analogy of an iceberg,” she says. On the surface are U.S. imperial units, but many industries already use metric measurements, including food labels, car speedometers, and thermometers.

Ultimately, says Benham, a full transition to the metric system won’t be possible until individuals take the plunge and decide to use it in their daily lives. That’s why she focuses on education at her job—and has switched to the metric system in her daily life, setting her smartphone to measure length in kilometers instead of miles and using degrees Celsius instead of Fahrenheit.

“We have the technology,” she says. “Change is going to happen. It’s just a slower change than a mandatory approach.”

Are you ready to change? If so, Benham recommends a glimpse at NIST’s online tools, which include tips on incorporating metric measurements into everything from cooking to gardening and health. “Once you overcome the hurdle of learning more units, it’s easy,” she explains. And widespread metrification could be easy too—if the American public only had the will.

By Erin Blakemore

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