It is often said that the United States “never adopted” the metric system. The truth is stranger: metric has been legal in the US since 1866, and US units have been defined in metric terms since 1893. What the country never did was switch everyday life over.
Legal since 1866, official underneath since 1893
The Metric Act of 1866 made it lawful to use metric units in any contract or trade. In 1875 the US was one of the seventeen original signers of the Metre Convention, and in 1890 it received its own copies of the international prototype meter and kilogram.
In 1893 Thomas Mendenhall, superintendent of the US Coast and Geodetic Survey (which then kept the national standards), issued what became known as the Mendenhall Order: from then on, the US yard and pound would be defined from the metric prototypes rather than from British standards. Every foot and pound in America has been, quietly, a fraction of a meter or a kilogram ever since.
The 1959 agreement
Small differences remained between American, British and Canadian inches. In 1959 these countries agreed on the international yard and pound: one yard is exactly 0.9144 m and one pound exactly 0.45359237 kg. That is why 1 inch is exactly 2.54 cm, the number the length converter relies on.
The push that stalled
In 1975 Congress passed the Metric Conversion Act, which declared metric the preferred system but made the switch voluntary. A US Metric Board was set up to coordinate it and was abolished in 1982. Highway signs in kilometers, tried in a few places, were mostly taken down. In 1988 a trade law named metric the preferred system for federal agencies and trade, and since the 1990s most packaged consumer goods have carried both metric and US quantities, which is why a soda can says both 12 fl oz and 355 mL.
Industries that trade globally, such as science, medicine, the military and car manufacturing, largely work in metric. Daily life (road signs, recipes, body weight, the weather forecast) stayed with miles, cups, pounds and degrees Fahrenheit.
When mixing units goes wrong
The most famous cost of two systems is NASA’s Mars Climate Orbiter, lost in September 1999 as it approached Mars. One team’s software reported thruster impulse in pound-force seconds while another expected newton-seconds, and the error steered the spacecraft far too close to the planet. The lesson is not that one system is better, but that every number needs its unit attached, and that conversions must be exact.
US customary is not “imperial”
US units are sometimes called imperial, but they are not the same. The British imperial system was reformed in 1824 while the US kept older English definitions, so a US gallon is about 17% smaller than an imperial gallon, and a US pint holds 16 fluid ounces instead of 20. Inches, feet, miles and pounds match, because the 1959 agreement unified them. See why the gallons differ, or convert between them with the weight and volume converters.
Try it
- Length & distance converter — mm to miles, drawn on a dual ruler
- Temperature converter — °C, °F and K on one thermometer
- Weight & mass converter — grams to tons, on a balance