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. Since everyday weights and measures are mostly non-SI, children in U.S. public schools are generally taught customary units before SI, although many schools are now attempting to teach SI units at an earlier age.
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 the U.S. customary units, since they often shared the same Germanic or Roman origins. Frequently, however, these units designated quite different sizes. For example, in different countries, the mile ranged from one half to five U.S. miles, and even foot and pound had varying definitions. Even within the United States, at least through the 19th century, the customary units of measure were sometimes just as variable. Eventually, most countries, including the United States, redefined their customary units in terms of SI units like kilogram and meter. Often, though, unlike English units, they were rounded to "nice" SI values, leading to their use in colloquial speech, alongside SI terms, into the present day.
Historically, a wide range of non-SI units has been used in the United States, and on the British Isles before that, but many of these have fallen into disuse. This article deals only with the units commonly used or officially defined in the United States.
As of 2007, the U.S. customary units remain the last widely used official non-metric system of measurement as besides the United States only Liberia and Myanmar have not adopted the International System of Units as their primary or sole system of measurement.
|Exact relationships shown in boldface|
|1 inch (in)||25.4 mm|
|1 foot (ft)||12 in||0.3048 m|
|1 yard (yd)||3 ft||0.9144 m|
|1 mile (mi)||5,280 ft||1.609344 km|
|1 link (li)||ft||0.201168 m|
|1 foot (survey)||m||0.3048006 m|
|1 rod (rd)||25 li||5.029210 m|
|1 chain (ch)||4 rd||20.1168 m|
|1 mile (survey)||80 ch||1.609347 km|
One inch international measure is exactly 25.4 millimeters, while one inch U.S. survey measure is defined so that 39.37 inches is exactly 1 meter. For most applications, the difference is insignificant. (One international inch is exactly 0.999998 of a U.S. survey inch, for a difference of about 3 millimeters per mile). International measure is used for everyday use, engineering, and commerce in the United States, while survey measure is used only for surveying.
International measure, agreed in 1959, uses the same definition of the units involved as is used in the UK and other Commonwealth countries. Before that date, those other countries still used separate standards. U.S. survey measure uses an older definition of the units (specified by the Mendenhall Order in 1893) which the United States used prior to adopting international measure. Previous to this agreement, the U.S. standard was identical to survey measure.
|Volume in general|
|1 cubic inch (cu in)||16.387 064 mL|
|1 cubic foot (cu ft)||1 728 cu in||28.316 846 592 L|
|1 cubic yard (cu yd)||27 cu ft||764.554 857 984 L|
0.764 554 857 984 m³
Other than the cubic foot, cubic inch and cubic yard, these units are differently sized from the units in the Imperial system, although the names of the units are similar. Also, while the U.S. has separate systems for measuring the volumes of liquids and dry material, the Imperial system has one set of units for both.
Technically speaking, since these units are defined in terms of the inch, it would make a difference whether international or survey measure was used. However, in practice, the difference between the two definitions would be imperceptible, and in any case in defining volumes international measure is used.
|1 minim (min)||0.061 611 52 mL|
|1 fluid dram (fl dr)||60 min||3.696 691 mL|
|1 fluid ounce (fl oz)||8 fl dr||29.573 53 mL|
|1 gill (gi)||4 fl oz||118.294 1 mL|
|1 cup (cp) or (c)||2 gi or 8 fl oz||236.588 2 mL|
|1 (liquid) pint (pt)||2 cups or 4 gi or 16 fl oz||473.176 5 mL|
|1 (liquid) quart (qt)||2 pt or 32 fl oz||946.352 9 mL|
|1 (liquid) US gallon (US gal)||4 qt or 128 fl oz or 231 cu in||3.785 412 L|
|1 beer barrel (bbl)||31 gal||117.347 8 L|
|1 oil barrel (bbl)||42 gal||158.987 3 L|
|1 hogshead||63 gal||238.481 0 L|
|1 (dry) pint (pt)||33.6003125 cu in (exactly)||550.610 5 mL|
|1 (dry) quart (qt)||2 pt||1.101 221 L|
|1 (dry) gallon||4 qt||4.404 884 L|
|1 peck (pk)||2 (dry) gallons||8.809 768 L|
|1 bushel (bu)||4 pk||35.239 07 L|
The Avoirdupois pound is legally defined as a measure of mass, but the name pound is also applied to measures of force. For instance, in many contexts, the pound avoirdupois is used as a unit of mass, but in the some contexts, the term "pound" is almost exclusively used to refer to "pound-force".
Troy weight, avoirdupois weight, and apothecaries weight are all defined in terms of the same basic unit, the grain, which is the same in all three systems. However, while each system has some overlap in the names of their units of measure (all have ounces and pounds), the relationship between the grain and these other units within each system varies. For example, in apothecaries and troy weight, the pound and ounce are the same, but are different from the pound and ounce in avoirdupois, in terms of their relationships to grains and to each other. The systems also have different units between the grain and ounce (apothecaries has scruple and dram, troy has pennyweight, and avoirdupois has just dram). The dram was once known as the drachm.
To alleviate confusion, it is typical when publishing non-avoirdupois weights to mention the name of the system along with the unit. Precious metals, for example, are often weighed in "troy ounces", because just "ounce" would be more likely to be assumed to mean an ounce avoirdupois.
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams. All the other units of mass are defined in terms of it.
For the pound and smaller units, the U.S. customary system and the British Imperial system are identical. However, they differ when dealing with units larger than the pound. The definition of the pound avoirdupois in the British Imperial system is identical to that in the U.S. customary system.
|1 grain (gr)||lb||64.798 91 mg|
|1 dram (dr)||gr||1.771 845 195 312 5 g|
|1 ounce (oz)||16 dr||28.349 523 125 g|
|1 pound (lb)||16 oz||453.592 37 g|
|1 hundredweight (cwt)||100 lb||45.359 237 kg|
|1 (short) ton||20 cwt||907.184 74 kg|
In the United States, only the ounce, pound and short ton — known in the country simply as the ton — are commonly used, though the hundredweight is still used in agriculture and shipping.
|1 pennyweight (dwt)||24 gr||1.555 173 84 g|
|1 troy ounce (oz t)||20 dwt||31.103 476 8 g|
|1 troy pound (lb t)||12 oz t||373.241 721 6 g|
|1 teaspoon (tsp)||5 mL|
|1 tablespoon (tbsp)||3 tsp||15 mL|
|1 cup||16 tbsp||240 mL|
|1 stick||4 oz||115 g|
For scientific matters the Rankine scale, an absolute temperature scale based upon Fahrenheit, saw some use. Modern scientists almost always use the Kelvin and Celsius scales, though U.S. medical practitioners often use the U.S. customary units for body temperature and weight.