The United States customary system, or American system and sometimes "English units", of units of measurement is the most commonly used system of measurement in the United States. It is similar but not identical to the obsolescent British Imperial units. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities, although the International System of Units (SI, often referred to as "metric") is universally used in science, and increasingly in medicine, government, and many sectors of industry. The vast majority of U.S. customary units have been defined in terms of the meter and the kilogram since the Mendenhall Order of 1893 (and, in practice, for many years before that date). These definitions were refined in 1959.
The U.S. customary units have common roots with the Imperial units which were used in the British Empire and are still used, mainly informally, in Britain and other countries. Many U.S. units are virtually identical to their Imperial counterparts, but the U.S. customary system developed from English units in use before the Imperial system was standardized in 1824, and there are several numerical differences from the Imperial system.
The U.S. system of units is similar to the Imperial system (in reference to the British Empire) which was used officially in the United Kingdom until 1995 (and which still has official usage there for a few purposes, and unofficial usage for many others). Both systems derive from the evolution of local units over the centuries, as a result of standardization efforts in England; 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.
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.
Units of length
|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)||1760 yd||1.609344 km|
|1 link (li)||33⁄50 ft or 7.92 in||0.2011684 m|
|1 (survey) foot (ft)||1200⁄3937 m||0.3048006 m|
|1 rod (rd)||25 li or 16.5 ft||5.029210 m|
|1 chain (ch)||4 rd||20.11684 m|
|1 furlong (fur)||10 ch||201.1684 m|
|1 (survey or statute) mile (mi)||8 fur||1.609347 km|
|1 league (lea)||3 mi||4.828042 km|
|1 fathom (ftm)||2 yd||1.8288 m|
|1 cable (cb)||120 ftm or 1.091 fur||219.456 m|
|1 nautical mile (NM)||8.439 cb or 1.151 mi||1.852 km|
The system for measuring length in the United States customary system is based on the inch, foot, yard, and mile, which are the only four customary length measurements in everyday use. Since July 1, 1959, these have been defined on the basis of 1 yard = 0.9144 meters except for some applications in surveying. This definition was agreed with the UK and other Commonwealth countries, and so is often termed international measure.
When international measure was introduced in the English-speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is 1 foot = 1200⁄3937 meters: this definition was retained for data derived from NAD27, but renamed the U.S. survey foot to distinguish it from the international foot. For most applications, the difference between the two definitions is insignificant — one international foot is exactly 0.999998 of a U.S. survey foot, for a difference of about 1⁄8 inch (3 mm) per mile — but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.
The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters. The SPCSs were also updated, but the National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states. All SPCSs are defined in meters, but seven states also have SPCSs defined in U.S. survey feet and an eighth state in international feet: the other 42 states use only meter-based SPCSs.
State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (2 ppm) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty-four states have legislated that surveying measures should be based on the U.S. survey foot, eight have legislated that they be made on the basis of the international foot, and eighteen have not specified the conversion factor from metric units.
Units of area
The most widely used area unit with a name unrelated to any length unit is the acre. The National Institute of Standards and Technology contends that customary area units are defined in terms of the square survey foot, not the square international foot. Conversion factors are based on Astin (July 27, 1968) and National Institute of Standards and Technology (2008).
|Exact relationships shown in boldface|
|1 square survey foot (sq ft or ft2)||144 square inches||0.09290341 m2|
|1 square chain (sq ch) or (ch2)||4356 feet2 (survey) or 16 sq rods||404.6873 m2|
|1 acre||43560 sq ft (survey) or 10 sq ch||4046.873 m2|
|1 section||640 acres or 1 sq mi (survey)||2.589998 km2|
|1 survey township (twp)||36 sections or 4 sq leagues||93.23993 km2|
Units of capacity and volume
|Volume in general|
|1 cubic inch (cu in) or (in3)||16.387064 mL|
|1 cubic foot (cu ft) or (ft3)||1728 cu in||28.31685 L|
|1 cubic yard (cu yd) or (yd3)||27 cu ft||764.559 L|
|1 acre foot (acre ft)||43560 cu ft
1613.333 cu yd
The cubic inch, cubic foot and cubic yard are commonly used for measuring volume. In addition, there is one group of units for measuring volumes of liquids, and one for measuring volumes of dry material.
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.
|Most common measures shown in italic font|
Exact conversions in bold font
|1 minim (min)||~ 1 drop or 0.95 grain of water||61.61152 μL|
|1 US fluid dram (fl dr)||60 min||3.696691 mL|
|1 teaspoon (tsp)||80 min||4.928921 mL|
|1 tablespoon (Tbsp)||3 tsp or 4 fl dr||14.78676 mL|
|1 US fluid ounce (fl oz)||2 Tbsp or 1.041 oz av of water||29.57353 mL|
|1 jigger (jig)||3 Tbsp||44.36028 mL|
|1 US gill (gi)||4 fl oz||118.2941 mL|
|1 US cup (cp)||2 gi or 8 fl oz||236.5882 mL|
|1 (liquid) US pint (pt)||2 cp or 16.65 oz av of water||473.1765 mL|
|1 (liquid) US quart (qt)||2 pt||0.9463529 L|
|1 (liquid) US gallon (gal)||4 qt or 231 cu in||3.785412 L|
|1 (liquid) barrel (bbl)||31.5 gal or 1⁄2 hogshead||119.2405 L|
|1 oil barrel (bbl)||42 gal or 2⁄3 hogshead||158.9873 L|
|1 hogshead||63 gal or 8.421875 cu ft
or 524.7 Lbs of water
One fluid ounce is 1⁄16 of a U.S. pint, 1⁄32 of a U.S. quart, and 1⁄128 of a U.S. gallon. The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water, but in the U.S. it is defined as 1⁄128 of a U.S. gallon. Consequently, a fluid ounce of water weighs about 1.041 ounces avoirdupois.
The saying "a pint's a pound the world around" refers to 16 US fluid ounces of water weighing approximately one pound avoirdupois in the United States, but that is true only in the United States. In the rest of the English-speaking world, an Imperial pint—being 20 Imperial ounces of water—will weigh one and a quarter pounds. An Imperial pint is approximately 19.2 US fluid ounces. The Imperial pint is approximately 568.26 mL. The US liquid pint is approximately 473.2 mL.
There are varying standards for barrel for some specific commodities, including 31 gal for beer, 40 gal for whiskey or kerosene, and 42 gal for petroleum. The general standard for liquids is 31.5 gal or half a hogshead. The common 55 gallon size of drum for storing and transporting various products and wastes is sometimes confused with a barrel, though it is not a standard measure.
In the United States, single servings of beverages are usually measured in fluid ounces. Milk is usually sold in half pints (8 fluid ounces), pints, quarts, half gallons, and gallons. Water volume for sinks, bathtubs, ponds, swimming pools, etc., is usually stated in gallons or cubic feet. Quantities of gases are usually given in cubic feet (at one atmosphere).
Minims, drams and gill are rarely used currently.
|1 (dry) pint (pt)||33.60 cu in||0.5506105 L|
|1 (dry) quart (qt)||2 pt||1.101221 L|
|1 (dry) gallon (gal)||4 qt or 268.8025 cu in||4.404884 L|
|1 peck (pk)||2 gal||8.809768 L|
|1 bushel (bu)||4 pk or 1.244 cu ft||35.23907 L|
|1 (dry) barrel (bbl)||7056 cu in or 3.281 bu||115.6271 L|
Small fruits and vegetables are often sold in dry pints and dry quarts. The U.S. dry gallon is less commonly used, and was not included in the handbook which many states recognize as the authority on measurement law. However pecks, or bushels are sometimes used—particularly for grapes, apples and similar fruits in agricultural regions.
Units of mass
|Most common measures shown in italic font|
Exact conversions shown in bold font
|1 grain (gr)||1⁄7000 lb||64.79891 mg|
|1 dram (dr)||2711⁄32 gr||1.771845 g|
|1 ounce (oz)||16 dr||28.34952 g|
|1 pound (lb)||16 oz||453.59237 g|
|1 US hundredweight (cwt)||100 lb||45.359237 kg|
|1 (short) ton||20 cwt||907.18474 kg|
|1 grain (gr)||1⁄7000 lb av or 1⁄5760 lb t||64.79891 mg|
|1 pennyweight (dwt)||24 gr or 7.777 carats||1.555174 g|
|1 troy ounce (oz t)||20 dwt||31.10348 g|
|1 troy pound (lb t)||12 oz t or 13.17 oz av||373.2417 g|
There have historically been five different English systems of mass: tower, apothecaries', troy, avoirdupois, and metric. Of these, it is the avoirdupois system which is the most common system of weights in the U.S., although Troy weight is still used to weigh precious metals. Apothecaries weight—once used by pharmacies—has been largely replaced by metric measurements. Tower weight fell out of use in England (due to legal prohibition in 1527) centuries ago, and was never used in the United States. The Imperial system, which is still used for some measures in the U.K. and commonwealth countries, is based on avoirdupois, with variations from U.S. customary units larger than a pound.
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams by agreement between the U.S., the U.K. and other English speaking countries in 1959. Other units of mass are defined in terms of it.
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 some contexts, the term "pound" is used to refer to "pound-force".
Troy weight, avoirdupois weight, and apothecaries' weight are all built from 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 apothecary 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, sometimes spelled drachm). The dram in avoirdupois weighs just under half of the dram in apothecaries'. The fluid dram unit of volume is based on the weight of 1 dram of water in the apothecaries' system.
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.
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 Imperial system is identical to that in the U.S. customary system.
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. The grain is used to describe the mass of propellant and projectiles in small arms ammunition. It was also used to measure medicine and other very small masses.
In agricultural practice, a bushel is a fixed mass of grain, nominally based on dry volume units.
- 1 bushel (maize) = 56 lb ≈ 25.401 kg
- 1 bushel (wheat) = 60 lb ≈ 27.216 kg
- Main article: Cooking weights and measures
The most common practical cooking measures for both liquid and dry ingredients in the United States (and many other countries) are the teaspoon, tablespoon and cup, along with halves, thirds, quarters and eighths of these. Pounds, ounces, fluid ounces, and common sizes are also used, such as can (presumed size varies depending on product), jar, square (e.g. 1 oz avdp. of chocolate), stick (e.g. 4 oz avdp. butter) or fruit/vegetable (e.g. a half a lemon, two medium onions).
Units of temperature
Degrees Fahrenheit are used in the United States to measure temperatures in most non-scientific contexts. The Rankine scale of absolute temperature also saw some use in thermodynamics. Scientists world-wide use the Kelvin and Celsius scales. Several technical standards are expressed in Fahrenheit temperatures and U.S. medical practitioners often use degrees Fahrenheit for body temperature.
The relationship between the different temperature scales is linear, but the scales have different zero points so that conversion is not simply multiplication by a factor: pure water is defined to freeze at 32 °F = 0 °C and boil at 212 °F = 100 °C at 1 atm; the conversion formula is easily shown to be
It has been argued that the Fahrenheit scale is better suited to describing ambient temperatures because most places where humans live fall between zero and one hundred on this scale. This gives a division of 100 degrees with no negative numbers being used in most conditions.
- 1 Board-foot = 2.360 dm³
- 1 British thermal unit (Btu) ~ 1055 J
- 1 Calorie (cal) = 4.184 J (or kJ, depending on usage)
- 1 Hand = 10.16 cm
- 1 Horsepower ~ 746 W
- 1 R-value (ft²·°F·h/Btu) ~ 0.1761 RSI (K·m²/W)
- 1 Slug (mass) = 1 lbf·s²/ft
- Various combination units are in common use, including the foot-pound and the PSI; these are straightforwardly defined based on the above basic units.
- T. C. Mendenhall, Superintendent of Standard Weights and Measures, Order of April 5, 1893, published as Appendix 6 to the Report for 1893 of the Coast and Geodetic Survey.
- Astin, A.V., Karo, H. A. and Mueller, F. H. (June 25, 1959). Doc 59-5442, "Refinement of Values for the Yard and the Pound." Federal Register. When reading the document it helps to bear in mind that 999,998 = 3937 × 254.
- "English units of measurement." (2001-2007). The Columbia Encyclopedia 6th ed.
- Units of Measurement Regulations 1995 No. 1804
- Ed Tenner, (May 2005), "The Trouble with the Meter"
- Robyn Williams (February 8, 1998) "Trouble with the Metric System". Australian Radio National, Ockham's Razor.
- Roberts, R.W. (February 3, 1975). Federal Register republished in Barbrow, L.E. and Judson, L. V. (1976) Weights and Measures of the United States. National Bureau of Standards Special Publication 447. p. 36
- Astin, A. V. (July 27, 1968). Federal Register. Republished in Barbrow, L.E and Judson, L.V. Weights and Measures of the United States: A Brief History. National Bureau of Standards Special Publication 447. pp. 34–35.
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units (SI).
- The liter should be abbreviated with a capital L per National Institute of Standards and Technology. (1995.) Guide for the Use of the International System of Units (SI). Special Publication 811. http://physics.nist.gov/Pubs/SP811/sec06.html#6.1.2
- 93rd Conference on Weights and Measures. (2009). Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices (NIST Handbook 44). National Institute of Standards and Technology.
- Summary of State Laws and Regulations in Weights and Measures. (2005) National Institute of Standards and Technology.
- Source: Appendix C, NIST Handbook 44, 2006 edition (http://ts.nist.gov/WeightsAndMeasures/upload/AppendC-06-HB44-Final.pdf Appendix C PDF]).
- Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 (46 page PDF file or table of contents).
- Jacques J. Proot's Anglo-Saxon weights & measures page.
- Rowlett's A Dictionary of Units of Measurement
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