For decades, the United States has been one of the very few nations on the planet to not use the metric system as its primary measurement standard. Whereas other countries use derivatives of meters (for distance), liters (volume) and kilograms (mass) and they describe temperatures in degrees Celsius (also called centigrade), the U.S., as of 2018, remains firmly in the grip of the English, or Imperial, system. This is despite U.S. Congress having authorized – but not mandated – the use of the metric system in 1866.
The primary disadvantage of the English system is that, frankly, it is wondrously haphazard. There is nothing intuitive about it whatsoever. Just to name one example, to convert from inches to feet in linear distance requires dividing by 12, while translating feet to yards means dividing by three and calculating yards from miles requires dividing by 1,760. The advantages of the metric system are rooted in its being simply based on successive powers of 10 across all primary types of measurement, but its disadvantages in the U.S., as you'll soon see, are straightforward.
The English System of Measurement
Even calling the English measurement system as "system" at all is perhaps something of a reach; it's really more of a ragtag collection of units and labels that fit together roughly as neatly as a set of square pegs and an array of roundish holes. But the reason it's always dominated everyday life in the U.S. is fairly obvious: The United States was originally founded (though not settled) by people from Europe, specifically England. When the U.S. gained its independence in the later part of the 18th century, its new constitution allowed for the establishment of a national system of weights and measures, and by 1830 or so, the common English units had been propagated throughout a nascent and rapidly growing America.
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It was not long, in geopolitical terms, before the U.S. became the dominant force worldwide both militarily and in terms of trade. Meanwhile, Britain (tantamount to England, for purposes of measurements), though defeated in the Revolutionary War, was still a global force to be reckoned with. As a result, for a long spell, the U.S. was able to easily impose its system of gallons, pounds, miles, acres and virtually everything else on the rest of the world. This is no longer the case, with the European Union and East Asian nations (China, South Korea and Japan) now serving as major international trade forces, and so pressure on the U.S. to conform to a more user-friendly metric system has increased on this basis alone.
The Metric System: An Overview
The metric system was chiefly a product of French scientists in the wake of their nation's own revolution in 1789. Its fundamental unit of length was the meter, which, while similar to the yard used in the English system, was actually rooted in something concrete – namely, one-millionth of the distance from one of the Earth's poles to the equator. (In fact, this turned out to slightly off, but the unit was retained at its original length.) Similarly, 1 kilogram was defined as the mass of water that consumed a volume of 1 liter. 0 degrees and 100 degrees Celsius were established as the freezing and boiling points of water respectively.
In addition to these practical standards, units smaller or larger than meters, kilograms and liters were listed as decimal multiples or fractions of the original units, meaning that they were obtained by multiplying or dividing by 10 or some power of 10. This brought Greek prefixes such as milli-, centi-, deci-, deca-, hecto- and kilo- into the framework.
In the wake of the aforementioned 1866 U.S. legislation, American scientists, doctors and engineers readily gravitated toward the SI (Systeme Internationale, from the French) units of the metric system. The public at large, however, continued to hold firm, even in the face of official codification of the metric system the world over into and throughout the 20th century. Great Britain made the metric system its official system of measurement in 1965, and 10 years later, the Metric Conversion Act encouraged the adoption of the more streamlined standard. But encouragement is not the same thing as a mandate, and in the eyes of the general public, the English system remains the standard well into the 21st century. Chances are very good that if you told a randomly selected American that it was predicted to be 25 degrees Celsius tomorrow afternoon, he would have no idea whether he's be comfortable in a T-shirt or whether a parka would be wise. (Quick tip: Multiply degrees Celsius by 1.8 and add 32 to get the equivalent Fahrenheit degrees. This means 25 C equals (1.8)(25) + 32 = 77 F. For a rough estimate, double C and add 30 instead.)
Resistance to the Metric System in the U.S.
As you have likely surmised, much of the resistance to America joining the rest of the first world on the metric train, the benefits of the metric system notwithstanding, is the simple burden of practical work that would be required to bring this about. For example, consider the number of speed-limit signs within, say, 5 miles of your own neighborhood. Every single one of these would need to be replaced at some point. Now try to imagine how many such signs are scattered around the 3.5 million square miles (a little shy of 10 million square kilometers, if you're counting) of the United States. That's an awful lot of metal, and that's just one example of a unit everyone is dialed into that would have to be retired in favor of the more internationally popular alternative.
Long before highways, or even cars, however, some of America's technical folk were averse to parting ways with certain English units, one of them being the inch. In particular, engineers who worked with tools such as screws were – and remain – attached to the "times two" format of this kind of equipment, which traditionally comes in units of halves, quarters, eighths and sixteenths of an inch. Dividing or multiplying by 10 when it comes to screws just isn't practical, and probably never will be. So, while it's easy to dismiss America's collective balking at the metric system as a the result of a combination of laziness and innumeracy, there exist plenty of pragmatic hurdles to making the metric jump.
Drawbacks of the Status Quo
While there would certainly be growing pains in forcefully forsaking the English system of weight and measures, these would undeniably be justified by the many advantages of fully adopting, rather than dancing around, the metric system. One example is in public health. In the spring of 2018, a hospital in New Hampshire and its affiliated clinics switched its electronic medical records system to metric units, a change that was primarily fueled by a desire to minimize the risk of medication dosage errors, a perennial bane in health care. Traditionally, medication dosages are given in milligrams of medication per kilogram of patient body weight. But when pounds are used for the patient weight, this can introduce mistakes because a kilogram is 2.2 pounds, sometimes leading to people being given over twice the amount of medication actually prescribed – a situation that can lead to dangerous levels of medication toxicity. According to staff, the patients quickly learned to adjust to their "new" weights, suggesting that Americans really could adjust to broad-scale adoption of SI units in their everyday and professional lives.