History
The U.S. system of units is similar to the British imperial system. Both systems derive from the evolution of local units over the centuries, as a result of standardization efforts in the United Kingdom; the local units themselves mostly trace back to Roman and Anglo-Saxon units. Today, these units are defined in terms of SI units.
In the Omnibus Trade and Competitiveness Act of 1988, the United States government designated the metric system of measurement as "the preferred system of weights and measures for U.S. trade and commerce". The legislation states that the federal government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement. This process of legislation and conversion is known as metrication, and in the U.S. is most evident in labeling requirements on food products, where SI units are almost always presented alongside customary units.
However, metrication in the United States has been less forcefully imposed than in other countries, and has encountered more resistance from industrial and consumer market forces. So customary units are still widely used on consumer products and in industrial manufacturing; only in military, medical, and scientific contexts are SI units generally the norm.
The customary system was championed by the United States-based International Institute for Preserving and Perfecting Weights and Measures in the late 19th century. Advocates of the customary system saw the French Revolutionary, or metric, system as atheistic. An auxiliary of the Institute in Ohio published a poem with wording such as "down with every "metric" scheme" and "A perfect inch, a perfect pint". One adherent of the customary system called it "a just weight and a just measure, which alone are acceptable to the Lord."
There are anecdotal objections to the use of metric units in carpentry and the building trades, on the basis that it is easier to remember an integer number of inches plus a fraction than a measurement in millimeters, or that inch measurements are more suitable when distances are frequently divided by two.
Other countries had (or still have, unofficially) customary units of their own, sometimes very similar in name and measure to U.S. customary units, since they often share the same Germanic or Roman origins. Frequently, however, these units designate quite different sizes. For example, the mile ranged by country from one half to five U.S. miles; even foot and pound had varying definitions. Until the twentieth century the customary units of measure in the United States were sometimes just as variable. Historically, a wide range of non-SI units were used in the United States and in Britain, but many have fallen into disuse. This article deals only with the units commonly used or officially defined in the United States.
Read more about this topic: United States Customary Units
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—Attributed to Georges Clemenceau (1841–1929)
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—Ralph Waldo Emerson (1803–1882)