United States customary units

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A table of weights from the secretaries of the different states, showing the number of pounds which their laws recognize as a bushel of different commodities. c. 1854

The U.S. Standard Meter, in use until 1960: many U.S. customary units are defined in terms of the SI meter.

The United States customary system of units of measurement (also called the English, Imperial or American) is the most commonly used system of measurement in the United States. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities, although there is increasing use of the International System of Units (SI, usually referred to as "metric" in the U.S.) in science, medicine, government, and many sectors of industry.^[1] 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).^[2] These definitions were refined in 1959.^[3]

The U.S. customary units show many similarities to the Imperial units which were used in the British Empire, and the two systems share common roots. However the Imperial system was not standardized until 1824: the U.S. customary system developed from earlier English units, and there are several differences from the Imperial system.

[edit] History

The U.S. system of units is similar to the Imperial system (in reference to the British Empire^[4]) which was used officially in the United Kingdom until 1995^[5] (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,^[6] 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,^[7] or that inch measurements are more suitable when distances are frequently divided by two.^[6]

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. Within the United States—at least through the 19th century—the customary units of measure were sometimes just as variable. 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.

[edit] Units of length

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.^[3] 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.^[3] 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.^[9]

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.^[9]

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.^[9]

[edit] 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.^[8] Conversion factors are based on Astin (July 27, 1968)^[10] and National Institute of Standards and Technology (2008).^[11]

[edit] Units of capacity and volume

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.

[edit] Fluid volume

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.

[edit] Dry volume

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.^[13][14] However pecks, or bushels are sometimes used—particularly for grapes, apples and similar fruits in agricultural regions.

[edit] Units of mass

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.

[edit] Grain measures

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

[edit] Cooking measures

The most common cooking measures in the United States include the teaspoon, tablespoon and cup, along with halves, thirds, quarters and eighths of these. These are defined as volumes of fluid, although they are used as measures of dry ingredients also. Pounds, ounces and fluid ounces are used in recipes occasionally. Common package sizes are also used, such as can (presumed size varies depending on product), jar, square (e.g. 1 oz avdp. of chocolate), or stick (e.g. 4 oz avdp. butter, which is close to 4 fluid ounces).

[edit] 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. Modern U.S. scientists mostly use the Kelvin and Celsius scales, although several technical standards are expressed in Fahrenheit temperatures and U.S. medical practitioners often use degrees Fahrenheit for body temperature.

The different temperature scales do not have a simple conversion factor as they have different zero points.

• Pure water freezes at 32 °F and boils at 212 °F at 1 atm.
• Water saturated with common salt freezes at −6.02 °F.
• Conversion formula:

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.^[15] One degree Fahrenheit is a smaller increment than a degree Celsius, which in theory lends more precision when describing meteorological conditions in whole degrees (as is most common in weather reports), but given the level of accuracy in meteorological forecasting this is probably not significant.

[edit] Other units

• 1 Board-foot = 2.360 dm³
• 1 British thermal unit (Btu) ~ 1055 J
• 1 Calorie = 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 R_SI (K·m²/W)
• 1 Slug (mass) = 1 lb_f·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.

[edit] See also

• Comparison of the imperial and US customary measurement systems
• Conversion of units and approximate conversion of units
• History of measurement, systems and units of measurement
• Metric system in general and the International System of Units (SI) in particular
• Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States

[edit] References

[edit] External links

• 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