A short history of two systems
The metric system was born of the French Revolution. In 1790, the French National Assembly commissioned a new system of weights and measures to replace the chaotic patchwork of regional units that varied from town to town. The units were to be based on nature, not on the length of a king's foot or the weight of a grain of wheat. The meter was defined as one ten-millionth of the distance from the equator to the North Pole, measured along the meridian through Paris. The gram was defined as the mass of one cubic centimeter of water at its maximum density. The liter was defined as one cubic decimeter. The system was decimal: every unit was related to every other by a power of ten, and the prefixes (kilo, centi, milli) were consistent across all quantities.
The imperial system, in contrast, evolved rather than was designed. The units that became the imperial system were codified in Britain by the Weights and Measures Act of 1824, which standardized a set of units that had been in use for centuries. The yard was said to be the distance from the nose to the outstretched fingertip of King Henry I, though this story is apocryphal. The pound was descended from the Roman libra (hence the abbreviation lb). The gallon was originally the volume of eight pounds of wheat. None of the units were related to each other by a consistent ratio; there were 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound, 14 pounds in a stone, 8 stones in a hundredweight. The system was a museum of medieval trade customs.
The metric system spread across continental Europe in the 19th century, partly through Napoleonic conquest and partly through the obvious convenience of decimal units. By 1900, most of the world had adopted it. Britain began metrication in 1965, completing it for most purposes by the late 1990s, though it retains imperial units for some uses (miles on road signs, pints in pubs). The United States, alone among major industrialized countries, never made the switch official. Liberia and Myanmar were the only other holdouts; Liberia has largely metricated in practice, and Myanmar is in transition.
The metric system, in principle
The metric system's strength is its internal consistency. Every unit of length, area, volume, mass, and force is related to every other by a power of ten, with the same set of prefixes scaling them up and down. One cubic centimeter of water weighs one gram (at maximum density, by definition). One liter of water weighs one kilogram. One calorie is the energy to heat one milliliter of water by one degree Celsius. These relationships make calculations trivial: a 10-meter swimming pool that is 2 meters deep and 5 meters wide holds 100 cubic meters of water, which weighs 100 metric tons. Try that calculation in feet, gallons, and pounds.
The SI system, the modern form of the metric system, rests on seven base units: the meter (length), the kilogram (mass), the second (time), the ampere (electric current), the kelvin (temperature), the mole (amount of substance), and the candela (luminous intensity). Every other SI unit is derived from these seven, with no arbitrary constants. As of the 2019 redefinition, all seven base units are defined in terms of fundamental physical constants — the speed of light, the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant, the cesium hyperfine transition frequency, and the luminous efficacy of green light. There is no longer a physical artifact anywhere in the world that defines a unit; the kilogram is no longer a platinum cylinder in a vault in Sèvres, it is a property of the universe.
The decimal structure means that conversions within the system are trivial. To convert kilometers to meters, multiply by 1,000. To convert grams to kilograms, divide by 1,000. To convert square meters to hectares, divide by 10,000. A child can learn the prefixes in an afternoon, and once learned, they apply uniformly to every quantity. The system is taught first in schools in nearly every country, which means that students learn one set of relationships and use them throughout their education and careers.
The imperial system, in practice
The imperial system's defenders argue that its units are human-scaled. A foot is about the length of a human foot. An inch is about the width of a thumb. A yard is about a stride. A cup is a comfortable amount to drink. A tablespoon is what fits on a spoon. A teaspoon is what fits on a smaller spoon. These approximations make rough estimation easy without a measuring tool, which is genuinely useful in cooking, carpentry, and other hands-on activities. The metric system's units, by contrast, are abstract: a meter is not a human length, a liter is not a human drink, a kilogram is not a comfortable weight to hold.
The imperial system's fractions are another argument in its favor. Inches divide naturally into halves, quarters, eighths, sixteenths, and thirty-seconds, which is how carpenters and machinists work. A piece of lumber is five eighths of an inch thick, not 15.875 millimeters thick. The fractional system makes the divisions visible: 5/8 is between 1/2 and 3/4, and you can see that it is closer to 1/2. The decimal equivalent, 0.625, does not show this as clearly. For work that involves repeated halving, the fractional system is faster.
The counter-argument is that the imperial system's units are inconsistent with each other. There are 12 inches in a foot and 3 feet in a yard, so there are 36 inches in a yard — but 1,760 yards in a mile, which is 5,280 feet, which is 63,360 inches. None of these numbers is a power of ten, and none is a multiple of the others. The conversion factors must be memorized; they cannot be derived. The imperial system also distinguishes between mass and force less carefully than the SI system: the pound is used for both, which works on Earth but breaks down for engineering calculations involving acceleration.
Why the US still hasn't switched
The United States came close to metricating in the 1970s. The Metric Conversion Act of 1975 established a Metric Board to coordinate the transition, and the change seemed imminent. The effort failed for several reasons. The public was unenthusiastic, viewing the change as an imposition. The costs were real: every speed limit sign, every gas pump, every grocery scale, every recipe, every construction drawing, every textbook would need to change. The Metric Board was disbanded in 1982 by the Reagan administration, and the effort was abandoned. The Omnibus Trade and Competitiveness Act of 1988 designated the metric system as the preferred system for trade and commerce, but did not mandate it.
The result is a hybrid. American scientists, doctors, and the military use the metric system. American engineers use a mix, often working in both systems and converting as needed. The construction industry uses imperial units almost exclusively. Consumer products are labeled in both systems, with metric units often in smaller type. Soft drinks are sold in 2-liter bottles (metric) and 12-ounce cans (imperial). Gasoline is sold by the gallon. Weather reports give temperatures in Fahrenheit. Car speedometers show miles per hour, with kilometers per hour in smaller type.
The cost of not metricating is real but diffuse. American manufacturers who export must produce two versions of every product, one for the domestic market and one for the metric world. American students must learn two systems, taking time from other subjects. Medical errors in the US are more common than in metricated countries, in part because of unit confusion between metric dosing and imperial patient weights. The Mars Climate Orbiter, lost in 1999 because one team used pound-seconds and another used newton-seconds, was a $327 million demonstration of the cost.
Where mixed systems cause real damage
The most serious damage from mixed systems is in healthcare. Medications are dosed in milligrams per kilogram of body weight, but American patients are weighed in pounds. A nurse who records a weight as 150 without specifying pounds or kilograms, and a doctor who assumes kilograms, will give the patient 2.2 times the intended dose. Pediatric dosing is especially dangerous, because children's doses are small and a 2.2x error can be fatal. Studies of medication errors in American hospitals have found unit confusion to be a recurring contributor, and several high-profile deaths have been traced to it.
Aviation is another area where mixed systems cause problems. The aviation industry standardized on metric units internationally in 1977, after an Air Canada flight ran out of fuel because of a pounds-versus-kilograms confusion. But some countries, including the United States, still use feet for altitude and nautical miles for distance, while others use meters and kilometers. Pilots flying internationally must be fluent in both systems, and occasional confusion produces near-misses. The Gimli Glider incident of 1983, where an Air Canada Boeing 767 ran out of fuel mid-flight because of a pounds-kilograms error in fuel loading, is the most famous example; the crew glided the aircraft to an abandoned airfield and landed without casualties, but only by extraordinary skill.
Construction and engineering projects across borders face the same issue. A European contractor building a US project must convert every drawing, every specification, every material order. Errors are common: a beam specified as 10 mm thick is manufactured as 10 inches thick, a fastener specified in metric is supplied in imperial, a floor area calculated in square meters is reported as square feet. The cost of these errors, in rework and delays, runs into billions of dollars per year globally.
Living and working across both
If you work in an international context, you will need to be fluent in both systems. The practical approach is to use the metric system as your primary, and convert to imperial only when communicating with someone who insists on it. Keep a conversion app or a printed conversion card handy. Memorize the rough conversions that come up most often: one inch is about 2.5 centimeters, one foot is about 30 centimeters, one meter is about 3.3 feet, one mile is about 1.6 kilometers, one kilogram is about 2.2 pounds, one liter is about a quarter of a gallon. These approximations are good enough for everyday use; use exact factors for any calculation that matters.
When writing for an international audience, give measurements in both systems. "The pool is 50 meters (164 feet) long" serves readers in both worlds. Use the primary system first and the conversion in parentheses, so that readers in the primary system can skip the conversion. Be consistent within a document: do not switch systems mid-paragraph without a clear reason. And always label the units: 150 is meaningless without pounds or kilograms, and the difference can be a matter of life and death.
For Americans learning the metric system, the fastest path is to live with it for a week. Set your phone's weather app to Celsius. Buy a kitchen scale that reads in grams. Measure your height in centimeters. Drive with your speedometer showing kilometers per hour (most US cars can switch). After a week of immersion, the units start to feel intuitive: 20°C is room temperature, 25°C is warm, 30°C is hot. A kilometer is a bit more than half a mile, so a 5K race is about 3.1 miles. A liter is a bit more than a quart, so a 2-liter bottle is about half a gallon. The system is not hard to learn; it just takes practice.