United States customary units

United States customary units are a system of measurement used across the United States and most of its territories, having been standardized and adopted in 1832. This system grew out of English units that were employed throughout the British Empire before American independence. After the U.S. broke away, the United Kingdom revised its own system in 1824 and officially adopted the imperial system in 1826, which altered the definitions of some units. As a result, while many U.S. units closely resemble their imperial equivalents, clear differences exist between the two.

Most U.S. customary units were redefined in relation to the meter and kilogram through the Mendenhall Order of 1893, and in practice, this had been happening for many years prior. These definitions were later refined by the international yard and pound agreement of 1959. In commercial activities, as well as for personal and social use, the U.S. relies on customary units. However, metric units are standard in science, medicine, many industrial sectors, and some government and military contexts. The International System of Units (SI), the modern metric system, is favored for many applications by the U.S. National Institute of Standards and Technology (NIST). For newer measurements lacking a traditional customary unit, international units are used, sometimes mixed with customary ones—for instance, electrical resistivity of wire expressed in ohms (SI) per thousand feet.

The 1832 customary system is based on the units used in the United Kingdom before the British imperial system took effect on January 1, 1826. Both systems trace back to English units, an older system that evolved over millennia before American independence, with roots in Roman and Anglo-Saxon units. In the late 19th century, the U.S.-based International Institute for Preserving and Perfecting Weights and Measures championed the customary system. Some of its advocates viewed the French Revolutionary, or metric, system as atheistic. The president of an Ohio auxiliary of the Institute wrote that traditional units were "a just weight and a just measure, which alone are acceptable to the Lord." His organization even published music for a song proclaiming "down with every 'metric' scheme."

The U.S. government passed the Metric Conversion Act of 1975, designating the metric system as "the preferred system of weights and measures for U.S. trade and commerce." This law states that the federal government must assist industry as it voluntarily converts to the metric system, a process called metrication. This is most visible in U.S. food labeling requirements, where SI units almost always appear alongside customary units. According to the CIA World Factbook, the United States is one of three nations—along with Liberia and Myanmar (Burma)—that have not adopted the metric system as their official system of weights and measures. In July 1991, President George H. W. Bush signed Executive Order 12770, which cited the Metric Conversion Act and directed executive branch departments and agencies to "take all appropriate measures within their authority" to use the metric system "as the preferred system of weights and measures for United States trade and commerce." It also authorized the Secretary of Commerce to charter an Interagency Council on Metric Policy (ICMP) to help coordinate government-wide implementation. However, metrication in the U.S. has remained limited.

U.S. customary units are widely used on consumer products and in industrial manufacturing. Metric units are standard in science, medicine, engineering, and many sectors of industry and government, including the military. Some anecdotal objections to metric units arise in carpentry and the building trades, where it is argued that remembering an integer number of inches plus a fraction is easier than a measurement in millimeters, or that foot-inch measurements are more suitable when distances are frequently divided into halves, thirds, and quarters, often in parallel. Tenner argues that the metric system lacks a colloquial equivalent to the foot. The term "United States customary units" was used by the former United States National Bureau of Standards, though "English units" is sometimes used in colloquial speech.

For measuring length, the U.S. customary system uses the inch, foot, yard, and mile—the only four customary length measurements in everyday use. From 1893, the foot was legally defined as exactly 1200⁄3937 m, approximately 0.3048006 m. Since July 1, 1959, length units have been defined based on 1 yd = 0.9144 m, making the foot exactly 0.3048 m. The U.S., the United Kingdom, and other Commonwealth countries agreed on this definition through the International Yard and Pound Agreement of 1958. At that time, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), constructed by triangulation using the foot definition from the Mendenhall Order of 1893 (1 ft = 1200⁄3937 m). This definition was retained for data derived from NAD27 but renamed the US survey foot to distinguish it from the international foot. For most applications, the difference between the two definitions is negligible—one international foot is exactly 0.999998 of a US survey foot, a difference of about 1⁄8 in (3 mm) per mile—but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can span hundreds of miles. The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters.