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A **system of measurement** is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units or SI (the modern form of the metric system), the British imperial system, and the United States customary system.

The French Revolution gave rise to the metric system, and this has spread around the world, replacing most customary units of measure. In most systems, length (distance), mass, and time are *base quantities*.

Later science developments showed that an electromagnetic quantity such as electric charge or electric current could be added to extend the set of base quantities. Gaussian units have only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such as power and speed, are derived from the base set: for example, speed is distance per unit time. Historically a wide range of units was used for the same type of quantity: in different contexts, length was measured in inches, feet, yards, fathoms, rods, chains, furlongs, miles, nautical miles, stadia, leagues, with conversion factors that were not powers of ten.

The preference for a more universal and consistent system only gradually spread with the growth of international trade and science. Changing a measurement system has costs in the near term, which results in resistance to such a change. The substantial benefit of conversion to a more rational and internationally consistent system of measurement has been recognized and promoted by scientists, engineers and politicians, and has resulted in most of the world adopting a commonly agreed metric system.

In antiquity, *systems of measurement* were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in *hands* and *knuckles*. The unifying characteristic is that there was some definition based on some standard. Eventually * cubits * and * strides * gave way to "customary units" to meet the needs of merchants and scientists.

In the metric system and other recent systems, underlying relationships between quantities, as expressed by formulae of physics such as Newton's laws of motion, is used to select a small number of base quantities for which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten, so for example where the unit of length is the metre; a distance of 1 m is 1,000 millimetres, or 0.001 kilometres.

Metrication is complete or nearly complete in almost all countries. US customary units are heavily used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma. U.S. units are used in limited contexts in Canada due to the large volume of trade; there is also considerable use of imperial weights and measures, despite *de jure* Canadian conversion to metric.

A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom – whose road signage legislation, for instance, only allows distance signs displaying imperial units (miles or yards)^{ [1] } – or Hong Kong.^{ [2] }

In the United States, metric units are used almost universally in science, widely in the military, and partially in industry, but customary units predominate in household use. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than grains, are used for medications. Some other non-SI units are still in international use, such as nautical miles and knots in aviation and shipping.

Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries.

Multiples and submultiples of metric units are related by powers of ten and their names are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.

In the early metric system there were two base units, the metre for length and the gram for mass. The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units.

Mesures usuelles (French for *customary measurements*) were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was used in France from 1812 to 1839.

A number of variations on the metric system have been in use. These include gravitational systems, the centimetre–gram–second systems (cgs) useful in science, the metre–tonne–second system (mts) once used in the USSR and the metre–kilogram–second system (mks). In some engineering fields, like computer-aided design, millimetre–gram–second (mmgs) is also used.^{[ citation needed ]}

The current international standard for the metric system is the International System of Units (*Système international d'unités* or SI). It is a system in which all units can be expressed in terms of seven units. The units that serves as the SI base units are the metre, kilogram, second, ampere, kelvin, mole, and candela.

Both imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the former British Empire and the British Commonwealth, but in all these countries they have been largely supplanted by the metric system. They are still used for some applications in the United Kingdom but have been mostly replaced by the metric system in commercial, scientific, and industrial applications. US customary units, however, are still the main system of measurement in the United States. While some steps towards metrication have been made (mainly in the late 1960s and early 1970s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development.

While imperial and US customary systems are closely related, there are a number of differences between them. Units of length and area (the inch, foot, yard, mile, etc.) have been identical since the adoption of the International Yard and Pound Agreement; however, America and, formerly, India retained older definitions for surveying purposes. This gave rise to the US survey foot for instance. The avoirdupois units of mass and weight differ for units larger than a pound (lb). The imperial system uses a stone of 14 lb, a long hundredweight of 112 lb and a long ton of 2240 lb. The stone is not used in the US and the hundredweights and tons are short: 100 lb and 2000 lb respectively.

Where these systems most notably differ is in their units of volume. A US fluid ounce (fl oz), about 29.6 millilitres (ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a US pint and 20 imp fl oz per imperial pint, the imperial pint is about 20% larger. The same is true of quarts, gallons, etc.; six US gallons are a little less than five imperial gallons.

The avoirdupois system served as the general system of mass and weight. In addition to this there are the troy and the apothecaries' systems. Troy weight was customarily used for precious metals, black powder and gemstones. The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system. The apothecaries' system was traditionally used in pharmacology, but has now been replaced by the metric system; it shared the same pound and ounce as the troy system but with different further subdivisions.

Natural units are units of measurement defined in terms of universal physical constants in such a manner that selected physical constants take on the numerical value of one when expressed in terms of those units. Natural units are so named because their definition relies on only properties of nature and not on any human construct. Varying systems of natural units are possible, depending on the choice of constants used.

Some examples are as follows:

- Geometrized unit systems are useful in relativistic physics. In these systems, speed of light and the gravitational constant are among the constants chosen.
- Planck units is system of geometrized units in which the reduced Planck constant is included in the list of defining constants. It is based on only properties of free space rather than of any object or particle.
- Stoney units is a system of geometrized units in which the Coulomb constant and the elementary charge are included.
- Hartree atomic units are a system of units used in atomic physics, particularly for describing the properties of electrons. The atomic units have been chosen to use several constants relating to the electron: the electron mass, the elementary charge, the Coulomb constant and the reduced Planck constant. The unit of energy in this system is the total energy of the electron in the Bohr atom and called the Hartree energy. The unit of length is the Bohr radius.

Non-standard measurement units also found in books, newspapers etc., include:

- The American football field, which has a playing area 100 yards (91.4 m) long by 160 feet (48.8 m) wide. This is often used by the American public media for the sizes of large buildings or parks. It is used both as a unit of length (100 yd or 91.4 m, the length of the playing field excluding goal areas) and as a unit of area (57,600 sq ft or 5,350 m
^{2}), about 1.32 acres (0.53 ha ). - British media also frequently uses the football pitch for equivalent purposes, although soccer pitches are not of a fixed size, but instead can vary within defined limits (100–130 yd or 91.4–118.9 m long, and 50–100 yd or 45.7–91.4 m wide, giving an area of 5,000 to 13,000 sq yd or 4,181 to 10,870 m
^{2}). However the UEFA Champions League field must be exactly 105 by 68 m (114.83 by 74.37 yd) giving an area of 7,140 m^{2}(0.714 ha) or 8,539 sq yd (1.764 acres ). For example, "HSS vessels are aluminium catamarans**about the size of a football pitch**."^{ [3] } - Larger areas are also expressed as a multiple of the areas of certain American states, or subdivisions of the UK etc.

- A ton of TNT equivalent, and its multiples the kiloton, the megaton, and the gigaton. Often used in stating the power of very energetic events such as explosions and volcanic events and earthquakes and asteroid impacts. A gram of TNT as a unit of energy has been defined as 1000 thermochemical calories (1,000 cal or 4,184 J).
- The atom bomb dropped on Hiroshima. Its energy yield is often used in the public media and popular books as a unit of energy. (Its yield was roughly 13 kilotons, or 60 TJ.)
- One stick of dynamite.

A unit of measurement that applies to money is called a unit of account in economics and unit of measure in accounting.^{ [4] } This is normally a currency issued by a country or a fraction thereof; for instance, the US dollar and US cent (1⁄100 of a dollar), or the euro and euro cent.

ISO 4217 is the international standard describing three letter codes (also known as the currency code) to define the names of currencies established by the International Organization for Standardization (ISO).

Throughout history, many official systems of measurement have been used. While no longer in official use, some of these **customary systems** are occasionally used in day-to-day life, for instance in cooking.

- Arabic
^{ [5] } - Afgan
- Cambodian
- Chinese
- Hebrew (Biblical and Talmudic)
- Hindu
- Indonesian
- Japanese
- Korean
- Omani
- Pakistani
- Philippine
- Mesopotamian
- Persian
- Singaporean
- Sri Lankan
- Syrian
- Taiwanese
- Tamil
- Thai
- Vietnamese
- Nepalese

Still in use:

- Ancient Greek
- Belgian
- Byzantine
- Czech
- Cypriot
- Danish
- Dutch
- English
- Estonian
- Finnish
- French (now)
- French (to 1795)
- German
- Greek
- Hungary
- Icelandic
- Irish
- Italian
- Latvian
- Luxembourgian
- Maltese
- Norwegian
- Polish
- Portuguese
- Roman
- Romanian
- Russian
- Scottish
- Serbian
- Slovak
- Spanish
- Swedish
- Switzerland
- Turkish
- Tatar
- Welsh

- ↑ "Statutory Instrument 2002 No. 3113 The Traffic Signs Regulations and General Directions 2002". Her Majesty's Stationery Office (HMSO). 2002. Retrieved 18 March 2010.
- ↑ HK Weights and Measures Ordinance
- ↑ Henry, Lesley-Anne (23 June 2007). "Sad sight of a superferry laid up due to soaring jet fuel bills".
*The Belfast Telegraph*. Archived from the original on 2008-10-06. - ↑ Financial Accounting Standards Research Initiative: The Unit of Account Issue
^{[ permanent dead link ]} - ↑ M. Ismail Marcinkowski,
*Measures and Weights in the Islamic World. An English Translation of Professor Walther Hinz's Handbook "Islamische Maße und Gewichte"*, with a foreword by Professor Bosworth, F.B.A. Kuala Lumpur, ISTAC, 2002, ISBN 983-9379-27-5. This work is an annotated translation of a work in German by the late German orientalist Walther Hinz, published in the*Handbuch der Orientalistik*, erste Abteilung, Ergänzungsband I, Heft 1, Leiden, The Netherlands: E. J. Brill, 1970.

- Tavernor, Robert (2007),
*Smoot's Ear: The Measure of Humanity*, ISBN 0-300-12492-9

Wikimedia Commons has media related to Customary units of measurement .

- CLDR – Unicode localization of currency, date, time, numbers
- A Dictionary of Units of Measurement
- Old units of measure
- Measures from Antiquity and the Bible Antiquity and the Bible at the Wayback Machine (archived May 10, 2008)
- Reasonover's Land Measures A Reference to Spanish and French land measures (and their English equivalents with conversion tables) used in North America
- The Unified Code for Units of Measure

**Density** is the substance's mass per unit of volume. The symbol most often used for density is *ρ*, although the Latin letter *D* can also be used. Mathematically, density is defined as mass divided by volume:

The **imperial system of units**, **imperial system** or **imperial units** is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed through a series of Weights and Measures Acts and amendments.

The **pound** or **pound-mass** is a unit of mass used in British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces. The international standard symbol for the avoirdupois pound is **lb**; an alternative symbol is **lb _{m}**,

The **International System of Units,** known by the international abbreviation **SI** in all languages and sometimes pleonastically as the **SI system**, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

**United States customary units** form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The **United States customary system** developed from English units which were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures was overhauled in 1824 to create the imperial system, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are significant differences between the systems.

In recipes, quantities of ingredients may be specified by mass, by volume, or by count.

**Troy weight** is a system of units of mass that originated in 15th-century England, and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight, the troy ounce, and the troy pound. The troy grain is equal to the grain unit of the avoirdupois system, but the troy ounce is heavier than the avoirdupois ounce, and the troy pound is lighter than the avoirdupois pound. One troy ounce equals exactly 31.1034768 grams.

A **grain** is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single ideal seed of a cereal. From the Bronze Age into the Renaissance, the average masses of wheat and barley grains were part of the legal definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate. Another source states that it was defined such that 252.458 units would balance 1 cubic inch (16 cm^{3}) of distilled water at an ambient air-water pressure and temperature of 30 inches of mercury (100 kPa) and 62 °F (17 °C) respectively. Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally.

The **ounce** is any of several different units of mass, weight or volume and is derived almost unchanged from the *uncia*, an Ancient Roman unit of measurement.

The **avoirdupois system** is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959.

A **fluid ounce** is a unit of volume typically used for measuring liquids. The British Imperial, the United States customary, and the United States food labeling fluid ounce are the only three that are still in common use, although various definitions have been used throughout history.

A **bushel** is an imperial and US customary unit of volume based upon an earlier measure of dry capacity. The old bushel is equal to 2 kennings (obsolete), 4 pecks, or 8 dry gallons, and was used mostly for agricultural products, such as wheat. In modern usage, the volume is nominal, with bushels denoting a mass defined differently for each commodity.

The **Mendenhall Order** marked a decision to change the fundamental standards of length and mass of the United States from the customary standards based on those of England to metric standards. It was issued on April 5, 1893, by Thomas Corwin Mendenhall, superintendent of the U.S. Coast and Geodetic Survey, with the approval of the United States Secretary of the Treasury, John Griffin Carlisle. The order was issued as the Survey's *Bulletin No. 26 – Fundamental Standards of Length and Mass*.

The **apothecaries' system**, or **apothecaries' weights and measures**, is a historical system of mass and volume units that were used by physicians and apothecaries for medical prescriptions and also sometimes by scientists. The English version of the system is closely related to the English troy system of weights, the pound and grain being exactly the same in both. It divides a pound into 12 ounces, an ounce into 8 drachms, and a drachm into 3 scruples of 20 grains each. This exact form of the system was used in the United Kingdom; in some of its former colonies, it survived well into the 20th century. The apothecaries' system of measures is a similar system of volume units based on the fluid ounce. For a long time, medical recipes were written in Latin, often using special symbols to denote weights and measures.

**English units** are the units of measurement used in England up to 1826, which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at different times, in different places, and for different applications.

The earliest recorded systems of weights and measures originate in the 3rd or 4th millennium BC. Even the very earliest civilizations needed measurement for purposes of agriculture, construction and trade. Early standard units might only have applied to a single community or small region, with every area developing its own standards for lengths, areas, volumes and masses. Often such systems were closely tied to one field of use, so that volume measures used, for example, for dry grains were unrelated to those for liquids, with neither bearing any particular relationship to units of length used for measuring cloth or land. With development of manufacturing technologies, and the growing importance of trade between communities and ultimately across the Earth, standardized weights and measures became critical. Starting in the 18th century, modernized, simplified and uniform systems of weights and measures were developed, with the fundamental units defined by ever more precise methods in the science of metrology. The discovery and application of electricity was one factor motivating the development of standardized internationally applicable units.

Both the British Imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from Germanic tribes and Roman units brought by William the Conqueror after the Norman Conquest of England in 1066.

The **foot–pound–second system** or **FPS system** is a system of units built on three fundamental units: the foot for length, the (avoirdupois) pound for either mass or force, and the second for time.

The **imperial and US customary measurement systems** are both derived from an earlier English system of measurement which in turn can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure.

The **quarter** was used as the name of several distinct English units based on ¼ sizes of some base unit.

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