Metrication in the United States is the gradual and still incomplete adoption of the International System of Units (SI). The metric system has been legal in trade since the Metric Act of 1866, the United States was an original signatory of the Metre Convention in 1875, and since the Mendenhall Order of 1893 customary units have been defined in terms of metric standards. Since 2023, the foot has had a single definition of exactly 0.3048 meters, following the retirement of the U.S. survey foot.
Federal policy has favored conversion since the Metric Conversion Act of 1975, which created the United States Metric Board to coordinate a voluntary changeover. Public resistance and the Board's lack of authority stalled the effort, and it was abolished in 1982. The Omnibus Trade and Competitiveness Act of 1988 made the metric system the "preferred system of weights and measures for United States trade and commerce", and Executive Order 12770 (1991) directed federal agencies to use it in procurement, grants and other business activities where economically feasible. Conversion has never been made mandatory, and there is strong public resistance to going further.
Use today splits by sector. Science, medicine, the military, pharmaceuticals, electronics and vehicle manufacturing are largely metric, and most packaged consumer goods must carry dual labeling. Customary units still dominate everyday life, including road signs, weather reports, cooking, construction and body measurements.
History
18th century
Immediately after gaining independence from Great Britain, the United States used a variety of units of measure, including Dutch units and English units. The 1789 Constitution grants Congress the authority to determine standards of measure, though Congress did not immediately use this authority to impose a uniform system. The United States was one of the first nations to adopt a decimal currency, under the Coinage Act of 1792.
In 1793, Thomas Jefferson requested equipment from France that could be used to evaluate the metric system. Joseph Dombey was sent with a standard kilogram, but his ship was blown off course by a storm and captured by British privateers in the Caribbean, and he died in captivity on Montserrat.
19th century
In the early 19th century, the U.S. Coast and Geodetic Survey, the government's surveying and map-making agency, used a physical meter standard, the "Committee Meter", brought from Switzerland.
In 1832, the Treasury Department adopted standards for the main customary units, including the yard, the avoirdupois pound, the 231-cubic-inch gallon and the Winchester bushel, based on work by Ferdinand Rudolph Hassler.
Shortly after the American Civil War, the 39th United States Congress protected the use of the metric system in commerce with the Metric Act of 1866 and supplied each state with a set of standard metric weights and measures.
In 1875, the United States became one of the original seventeen signatory nations to the Metre Convention, also known as the Treaty of the Metre. The treaty concluded five years of meetings in which the metric system was reformulated and its standards refined. It established the International Bureau of Weights and Measures (Bureau international des poids et mesures, BIPM) in Sèvres, France, to provide standards of measurement for worldwide use.
On July 4, 1876, Melvil Dewey (known for his Dewey Decimal Classification) incorporated the American Metric Bureau in Boston to sell rulers and other metric measuring tools. Dewey had hoped to make his fortune selling metric supplies.
Under the Mendenhall Order of 1893, metric standards developed through the BIPM were officially adopted as the fundamental standards for length and mass in the United States, though some metric standards were already in use. The foot, pound and other customary units have been defined in terms of metric units ever since.
The 1895 Constitution of Utah, in Article X, Section 11, required that "The Metric System shall be taught in the public schools of the State." This section was repealed, effective July 1, 1987.See also, Senate Joint Resolution No. 1, Laws 1986 (2nd S.S.), and approved at the general election on November 4, 1986, to become effective on July 1, 1987.
20th century
In 1959, the United States agreed with other English-speaking countries on the international yard and pound, fixing the yard at exactly 0.9144 meters and the pound at exactly 0.45359237 kilograms. The same notice named the older, slightly longer foot the "U.S. survey foot" and restricted it to geodetic surveying, with the intention that it would eventually be replaced.
The General Conference on Weights and Measures is the governing body for the modern metric system and comprises the signing nations of the Treaty of the Metre. In 1960 it approved an updated version of the metric system named (International System of Units), abbreviated SI.
On February 10, 1964, the National Bureau of Standards (now the National Institute of Standards and Technology) issued a statement that it would use the metric system, except where this would have an obvious detrimental effect.
In 1968, Congress authorized the U.S. Metric Study, a three-year study of systems of measurement in the United States, with emphasis on the feasibility of metrication. The United States Department of Commerce conducted the study. A 45-member advisory panel consulted and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials. The final report concluded that the U.S. would eventually join the rest of the world in using the metric system. It found that metric units were already used in many areas and that their use was increasing. Most participants believed conversion was in the country's best interests, particularly given the importance of foreign trade and the growing influence of technology.
The study recommended a carefully planned transition to predominantly metric use over a decade. Congress passed the Metric Conversion Act of 1975 "to coordinate and plan the increasing use of the metric system in the United States". Conversion was voluntary, and the United States Metric Board (USMB) was established for planning, coordination, and public education. The education effort raised awareness, but the public response included resistance, apathy, and confusion. Federal agencies also promoted the change directly to the public; the U.S. Office of Education, for example, produced a television public service announcement on metric education in 1978. In 1981, the USMB reported to Congress that it lacked the clear mandate needed to bring about national conversion. Because of this, and because the Reagan administration was looking to cut federal spending (White House advisor Lyn Nofziger was particularly active against the Board), the USMB was disbanded in the autumn of 1982.
The end of the USMB increased doubts that metrication would really happen. Public and private sector conversion slowed even as international competition and the demands of global markets increased.
The American National Metric Council (ANMC) was established in 1973 by the American National Standards Institute (ANSI) as a non-profit, tax-exempt organization for planning and coordinating metric activity in all sectors of the U.S. economy. It became a separately incorporated organization in 1976. The ANMC aimed to keep conversion voluntary and orderly, minimizing costs and maximizing benefits, and provided information, forums and assistance to its subscribers.
Unlike the USMB, which only implemented the policy set out in the Metric Conversion Act, the ANMC tried to coordinate metric planning across industrial sectors. Its committees submitted conversion plans for the chemical and instruments sectors, which the USMB later approved. From 1975 through 1987, the ANMC held a series of well-attended annual conferences. National Metric Conferences, jointly sponsored by the ANMC, the U.S. Metric Association (USMA), the Department of Commerce, and NIST, followed from 1989 through 1993.
National Metric Week, begun by the National Council of Teachers of Mathematics, was first held in 1976 during the week beginning on May 10. It continued to be held in May until 1983, when it was moved to the week of October 10 because the original date was too close to the end of the school year.
Congress gave new encouragement to industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This amended the Metric Conversion Act of 1975 and designated the metric system as "the preferred system of weights and measures for United States trade and commerce". The legislation states that the federal government has a responsibility to assist industry, especially small business, as it voluntarily converts, and it required most federal agencies to use the metric system in their procurement, grants, and other business-related activities by the end of 1992.
Executive Order 12770, signed by President George H. W. Bush on July 25, 1991, directed executive branch departments and agencies to "take all appropriate measures within their authority" to use the metric system in procurement, grants, and other business-related activities, to the extent economically feasible, by September 30, 1992. Metric use was not required where it was impractical or likely to cause significant inefficiencies or loss of markets to U.S. firms. The order made the Secretary of Commerce responsible for coordinating implementation across the government and authorized an Interagency Council on Metric Policy.
Some members of Congress attempted to ban use of the metric system on federal highways in 1992 and 1993. These bills were unpopular in the House of Representatives and failed before a vote.
While not mandating metric use in the private sector, the federal government has sought to act as a catalyst for conversion in trade, industry, and commerce. Exceptions were made for the highway and construction industries. The Department of Transportation planned to require metric units by 2000, but this plan was canceled by the Transportation Equity Act for the 21st Century (TEA-21) of 1998. By then, 43 state transportation departments had begun converting to metric, and in 1999 the Federal Highway Administration reported that most had substantially converted their design and construction work, warning that "reversion to inch-pound units by some States will perpetuate a confusing mix of measurement systems". Many states nevertheless reverted over the following decade. The California Department of Transportation, which had adopted metric units as its preferred system in 1993, released its 2006 standards in customary units and required customary units for nearly all projects submitted after June 30, 2007. The Florida Department of Transportation stopped using its metric design standards for projects let from July 2006.
The use of two unit systems caused the loss of the Mars Climate Orbiter on September 23, 1999. NASA specified metric units, but ground software supplied by Lockheed Martin, the spacecraft's prime contractor, reported thruster impulse in pound-force seconds rather than newton-seconds. Because one pound-force second is about 4.45 newton-seconds, the navigation team's models understated the effect of each thruster firing by that factor. The spacecraft was intended to reach an altitude of about above Mars, but instead descended to about and is presumed to have been destroyed in the Martian atmosphere.
21st century