View source for Systems of measurement

A '''system of measurement''' is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of [[trade]] and internal [[commerce]]. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning ''all other needed units can be derived from them'', whereas in the early and most historic eras, the units were given by fiat (See [[Statutory law]]) by the ruling entities and were not necessarily well inter-related or self-consistent. 

<blockquote>
''Although we might suggest that the Egyptians had discovered the art of measurement, it is really only with the Greeks that the science of measurement begins to appear. The Greeks' knowledge of geometry, and their early experimentation with weights and measures, soon began to place their measurement system on a more scientific basis. By comparison, Roman science, which came later, was not as advanced...''<ref>Quoted from the [http://www.sciencetech.technomuses.ca/english/collection/measurement2.cfm Canada Science and Technology Museum website]</ref>
</blockquote>

The [[French Revolution]] gave rise to a scientific system, and there has been steady significant pressure since to convert to a scientific basis from so called customary units of measure. In most systems, [[length]] (distance), [[weight]], and [[time]] are ''fundamental quantities''; or as has been now accepted as better in science and engineering, the substitution of [[mass]] for [[weight]], as a better more basic [[parameter]]. Some systems have changed to recognize the improved relationship, notably the [[1824]] legal changes to the [[imperial units|imperial system]].

{{TOCnestright}}
Later science developments showed that either [[electric charge]] or [[electric current]] must be added to complete ''the minimum set of fundamental quantities'' by which all other [[Metrology|metrological units]] may be defined. Other quantities, such as [[Power (physics)|power]], [[speed]], etc. are derived from the fundamental set; for example, speed is distance divided by time. Historically a wide range of units were used for the same quantity; for example, in several cultural settings, length was measured in [[inch]]es, [[Foot (unit of length)|feet]], [[yards]], [[fathom]]s, [[Rod (unit)|rods]], [[Chain (unit)|chain]]s, [[furlong]]s, [[mile]]s, [[nautical mile]]s, [[stadia]], [[league]]s, with conversion factors which are not simple powers of ten or even always simple fractions within a given customary system.

Nor were they necessarily the same units (or equal [[units of measure|units]]) between different members of similar cultural backgrounds. It must be understood by the modern reader that historically, measurement systems were perfectly adequate within their own cultural [[Social environment|milieu]], and ''the understanding'' that a better more universal system (based on more rationale and fundamental units) only gradually spread with the maturation and appreciation of the rigor characteristic of [[Classical mechanics|Newtonian physics]]. Moreover, changing one's measurement system has real fiscal and cultural costs.

Once the analysis tools within that field were appreciated and came into widespread use in the nascent sciences, especially in the utilitarian subfields of [[applied science]] like [[civil engineering|civil]] and [[mechanical engineering]], conversion to a common basis had no impetus. It was only after the appreciation of these needs and the appreciation of the difficulties of converting between numerous national customary systems became widespread could there be any serious justification for an international effort of standardization. Credit the [[French Revolution]]ary spirit for taking the first significant and radical step down that road.

In antiquity, ''systems of measurement'' were defined locally, the different units were 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 per custom like 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'', however egocentric or amusing it may now seem viewed with eyes used to modern precision. Eventually ''[[cubit]]s'' and ''[[yard|strides]]'' gave way under need and demand from merchants and evolved to ''customary units.

In the metric system and other recent systems, a single basic unit is used for each fundamental quantity. Often secondary units (multiples and submultiples) are used which convert to the basic units by multiplying by powers of ten, i.e., by simply moving the [[decimal point]]. Thus the basic metric unit of length is the [[metre]] or [[meter]]; a distance of 1.234&nbsp;m is 1234.0&nbsp;millimetres, or 0.001234&nbsp;kilometres.

[[Image:MetricImperialUSCustomaryUnits.jpg|thumb|375px|A baby bottle that measures in three measurement systems&mdash;imperial (U.K.), U.S. customary, and metric.]]

== ''Approximate'' conversion of units ==
'''Approximate conversion of units''' often needs to be done without calculator, computer, or sometimes even longhand. This article is an informal listing of some simple conversions between (and within) the [[Imperial unit|imperial system]], the [[U.S. customary units|U.S. customary system]] and the [[metric system]]/[[SI]].  When people say "a foot long" or "ten kilometres from here", they rarely mean "0.3048 m" or "6.21571 miles".  

These are all rough approximations; for more accurate information, see [[Conversion of units]]. 

Exact conversions can be performed in [[Google]] by typing the source and target units directly in the search box, using the keyword ''in''. 

For example; On entering <br>'''34 acres in hectares'''

in Google search box there is returned: <br>'''34 acres = 13.7593118 hectares.'''

The user should be conscious of the [[significant figures]] in the result generated.

== Metric system ==

[[Metric system]]s of units have evolved since the adoption of the first well-defined system in [[France]] in [[1791]]. During this evolution the use of these systems spread throughout the world, first to the non-English-speaking countries, and more recently to the English speaking countries.

Multiples and submultiples of metric units are related by powers of ten; the names for these are formed with [[SI prefix|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 fundamental or base units, the [[metre]] and the [[gram]], for length and mass. The other units of length and mass, and all units of area, volume, and compound units such as density were derived from these two fundamental units.

[[Mesures usuelles]] ([[French language|French]] for ''customary measurements'') were a system of [[measurement]] introduced to act as 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 [[kilogram-force|gravitational systems]], the [[centimetre gram second system of units|centimetre-gram-second systems]] (cgs) useful in science, the [[metre-tonne-second system of units|metre-tonne-second system]] (mts) once used in the [[USSR]] and the [[SI|metre-kilogram-second system]] of units (mks) most commonly used today.

The current international standard metric system is the [[SI|International System of Units]] (''Système international d'unités'' or SI) It is an mks system based on the [[metre]], [[kilogram]] and [[second]] as well as the [[kelvin]], [[ampere]], [[candela]], and [[mole (unit)|mole]].

The [[SI]] includes two classes of units which are defined and agreed internationally.
The first of these classes are the seven [[SI base unit]]s for length, mass, time, temperature, electric current, luminous intensity and amount of substance. The second of these are the [[SI derived unit]]s. These derived units are defined in terms of the seven base units. All other quantities (e.g. work, force, power) are expressed in terms of [[SI derived unit]]s.

== Imperial and U.S. customary units ==
Both the [[Imperial unit]]s and [[United States customary units|U.S. customary units]] derive from earlier [[English unit]]s. Imperial units were mostly used in the [[Commonwealth of Nations|British Commonwealth]] and the former [[British Empire]]. They are still used in common household applications to some extent and so are also sometimes called '''common units''', but have now been mostly replaced by the metric system in [[commerce|commercial]], [[scientific]], and [[industry|industrial]] applications.  

Contrarily, however, U.S. customary units are still the main system of measurement in the [[United States]]. While some steps towards [[metrication]] have been made (mainly in the late [[1960]]s and early [[1970]]s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development. The effort is proceeding slowly due to the overwhelming financial cost of converting the existing infrastructure. U.S. companies which trade internationally are more likely to use the metric system due to international standards and certifications such as [[ISO9000]]. The metric system is preferred in certain fields such as [[science]], [[medicine]] and [[technology]]. The building profession uses US customary units, though architects working internationally are increasingly adapting to the metric system.

These two systems are closely related. There are, however, a number of [[Comparison of the imperial and US customary measurement systems|differences between them]]. Units of length and area (the [[inch]], [[foot]], [[yard]], [[mile]] etc.) are identical except for surveying purposes. The [[Avoirdupois]] units of mass and weight differ for units larger than a [[Pound (weight)|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 U.S. and the hundredweights and tons are short being 100 lb. and 2000 lb. respectively.

Where these systems most notably differ is in their respective units of volume. A U.S. [[fluid ounce]] (fl. oz.) is slightly larger than its Imperial equivalent (the former being approximately 29.6 [[millilitre]]s (ml) and the latter 28.4 ml). However, as there are 16 U.S. fl. oz. to a U.S. [[pint]] as opposed to the 20 Imperial fl. oz. per Imperial pint, these pints are quite different in volume. The same is true of [[quart]]s, [[gallon]]s, etc. Six U.S. gallons are a little less than five Imperial gallons.

Mentioned above was the [[Avoirdupois]] system which has served as the general system of mass and weight. In addition to this there are the [[Troy weight|Troy]] and the [[Apothecaries' system]]s. Troy weight was customarily used for [[precious metal]]s, [[black powder]] and [[gemstone]]s. 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 twelve ounces opposed to the sixteen ounces per pound of the Avoirdupois system. The Apothecaries' system; traditionally used in [[pharmacology]], now replaced by the metric system; shares same pound and ounce as the troy system but with different further subdivisions.

== Natural units ==
[[Natural units]] are [[Physics|physical]] [[units of measurement]] defined in terms of universal [[physical constants]] in such a manner that some chosen physical constants take on the numerical value of one when expressed in terms of a particular set of natural units.   Natural units are natural because the origin of their definition comes only from properties of [[nature]] and not from any human construct. Various systems of natural units are possible. Below are listed some examples.

* [[Geometrized unit system|Geometric unit systems]] are useful in [[Theory of relativity|relativistic physics]]. In these systems the base physical units are chosen so that the [[speed of light]] and the [[gravitational constant]] are set equal to unity.
* [[Planck units]] are a form of geometric units obtained by also setting [[Boltzmann's constant]], the [[Coulomb's law|Coulomb force constant]] and [[Dirac's constant]] to unity. They might be considered unique in that they are based only on properties of [[free space]] rather than any prototype, object or particle.
* [[Natural units#Stoney units|Stoney units]] are similar to Planck units but set the [[elementary charge]] to unity and allow Dirac's constant to float.
* [[Natural units#"Schrödinger" units|"Schrödinger" units]] are also similar to Planck units and set the elementary charge to unity too but allow the speed of light to float.
* [[Atomic units]] (au) are a convenstupid wikipediaient system of units of measurement used in [[atomic physics]], particularly for describing the properties of [[electron]]s. The atomic units have been chosen such that the fundamental electron properties are all equal to one atomic unit. They are similar to "Schrödinger" units but set the [[electron mass]] to unity and allow the gravitational constant to float. The unit [[energy]] in this system is the total energy of the [[electron]] in the [[Bohr atom]] and called the [[Hartree energy]]. The unit length is the [[Bohr radius]].
* [[Natural units#Electronic system of units|Electronic units]] are similar to Stoney units but set the [[electron mass]] to unity and allow the gravitational constant to float. They are also similar to Atomic units but set the speed of light to unity and allow Dirac's constant to float.
* [[Natural units#Quantum electrodynamical system of units|Quantum electrodynamical units]] are similar to the electronic system of units except that the [[proton mass]] is normalised rather than the electron mass.

== Non-standard units ==
[[List of unusual units of measurement|Non-standard measurement units]], sometimes found in books, newspapers etc., include:

===Area===
* The (American) [[American football#Field and players|football field]], which has a playing area 120 [[yard]]s long by 160 feet wide (109.7&nbsp;m&nbsp;× 48.8&nbsp;m). This is often used by the [[United States|American]] public media for the sizes of large buildings or parks: easily walkable but !!!! non-trivial distances. Note that it is used as a unit of length (100&nbsp;yd&nbsp;/ 91.4&nbsp;m, the length of the playing field excluding goal areas) or area (57,600&nbsp;sq&nbsp;ft&nbsp;/ 5,351&nbsp;m<sup>2</sup>). 
* British media also frequently uses the [[football pitch]] for equivalent purposes, although [[football (soccer)|Association Football (Soccer)]] pitches are not of a fixed size, but instead can vary within defined limits (100-130 yards long, and 50-100 yards wide, giving an area of 5,000 to 13,000 sq&nbsp;yd). Example: [http://www.belfasttelegraph.co.uk/news/local-national/article2698634.ece HSS vessels are aluminium catamarans '''about the size of a football pitch'''... - Belfast Telegraph 23 June 2007]
* In the U.S. a small circular area is often described as "the size of a [[Dime (United States coin)|dime]]". Example: [http://www.sciencedaily.com/upi/index.php?feed=Science&article=UPI-1-20070621-10330900-bc-us-brainscanner.xml The "brain scope" developed at Duke University is inserted into a '''dime-sized hole''' in the skull...  -Science Daily]

=== Energy ===
* A ton of [[Trinitrotoluene|TNT]], and its multiples the [[kiloton]] and the [[megaton]] and the [[gigaton]]. Often used in stating the power of very energetic events such as [[explosion]]s and [[volcanic]] events and [[earthquakes]] and [[asteroid]] impacts. A [[gram]] of TNT as a unit of [[energy]] has been defined as 1000 thermochemical [[calories]] = roughly 4184 [[joules]].
* The [[Hiroshima]] [[atom bomb]]. Its force is often used in the [[public media]] and popular [[books]] as a unit of energy. (Its yield was roughly 13 kilotons / 60 TJ.)

=== Mass ===
* The weight of an [[elephant]]. It is often used as a unit of weight in popular books about very big animals such as [[dinosaur]]s. This unit needs to be defined, as the real weight of elephants varies much with age, sex and species. An average adult male African elephant weighs 11,000 lb (5,000 kg).<ref>[http://www.pittsburghzoo.org/wildlife_lookUpAnimal_detail.asp?categoryname=&animal=15 Pittsburgh Zoo & PPG Aquarium<!-- Bot generated title -->]</ref>
* [[Bald eagles]]. Example: [http://news.nationalgeographic.com/news/2007/07/070702-biggest-bird.html Largest Flying Bird ...Argentavis was no ostrich. Despite '''weighing as much as 16 bald eagles'''... - National Geographic News 2 Jul 2007] Argentavis was described elsewhere in the article as weighing 155 pounds (70 kg), so the weight used for a bald eagle in this comparison would be 10 lb (4.5 kg).

=== Vertical distance ===
* The "height of a London Bus" is used by British media to describe height. Example: [http://www.guardian.co.uk/Archive/Article/0,4273,3937057,00.html ... tsunami three times the '''height of a London bus''' that battered the north coast of New Guinea, wiping out four villages and killing more than 3,000 people - Guardian Unlimited]

=== Volume ===
*Nuts, fruit and vegetables are sometimes used to indicate volume. Example: [http://www.thedesertsun.com/apps/pbcs.dll/article?AID=/20070706/GETPUBLISHED/707060325/1050 Facts about Arthritis ...The size of the nodules can range from that of a '''pea''' to '''walnut''' size. - The Desert Sun, Palm Springs CA 6 July 2007]
* Larger volumes can be indicated as being "the size of a house". Example: [http://www.spaceflightnow.com/delta/d325/070702targets.html Asteroids can range from '''the size of a house''' to objects the size of Vesta. - Spaceflight Now quoting NASA press kit]

== Units of currency ==
A unit of measurement that applies to [[money]] is called a [[unit of account]]. This is normally a [[currency]] issued by a [[country]] or a fraction thereof; for instance, the [[United States dollar|U.S. dollar]] and U.S. 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 (IOS).

== Historical systems of measurement ==
{{Main|History of measurement}}

Prior to the widespread adoption of the metric system many different systems of official measurement had been in use, many of these remain today, at least in part, in traditional or customary use. Many of these were related to some extent or other. Often they were based on the dimensions of the human body. When the world turned to trade between [[city-states]] better systems were needed to enable that mercantile activity. Over time, the evolution continued as transportation continued to shrink the world, and so what was once an artifact of a pocket kingdom matured into something that was at least workable. Despite the growth and adoption of modern systems like [[SI]] around the world for business and governance, such '''''customary systems''''' are still commonly used in day to day life for everyday ordinary household tasks around the world, most notably, in [[cooking]] and [[cookbook]]s.

Throughout the [[history of measurement]], many of the units that have been used in [[Europe]] and around the [[Mediterranean]] are variations on older systems originating in the [[ancient Near East]].

=== Medieval European measurements ===
Medieval European systems of measurement evolved during the [[Middle Ages]] (or European [[Dark Ages]]) due to the agriculture-intensive way of life. These systems may also be referred to as ''feudal measurement systems''. The measurements were approximate and variable. The measures can be categorized by ever expanding commercial, political and religious spheres of influence.

==== Eastern European ====
In Eastern Europe traditional standards of measure were predominantly of Greek origin
* [[Old Polish units of measurement|Polish system]]
* [[Romanian units of measurement|Romanian system]]
* [[Obsolete Russian weights and measures|Russian system]]  
* [[Obsolete Tatar weights and measures|Tatar system]]

==== Western and Northern European ====
In Western and Northern Europe traditional standards of measure were predominantly of Roman origin:
* [[Danish unit|Danish system]] 
* [[Dutch unit|Dutch system]]
* [[English unit|English system]]
* [[Finnish unit|Finnish system]] 
* [[French unit|French system]]
* [[German unit|German system]] 
* [[Norwegian units of measure|Norwegian system]] 
* [[Obsolete Scottish units of measurement|Scottish system]]
* [[Spanish unit|Portuguese and Spanish system]]
* [[Swedish unit|Swedish system]]

== See also ==
[[Dutch units of measurement]]

== Notes and References ==
 {{refs}}
* Tavernor, Robert (2007), ''Smoot's Ear: The Measure of Humanity'', ISBN 030-0124-92-9

== External links ==
* [http://www.chipkin.com/a/resources/SIConverter/ A utility to Convert Different Units]
* [http://www.unicode.org/cldr/index.html CLDR - Unicode localization of currency, date, time, numbers]
* [http://www.unc.edu/~rowlett/units/ Dictionary of Units of Measurement]
* [http://www.du.edu/~jcalvert/tech/oldleng.htm Old units of measure]
* [http://members.aol.com/JackProot/met/antbible.html Measures from Antiquity and the Bible]
* Reasonover's Land Measures [http://www.booksontexas.com/Reasonover_s_Land_Measures_p/reas05.htm A Reference to Spanish and French land measures (and their English equivalents with conversion tables) used in North America]
* [http://aurora.regenstrief.org/~schadow/units/UCUM/ucum.html The Unified Code for Units of Measure]

{{systems}}
{{systems of measurement}}

[[Category:Units of area]]
[[Category:Obsolete units of measure|*]]
[[Category:Units of measure]]