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Gallon - A gallon is a measure of volume of approximately four litres. Historically it has had many different definitions, but there are three definitions in current use. These are the U.S. liquid gallon (approx. 3.8 litres) and the U.S. dry gallon (approx. 4.4 L) which are in use in the United States, and the Imperial (UK) gallon (approx. 4.5 L) which is in unofficial use within the United Kingdom. One or other version is sometimes found in other English-speaking countries.

The U.S. liquid gallon is legally defined as 231 cubic inches, and is equal to exactly 3.785 411 784 litres (1 L = 10-3 m3) or 0.133 680 555... cubic feet. This is the most common definition of a gallon in the United States. The U.S. fluid ounce is defined as 1/128 of a U.S. gallon. The U.S. dry gallon is one-eighth of a U.S. Winchester bushel of 2150.42 cubic inches, thus it is equal to exactly 268.8025 cubic inches or 4.404 883 770 86 litres. The U.S. dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck. The Imperial (UK) gallon was legally defined as 4.546 09 litres. This definition is used in some Commonwealth countries and Ireland, and is based on the volume of 10 pounds of water at 62 °F. (A U.S. liquid gallon of water weighs about 8.33 pounds at the same temperature.) The Imperial fluid ounce is defined as 1/160 of an Imperial gallon. As of 1 January 2000 it ceased to be legal within the United Kingdom for economic, health, safety or administrative purposes.

The corn or dry gallon was used in the United States until recently for grain and other dry commodities. It is one eighth of the (Winchester) bushel, originally a cylindrical measure of 18.5 inches in diameter and 8 inches depth. That made the dry gallon 268.80252 cubic inches. The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 cubic inches exactly, making its gallon 268.8025 cubic inches exactly (4.40488377086 L). In previous centuries there had been a corn gallon of around 271 to 272 cubic inches.

The wine, fluid, or liquid gallon is the standard U.S. gallon since the early 19th century. The wine gallon, which some sources relate to the volume occupied by eight medieval merchant pounds of wine, was at one time defined as the volume of a cylinder six inches deep and seven inches in diameter, i.e. 230.90706 cubic inches. It had been redefined during the reign of Queen Anne, in 1706, as 231 cubic inches exactly (3 x 7 x 11 in), which is the result of the earlier definition with pi approximated to 22/7. Although the wine gallon had been used for centuries for import duty purposes there was no legal standard of it in the Exchequer and a smaller gallon (224 cu in) was actually in use, so this statute became necessary. It remains the U.S. definition today.

The original ratio between corn and wine gallon is 1369:1176, but 268.8:231 is exactly 64:55 or ca. 13:11. This approximation is still applicable, although the ratio of 1.164115646 slightly changed to 1.163647186 with current definitions (268.8025:231 = 107521:92400, approx. 1351:1161). In some contexts it is or was necessary to disambiguate between those two U.S. gallons, so "liquid" or "fluid" and "dry" respectively are then added to the name.

In 1824, Britain adopted a close approximation to the ale gallon known as the Imperial gallon and abolished all other gallons in favour of it. Inspired by the kilogram-litre relationship, the Imperial gallon was based on the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury and at a temperature of 62 °F. In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density 0.998859 grams per millilitre weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL. This works out at approximately 4.5460903 L (277.4416 cu in). The metric definition of exactly 4.54609 cubic decimetres (also 4.54609 L after the litre was redefined in 1964, ca. 277.419433 cu in) was adopted shortly afterwards in Canada; for several years, the conventional value of 4.546092 L was used in the United Kingdom, until the Canadian convention was adopted in 1985.

Before and into the 19th century there were also several other gallons in use, with varying definitions. These are summarized in the table below. During some eras, the gallon was based on an exact conversion with a linear measure cubed. Other eras, the gallon was based on a rational approximation to the volume of a cylinder that could be used as a standard container, such as a basket, barrel, or jar. Other definitions were based on the density of a commodity, occasionally water, but more often a more marketable good such as wine or oats. Given these options and the variety of cultures that have used the gallon, it is not surprising that the exact value has drifted over the centuries.

Liter - The litre or liter (see spelling differences) is a unit of volume. There are two official symbols: the Latin letter L in lower and upper case (l and L). The lower case L is also often written as a cursive l, though this symbol has no official approval by any international bureau. Although the litre is not an SI unit, it is accepted for use with the SI, and has appeared in several versions of the metric system. The official SI unit of volume is the cubic metre (m3). One litre is equal to 0.001 cubic metre and is denoted as 1 cubic decimetre (dm3). The word litre is derived from an older French unit, the litron, whose name came from Greek via Latin. The original French metric system used the litre as a base unit. The spelling of the word used by the International Bureau of Weights and Measures is "litre". This spelling is also the usual one in English in most English speaking countries, but the usual American English spelling is "liter", which is officially endorsed by the United States.

Litres are most commonly used for items (such as fluids and berries) which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water. One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 degrees Celsius. Similarly: 1 millilitre of water has about 1 g of mass; 1,000 litres of water has about 1,000 kg of mass. This relationship is because the gram was originally defined as the mass of 1 mL of water. However, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, pressure.

In 1795, the litre was introduced in France as one of the new "Republican Measures", and defined as one cubic decimetre. In 1879, the CIPM adopted the definition of the litre, and the symbol l (lowercase letter L). In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (3.98 °C) under a pressure of 1 atm. This made the litre equal to about 1.000 028 dm3 (earlier reference works usually put it at 1.000 027 dm3). In 1964, at the 12th CGPM conference, the original definition was reverted to, and thus the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm3. In 1979, at the 16th CGPM conference, the alternative symbol L (uppercase letter L) was adopted. It also expressed a preference that in the future only one of these two symbols should be retained, but in 1990 said it was still too early to do so.

In spoken English, the abbreviation "mL" (for millilitre) is often pronounced as "mil", which is homophonous with the colloquial term "mil", which is intended to mean "one thousandth of a metre". This generally does not create confusion, because the context is usually sufficient - one being a volume, the other a linear measurement. The colloquial use of "mil" for millimetre for an ambiguous topic as in "5 mils of rain fell since 9am" may, however, be confusing. And in the United States a term of the same spelling and pronunciation means a thousandth of an inch.

The abbreviation cc (for cubic centimetre, equal to a millilitre or mL) is a unit of the cgs system, that preceded the MKS system, that later evolved into the SI system. The abbreviation cc is still commonly used in many fields including (for example) sizing for motorcycle and related sports for small combustion engine displacement; larger engines, such as automobile engines, do have their displacement measured in litres.

In European countries where the metric system was established well before the adoption of the SI standard, there is still carry-over of usage from the precursor cgs and MKS systems. In the SI system, use of prefixes for multiples of 1,000 is preferred and all other multiples discouraged. However, in countries where these other multiples were already established, their use remains common. In particular, use of the centi (10-2), deci (10-1), deca (10+1), and hecto (10+2) prefixes are still common. For example, in many European countries, the hectolitre is the typical unit for production and export volumes of beverages (milk, beer, soft drinks, etc) and for measuring the size of the catch and quotas for fishing boats; decilitres are found in cookbooks; centilitres indicate the capacity of drinking glasses and of small bottles. In colloquial Dutch in Belgium, a 'vijfentwintiger' and a 'drieendertiger' (literally 'twenty-fiver' and 'thirty-threer') are the common beer glasses, the corresponding bottles mention 25 cL or 33 cL. Bottles may also be 75 cL or half size at 37.5 cL for 'artisanal' brews or 70 cL for wines or spirits. Cans come in 25 cL, 33 cL and 50 cL aka 0.5 L. Family size bottles as for soft drinks or drinking water use the litre (0.5 L, 1 L, 1.5 L, 2 L), and so do beer barrels (50 L, or the half sized 25 L). This unit is most common for all other household size containers of liquids, from thermocans, by buckets, to bath tubs; as well as for fuel tanks and consumption for heating or by vehicles.

In countries where the metric system was adopted as the official measuring system after the SI standard was established, common usage more closely follow contemporary SI conventions. For example, in Canada where the metric system is now in widespread use, consumer beverages are labelled almost exclusively using litres and millilitres. Hectolitres sometimes appear in industry, but centilitres and decilitres are rarely, if ever, used. Larger volumes are usually given in cubic metres (equivalent to 1 kL), or thousands or millions of cubic metres. The situation is similar in Australia, although kilolitres, megalitres and gigalitres are commonly used for measuring water consumption, reservoir capacities and river flows. For larger volumes of fluids, such as annual consumption of tap water, lorry (truck) tanks, or swimming pools, the cubic metre is the general unit, as it is generally for all volumes of a non-liquid nature.

Centimeter - A centimetre (American spelling: centimeter, symbol cm) is a unit of length in the metric system, equal to one hundredth of a metre, which is the current SI base unit of length. Centi is the SI prefix for a factor of 10-2. Hence a centimetre can be written as 10 x 1003 m (engineering notation) or 1E-2 m (scientific E notation) - meaning 10 x 101 mm or 1 m/100 respectively, centimetre-gram-second system of units.

Though for many physical quantities, SI prefixes for factors of 103-like milli- and kilo- are often preferred by technicians, the centimetre remains a practical unit of length for many everyday measurements. A centimetre is approximately the width of the fingernail of an adult person.

Petroleum Barrel - The standard barrel of crude oil or other petroleum product (abbreviated bbl) is 42 US gallons (34.972 Imperial gallons or 158.987 L). This measurement originated in the early Pennsylvania oil fields, and permitted both British and American merchants to refer to the same unit, based on the old English wine measure, the tierce.

Earlier, another size of whiskey barrel was the most common size; this was the 40 US gallons (33.3 imp gal; 151.4 L) barrel for proof spirits, which was of the same volume as 5 US bushels. However, by 1866 the oil barrel was standardized at 42 US gallons.

Oil has not actually been shipped in barrels since the introduction of oil tankers, but the 42-US-gallon size is still used as a unit for measurement, pricing, and in tax and regulatory codes. Each barrel is refined into about 19.74 gallons of gasoline, the rest becoming other products such as jet fuel and heating oil, using fractional distillation. The current standard volume for barrels for chemicals and food is 55 US gallons (46 imp gal; 208 L).

Feet - A foot (plural: feet or foot; symbol or abbreviation: ft or, sometimes, ' - the prime symbol) is a non-SI unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system, but in each is around a quarter to a third of a meter. The most commonly used foot today is the international foot. There are three feet in a yard and 12 inches in a foot.

The popular belief is that the original standard was the length of a man's foot. In rural regions and without calibrated rulers, many units of measurement were in fact based on the length of some part of body of the person measuring (or for example the area that could be ploughed in a day). In that sense, the human foot was no doubt the origin of the measuring unit called a "foot" and was also for a long time the definition of its length. To prevent discord and enable trade, many towns decided on a standard length and displayed this publicly. In order to enable simultaneous use of the different units of length based on different parts of the human body and other "natural" units of length, the different units were redefined as multiples of each other, whereby their lengths no longer corresponded to the original "natural" standards. This process of national standardization began in Scotland in 1150 and in England in 1303, but many different regional standards had existed in both these countries long before.

Effective July 1, 1959 the length of the international yard in the United States and countries of the Commonwealth of Nations was defined as 0.9144 meters. Consequently, the international foot is defined to be equal to 0.3048 meters (equivalent to 304.8 millimeters). This was 2 ppm shorter than the previous U.S definition and 1.7 ppm longer than the previous British definition.

The international standard symbol for a foot is "ft" (see ISO 31-1, Annex A). In some cases, the foot is denoted by a prime, which is often approximated by an apostrophe, and the inch by a double prime; for example, 2 feet 4 inches is sometimes denoted as 2' 4"". This use can cause confusion, because the prime and double prime are also international standard symbols for arcminutes and arcseconds.

Meter - The metre or meter (from the Greek /'metron/) is a unit of proper length. It is the basic unit of length in the metric system and in the International System of Units (SI), used around the world for general and scientific purposes. Historically, the metre was defined by the French Academy of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1/10,000,000 of the distance from the equator to the north pole through Paris. In 1983, it was redefined by the International Bureau of Weights and Measures (BIPM) as the distance travelled by light in free space in 1/299,792,458 of a second. The symbol for metre is a lower case m. Decimal multiples and submultiples of the metre, such as kilometre (1000 metres) and centimetre (1/100 metre), are indicated by adding SI prefixes to metre.

Meridional definition - In the eighteenth century, there were two favoured approaches to the definition of the standard unit of length. One approach suggested defining the metre as the length of a pendulum with a half-period of one second, a 'seconds pendulum'. The other approach suggested defining the metre as one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the Equator to the North Pole. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.

In order to establish a universally accepted foundation for the definition of the metre, measurements of this meridian more accurate than those available at that time were imperative. The Bureau des Longitudes commissioned an expedition led by Delambre and Pierre Mechain, lasting from 1792 to 1799, which measured the length of the meridian between Dunkerque and Barcelona. This portion of the meridian, which also passes through Paris, was to serve as the basis for the length of the half meridian, connecting the North Pole with the Equator.

However, in 1793, France adopted as its official unit of length a metre based on provisional results from the expedition as its official unit of length. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the Earth, this length became the standard. The circumference of the Earth through the poles is therefore slightly more than forty million metres.

In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Metre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sevres, France. This new organisation would preserve the new prototype metre and kilogram standards when constructed, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conference Generale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice.

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.

Cup - The cup is a unit of measurement for volume, used in cooking to measure bulk foods, such as granulated sugar (dry measurement), and liquids (fluid measurement). It is in common use in the United States and American-influenced nations, such as Japan. This cup is hardly ever used in the United Kingdom or the rest of Europe, however an informal cup referring to the volume of an average coffee cup (and thus noticeably different from the U.S. cup) is frequently used in recipes in other countries such as Germany (and even in older Swedish recipes).

There is no internationally agreed standard definition of the cup, whose modern volume ranges between 200 and 284 millilitres. The cup sizes generally used in the many Commonwealth countries and the United States differ only by about 44 mL (1.5 fl oz), but when all ingredients in recipes are measured with the same cup, the proportions are identical.

In Europe, cooking recipes normally state any liquid volumes larger than a few tablespoons in millilitres, the scale found on most measuring cups worldwide. Non-liquid ingredients are normally weighed in grams instead, using a kitchen scale, rather than measured in cups. Some recipes in Europe use the decilitre (1 dl = 100 mL) as a cup-like measure. For example, where an American customary recipe might specify "1 cup of sugar and 2 cups of milk", a European recipe might specify "200 g sugar and 500 mL of milk" (or 1/2 litre or 5 decilitres). Conversion between the two measures must take into account the density of the ingredients. Many European measuring cups have additional scales for common ingredients like sugar, flour, or rice to make the process easier.

Milliliter - A milliliter (ml) is 1/1000 or 10^-3 of a liter.

Inch - An inch (plural: inches; symbol or abbreviation: in or, sometimes, " - a double prime) is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in the United States, and is widely used in the United Kingdom, and Canada, despite the introduction of metric to the latter two in the 1960s and 1970s, respectively. The inch is still commonly used informally, although somewhat less, in other Commonwealth nations such as Australia; an example being the long standing tradition of measuring the height of newborn children in inches rather than centimetres.

Effective July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be 0.9144 meters. Consequently, the international inch is defined to be equal to 25.4 millimeters. The international standard symbol for inch is in (see ISO 31-1, Annex A). In some cases, the inch is denoted by a double prime, which is often approximated by double quotes, and the foot by a prime, which is often approximated by an apostrophe.

Fluid Ounce - A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume equal to about 29 ml. It is used in both the imperial and the US customary systems, and it is sometimes referred to simply as an ounce in cases where no confusion with the unit of weight (also called an ounce) is likely to occur.

Pint - The pint is an English unit of volume or capacity in the imperial system and United States customary units. The imperial version is 20 imperial fluid ounces and is equivalent to 568.26 mL, while the U.S. version is 16 U.S. fluid ounces and is equivalent to 473 mL. Pints are commonly abbreviated as either "p" or "pt".

As with other measurement units used in the imperial system and USA, the pint used to be a common measure throughout Europe (differing in exact value from country to country) but was replaced in much of Europe with the metric system during the nineteenth century.

Quart - The quart is an imperial and US customary unit of volume equal to a quarter of a gallon, two pints, or four cups. Since gallons of various sizes have historically been in use, quarts of various sizes have also existed. Three of these quarts remain in current use, all approximately equal to one litre.

Tablespoon - A tablespoon is a type of large spoon usually used for serving. A tablespoonful, an amount approximately equal to the capacity of one tablespoon, is commonly used as a measure of volume used in cooking. It is abbreviated in English as T., tb., tblspn, tbs. or tbsp. Canada, Japan, New Zealand, South Africa and the UK define 1 level tablespoon as 15 ml.

Teaspoon - In some countries, a teaspoonful - as much as one teaspoon can hold - is used as a unit of volume, especially in cooking recipes and pharmaceutic prescriptions. It is abbreviated in English as t., ts., tsp. or tspn. (German and Dutch: TL, from Teeloffel or Theelepel).

For classic purposes in the United States, one teaspoon is precisely 1/768 of a U.S. liquid gallon (see United States customary units). This is precisely 4.92892159375 mL and 231/768 (0.30078125) cubic inch. It is also equal to 1/3 tablespoon, 1/6 U.S. fl. oz, and 1/48 of U.S. customary cup. For nutritional labeling purposes on food packages in the U.S., the teaspoon is, by Federal regulations, rounded to precisely 5 mL, per 21CFR101.9(b)(5)(viii).

Common teaspoons such as bar spoons for measuring ingredients and stirring mixed drinks are often not designed to contain a standard volume. In practice, they may hold anything between 2.5 mL and 6 mL of liquid, so caution must be employed when using a teaspoon to measure a prescribed dose of medicine. For this reason and in order to avoid dispensing errors, special measuring spoons are available that hold exactly 5 mL. The common teaspoon is always smaller than the tablespoon.