Americans have long used diverse weights and measures, reflecting their diverse origins. From colonial times, converting from one unit to another was part of courses in commercial arithmetic. The United States Constitution explicitly granted authority to establish uniform national weights and measures to the federal government. By the 1830s, standards based on British units of length, area, and volume were distributed to the states, and from there to local authorities.
In the meantime, a new system of weights and measures, developed by French scientists following the Revolution of 1789, gained increasing international use. This metric system had as its fundamental unit of length the meter, defined to be a small fraction of the length of a meridian of the earth. Other units of length were decimal multiples or fractions of the meter. Decimal relations also applied to units of area and volume. Units of mass were masses of a given volumes of water. Temperatures were also divided in decimal units, with 0 as the freezing point of water and 100 as the boiling point.
In 1866, the Congress and President approved legislation making the metric system legal but not mandatory in the United States. New metric standards were prepared for the states. Educators developed apparatus for teaching the system. It was widely adopted by scientists and doctors in their work. Some strongly opposed metric units, and many were content to use familiar measures.
Meanwhile, most of the rest of the world went metric. After Great Britain voted to adopt the metric system in 1965, there was growing pressure to make it mandatory in the United States. The Metric Conversion Act of 1975 encouraged adoption of the system. New teaching devices informed both adults and children about it.
Attempts to make metric units mandatory nationwide were reversed in the early 1980s. However, teaching the system continued. Most immigrants now coming to the United States have grown up with the metric system, creating further incentives for its use.