United States customary units

United States customary units
Name	United States customary units
Caption	Common units of the system
Standard	Customary law
Units	Inch, Foot, Yard, Mile, Pound, Ounce, Gallon, Bushel

United States customary units. The system of weights and measures traditionally used in the United States and its territories. It evolved from the English units brought by British colonists and was standardized after the American Revolution. While largely superseded by the International System of Units in science, medicine, and many sectors of Commerce, it remains the primary system for everyday use in the United States.

History

The system's origins lie in the English units used throughout the British Empire prior to American independence. Early standardization efforts in the Thirteen Colonies were local and inconsistent. Following the American Revolution, figures like Secretary of State Thomas Jefferson proposed decimal-based measurement reform, but these plans were not adopted. The Constitution of the United States granted Congress the power to "fix the Standard of Weights and Measures," leading to the foundational Mendenhall Order of 1893, which defined customary units in terms of the metric system. Further refinement came with the International Yard and Pound Agreement of 1959, which aligned U.S. and Commonwealth definitions. Key regulatory bodies include the National Institute of Standards and Technology and its predecessor, the National Bureau of Standards.

Units of length

The primary units for measuring distance are the inch, foot, yard, and mile. Twelve inches equal one foot, three feet equal one yard, and 5,280 feet equal one statute mile. Surveyors historically used the U.S. survey foot and U.S. survey mile, which had slightly different definitions until being officially deprecated in favor of the international foot in 2022. Nautical measurements employ the nautical mile, linked to the latitude of one minute of arc. Smaller measurements, such as the thou (one-thousandth of an inch), are used in fields like machining and engineering.

Units of mass and weight

The common unit for mass is the avoirdupois pound, defined as exactly 0.45359237 kilograms. Sixteen ounces make one pound, and 2,000 pounds constitute a short ton. The troy ounce and troy pound are used for precious metals like gold and silver. In apothecaries' measure, used historically in pharmacy, units include the grain, scruple, and dram. The grain is the same in both avoirdupois and troy systems.

Units of capacity and volume

These units differ for liquids and dry goods. The common liquid measures are the fluid ounce, cup, pint, quart, and gallon. The U.S. liquid gallon is defined as 231 cubic inches. For dry volumes, units like the dry pint, dry quart, peck, and bushel are used, with the bushel defined as 2,150.42 cubic inches. The barrel is a standard unit of volume in industries like petroleum, though its exact size varies by context, such as for beer or crude oil.

Units of area

Area is typically expressed as the square of linear units, such as square inch, square foot, square yard, and acre. One acre is defined as 43,560 square feet. Larger land areas in the Public Land Survey System are measured in townships and sections. The rood is an older unit equal to one-quarter of an acre. Surveying and real estate transactions, particularly for agricultural land, heavily rely on the acre.

Relationship to the metric system

Since the Mendenhall Order, all U.S. customary units have been legally defined by metric standards maintained by the National Institute of Standards and Technology. The Metric Conversion Act of 1975 declared the International System of Units as the preferred system for U.S. trade and commerce, but did not mandate its exclusive use. Industries like automotive, aerospace, science, and the military use metric units extensively. However, customary units dominate everyday contexts such as road signs, weather reports, cooking, and real estate. This dual-system usage often requires conversions, governed by standards from organizations like ASTM International and the Society of Automotive Engineers.

Category:Systems of units Category:Customary units of measurement Category:United States culture