United States customary units

U.S. customary units, also known in the United States as English units or Imperial units (in reference to the British Empire) (but see English unit for British standards) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units &mdash; the modern metric system). All units are defined in terms of SI base units, but at ratios inconvenient for conversion.

History
The U.S. system of units is similar to the Imperial system 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. 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 US miles, and even foot and pound had varying definitions. Even within the USA, 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 have been used in the United States, and on the British Isles before that, but many of these have fallen into disuse. This article only deals 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.

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. However, for each of these units there exist two slightly different definitions, yielding two different systems of measure - international measure, and U.S. survey measure. The relationships between the different units within each measure is the same, but each measure has a slightly different definition in terms of metric units.

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 (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 US standard was identical to survey measure.

Units of area
The units of area in the U.S. customary system are mostly based on the units of length squared, e.g., square inch (sq in, 645.16 mm²). Since the U.S. customary system has two differing definitions of the foot (international and survey), there are also two differing definitions for the square foot.

Americans generally measure rooms and buildings in square feet; carpet in square yards; property in acres; and territory in square miles. The square rod and square chain are rarely used.

The Public Land Survey System, used to divide land in most of the United States, uses the survey mile to divide land into regular square parcels, such as "sections" and "townships". However, these terms are not used for general area measurement, but are instead used to indicate particular parcels of land in a rectangular grid.

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.

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.

Liquid volume
Most important measures shown in bold font

The gill—pronounced —is basically a paper unit which is not actually used in the United States any more. Minims and drams are also rarely used. Single servings of beverages are usually measured in fluid ounces. Milk is usually sold in cups or half pints (8 fluid ounces), pints, quarts and gallons. Water volume for sinks, bathtubs, ponds, swimming pools, etc., is usually described in gallons. The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water, but in the US it is defined as the 128th part of a US gallon, so a fluid ounce of water weighs about 1.043 ounces. The saying "a pint's a pound the world around" refers to 16 fluid ounces being one pint in the United States, but ironically it is only approximately true in the US. In the rest of the English-speaking world 20 fluid ounces make a pint, so it is said that "A pint of water weighs a pound and a quarter".

Dry volume
Small fruits, berries, and vegetables are often sold in dry pints and dry quarts. Most Americans do not have experience with dry gallons, however pecks, or bushels are sometimes used particularly for apples and similar fruits in agricultural regions.

Units of mass
There have historically been four different English systems of mass: Tower weight, Troy weight, Avoirdupois system, and Apothecaries' system. Tower weight fell out of use (due to legal prohibition) centuries ago, and was never used in the United States. Troy weight is still used to weigh precious metals. Apothecaries weight, once used in pharmacy, has been largely replaced by metric measurements. Avoirdupois mass, being directly related to the kilogram, is used to define mass in the U.S. customary system.

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 realm of physics, the term "pound" means "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.

Avoirdupois weight
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.

Troy weight
The grain has the same definition as for Avoirdupois weight.

Cooking measures
The most common cooking weights and measures in the United States are as follows: The SI equivalents have been rounded to reasonable values in the table above, because cooking is not a precise science.

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

Units of temperature
Degrees Fahrenheit are used in the United States to measure temperatures in most non-scientific contexts. 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 medical practitioners often use customary units for body temperature and weight.
 * 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: $$F = \frac{9}{5}C + 32$$
 * For a rough conversion formula that is easier to perform in one's head, American travelers to regions that use Celsius may prefer $$F \approx 2C + 30\,$$ which is identical to the real formula when F=50 and is accurate to within 5 degrees Fahrenheit when $$5 \le F \le 95$$.

Other units

 * British thermal unit (~ 1055 J)
 * Horsepower (~ 746 W)
 * Hand (10.16 cm)
 * Calorie (4.184 J or kJ, depending on usage)
 * Board-foot (2.360 dm³)
 * R-value (0.176 K·m²/W)