TheInfoList

A unit of measurement is a definite
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
of a
quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value in terms of a unit of measu ...
, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a
length Length is a measure of distance. In the International System of Quantities The International System of Quantities (ISQ) is a set of quantities and the equation In mathematics, an equation is a statement that asserts the equality (mathema ...

is a
physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''numerical value'' ...
. The
metre The metre (British English, Commonwealth spelling) or meter (American English, American spelling; American and British English spelling differences#-re, -er, see spelling differences) (from the French unit , from the Greek noun , "measure", and ...
is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre". Measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind. The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of
systems of units A system of measurement is a collection of units of measurement A unit of measurement is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of ...
used to be very common. Now there is a global standard, the
International System of Units International is an adjective (also used as a noun) meaning "between nations". International may also refer to: Music Albums * International (Kevin Michael album), ''International'' (Kevin Michael album), 2011 * International (New Order album), '' ...
(SI), the modern form of the
metric system The metric system is a system of measurement 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 ...

. In trade, weights and measures is often a subject of governmental regulation, to ensure fairness and transparency. The
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry, ...
(BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI).
Metrology 290px, alt=Man in white standing in front of a large machine, A scientist stands in front of the Microarcsecond Metrology (MAM) testbed. Metrology is the scientific study of measurement ' Measurement is the number, numerical quantification (s ...

is the science of developing nationally and internationally accepted units of measurement. In
physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...

and metrology, units are standards for
measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, which can be used to compare with other objects or events. The scope and application of meas ...

of physical quantities that need clear definitions to be useful.
ReproducibilityReproducibility is a major principle of the scientific method The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th century. It involves ...
of experimental results is central to the
scientific method The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th century. It involves careful observation, applying rigorous skepticism about what ...

. A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures historically developed for commercial purposes.
Science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictions about the u ...

,
medicine Medicine is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations ...

, and
engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

often use larger and smaller units of measurement than those used in everyday life. The judicious selection of the units of measurement can aid researchers in
problem solving Problem solving consists of using generic or ad hoc Ad hoc is a List of Latin phrases, Latin phrase meaning literally 'to this'. In English, it generally signifies a solution designed for a specific problem or task, non-Generalization, gene ...

(see, for example,
dimensional analysis In engineering and science, dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base quantity, base quantities (such as length, mass, time, and electric current) and units of measure ...
). In the
social sciences Social science is the branch Image:Tree Leaves.JPG, The branches and leaves of a tree. A branch ( or , ) or tree branch (sometimes referred to in botany as a ramus) is a woody structural member connected to but not part of the central tru ...

, there are no standard units of measurement and the theory and practice of measurement is studied in
psychometrics Psychometrics is a field of study concerned with the theory and technique of psychological measurement. As defined by the US National Council on Measurement in Education (NCME), psychometrics refers to psychological measurement. Generally, it r ...
and the
theory of conjoint measurement The theory of conjoint measurement (also known as conjoint measurement or additive conjoint measurement) is a general, formal theory of continuous quantity. It was independently discovered by the French economist Gérard Debreu (1960) and by the A ...
.

# History

A unit of measurement is a standardised
quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value in terms of a unit of measu ...
of a physical property, used as a factor to express occurring quantities of that property. Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials. The earliest known uniform systems of measurement seem to have all been created sometime in the 4th and 3rd millennia BC among the ancient peoples of
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the ...

,
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identif ...
and the
Indus Valley#REDIRECT Indus River#REDIRECT Indus River {{Redirect category shell, {{R from move {{R from miscapitalisation {{R unprintworthy ...
{{Redirect category shell, {{R from move {{R from miscapitalisation {{R unprintworthy ...
, and perhaps also
Elam Elam (; Linear Elamite: ''hatamti''; Cuneiform Cuneiform is a Logogram, logo-Syllabary, syllabic writing system, script that was used to write several languages of the Ancient Near East. The script was in active use from the early Bronz ...

in
Persia Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, also West Asia, is the westernmost subregion of Asia. It is entirely a part of the Greater Middle Ea ...

as well. Weights and measures are mentioned in the
Bible The Bible (from Koine Greek Koine Greek (, , Greek approximately ;. , , , lit. "Common Greek"), also known as Alexandrian dialect, common Attic, Hellenistic or Biblical Greek, was the koiné language, common supra-regional form of Greek ...
(Leviticus 19:35–36). It is a commandment to be honest and have fair measures. In the ''
Magna Carta (Medieval Latin for "Great Charter of Freedoms"), commonly called (also ''Magna Charta''; "Great Charter"), is a Royal charter, royal charter of rights agreed to by King John of England at Runnymede, near Windsor, Berkshire, Windsor, on ...

'' of 1215 (The Great Charter) with the seal of
King John of the King of the Romans (variant used in the early modern period) File:Nezahualpiltzintli.jpg, Aztec King Nezahualpiltzintli of Texcoco King is the title given to a male monarch in a variety of contexts. The female equivalent is queen re ...

, put before him by the Barons of England, King John agreed in Clause 35 "There shall be one measure of wine throughout our whole realm, and one measure of ale and one measure of corn—namely, the London quart;—and one width of dyed and russet and hauberk cloths—namely, two ells below the selvage..." As of the 21st Century, multiple unit systems are used all over the world such as the United States Customary System, the British Customary System, and the International System. However, the United States is the only industrialized country that has not yet at least mostly converted to the Metric System. The systematic effort to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up such a unit system. This system was the precursor to the metric system which was quickly developed in France but did not take on universal acceptance until 1875 when The Metric Convention Treaty was signed by 17 nations. After this treaty was signed, a General Conference of Weights and Measures (CGPM) was established. The CGPM produced the current SI system which was adopted in 1954 at the 10th conference of weights and measures. Currently, the United States is a dual-system society which uses both the SI system and the US Customary system.

# Systems of units

The use of a single unit of measurement for some quantity has obvious drawbacks. For example, it is impractical to use the same unit for the distance between two cities and the length of a needle. Thus, historically they would develop independently. One way to make large numbers or small fractions easier to read, is to use unit prefixes. At some point in time though, the need to relate the two units might arise, and consequently the need to choose one unit as defining the other or vice versa. For example, an
inch Measuring tape with inches The inch (symbol: in or ″) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete pie ...
could be defined in terms of a barleycorn. A system of measurement is a collection of units of measurement and rules relating them to each other. As
science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictions about the u ...

progressed, a need arose to relate the measurement systems of different quantities, like length and weight and volume. The effort of attempting to relate different traditional systems between each other exposed many inconsistencies, and brought about the development of new units and systems. The system of units varies from country to country and some of the different system of units are CGS system of units, FPS system of units,
MKS system of units The MKS system of units is a physical system of measurement 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 t ...
and SI system of units. Among the different system of units using in the world, the most widely used and internationally accepted one is the
International System of Units International is an adjective (also used as a noun) meaning "between nations". International may also refer to: Music Albums * International (Kevin Michael album), ''International'' (Kevin Michael album), 2011 * International (New Order album), '' ...
, or SI system of units. In this SI units system, there are seven
SI base unit The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ot ...

s and three supplementary units. The base SI units are metre, kilogram, second, kelvin, ampere, candela and the mole and the three supplementary SI units are radian, steradian and becquerel. All other SI units can be derived from these base units. Systems of measurement in modern use include the
metric system The metric system is a system of measurement 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 ...

, the
imperial system The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thr ...
, and
United States customary units United States customary units (U.S. customary units) are a system of measurements commonly used in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous ...
.

Historically many of the systems of measurement which had been in use were to some extent based on the dimensions of the human body. As a result, units of measure could vary not only from location to location but from person to person.

## Metric systems

Metric system The metric system is a system of measurement 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 ...

s of units have evolved since the adoption of the original metric system in
France France (), officially the French Republic (french: link=no, République française), is a country primarily located in Western Europe, consisting of metropolitan France and Overseas France, several overseas regions and territories. The metro ...

in 1791. The current international standard metric system is the
International System of Units International is an adjective (also used as a noun) meaning "between nations". International may also refer to: Music Albums * International (Kevin Michael album), ''International'' (Kevin Michael album), 2011 * International (New Order album), '' ...

(abbreviated to SI). An important feature of modern systems is
standardization Standardization or standardisation is the process of implementing and developing technical standard A technical standard is an established norm or requirement for a repeatable technical task. It is usually a formal document that establishes un ...
. Each unit has a universally recognized size. Both the
imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed throu ...
and
US customary units United States customary units (U.S. customary units) are a system of measurements commonly used in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous ...
derive from earlier
English unit English units are the units of measurement A unit of measurement is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of ...
s. Imperial units were mostly used in the
British Commonwealth The Commonwealth of Nations, generally known simply as the Commonwealth, is a political association of 54 member states, almost all of which are former territories A territory is an administrative division, usually an area that is under the ...

and the former
British Empire The British Empire was composed of the dominions, Crown colony, colonies, protectorates, League of Nations mandate, mandates, and other Dependent territory, territories ruled or administered by the United Kingdom and its predecessor states. ...
. US customary units are still the main system of measurement used in the
United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal di ...

outside of science, medicine, many sectors of industry, and some of government and military, and despite Congress having legally authorised metric measure on 28 July 1866. Some steps towards US
metrication , as in almost all countries. Metrication or metrification is the act or process of converting to a metric system, metric system of measurement. All over the world, nations have transitioned from their local and traditional units of measurement ...

have been made, particularly the redefinition of basic US and imperial units to derive exactly from SI units. Since the
international yard and pound The international yard and pound are two units of measurement that were the subject of an agreement among representatives of six nations signed on 1 July 1959, namely the United States The United States of America (USA), commonly known as t ...
agreement of 1959 the US and imperial inch is now defined as exactly , and the US and imperial avoirdupois pound is now defined as exactly .

## Natural systems

While the above systems of units are based on arbitrary unit values, formalised as standards, some unit values occur naturally in science. Systems of units based on these are called
natural units In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
. Similar to natural units,
atomic units The Hartree atomic units are a system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described ...
(au) are a convenient
system of units A system of measurement is a collection of units of measurement A unit of measurement is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of ...
of measurement used in atomic physics. Also a great number of unusual and non-standard units may be encountered. These may include the
solar mass The solar mass () is a standard unit of mass in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial obje ...
(), the megaton (the energy released by detonating one million tons of
trinitrotoluene Trinitrotoluene () more commonly known as TNT, or more specifically 2,4,6-trinitrotoluene, is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) compo ...

, TNT) and the
electronvolt In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "P ...
.

## Legal control of weights and measures

To reduce the incidence of retail fraud, many national
statutes A statute is a formal written enactment of a legislative A legislature is a deliberative assembly with the authority In the fields of sociology Sociology is the study of society, human social behaviour, patterns of social relationships ...
have standard definitions of weights and measures that may be used (hence "
statute measure between 1855 and 1892, when the United States Department of the Treasury, Treasury Department formally adopted a metric standard. Bronze Yard No.11 was forged to be an exact copy of the British Imperial Standard Yard, which was ruined in 1834 ...
"), and these are verified by legal officers.

## Informal comparison to familiar concepts

In informal settings, a quantity may be described as multiples of that of a familiar entity, which can be easier to contextualize than a value in a formal unit system. For instance, a publication may describe an area in a foreign country as a number of multiples of the area of a region local to the readership. The propensity for certain concepts to be used frequently can give rise to loosely defined "systems" of units.

# Base and derived units

For most quantities a unit is necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given. But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the ''base units'' and the other units are ''derived units''. Thus base units are the units of the quantities which are independent of other quantities and they are the units of length, mass, time, electric current, temperature, luminous intensity and the amount of substance. Derived units are the units of the quantities which are derived from the base quantities and some of the derived units are the units of speed, work, acceleration, energy, pressure etc. Different systems of units are based on different choices of a set of related units including fundamental and derived units.

# Calculations with units of measurement

## Units as dimensions

Any value of a
physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''numerical value'' ...
is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity ''Z'' is expressed as the product of a unit and a numerical factor: : For example, let $Z$ be "2 candlesticks", then candlestick. The multiplication sign is usually left out, just as it is left out between variables in the scientific notation of formulas. The convention used to express quantities is referred to as
quantity calculusQuantity calculus is the formal method for describing the mathematical relations between ''abstract'' physical quantities A physical quantity is any phenomenon that can be measured with an instrument or be calculated for. A physical quantity can be e ...
. In formulas the unit can be treated as if it were a specific magnitude of a kind of physical
dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), m ...

: see
dimensional analysis In engineering and science, dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base quantity, base quantities (such as length, mass, time, and electric current) and units of measure ...
for more on this treatment. Units can only be added or subtracted if they are the same type; however units can always be multiplied or divided, as
George Gamow George Gamow (March 4, 1904 – August 19, 1968), born Georgiy Antonovich Gamov (russian: Георгий Антонович Гамов), was a Ukrainian-American theoretical physicist and cosmologist. He was an early advocate and developer of ...
used to explain. Let $Z_1$ be "2 candlesticks" and $Z_2$ "3 cabdrivers", then :"2 candlesticks" times "3 cabdrivers" $=$Z_2] = 6 candlestick $\times$ cabdriver. A distinction should be made between units and standards. A unit is fixed by its definition and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit and realizes that unit only under certain physical conditions. For example, a metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be exactly one metre long only at a certain temperature. There are certain rules that have to be used when dealing with units: * Treat units algebraically. Only add like terms. When a unit is divided by itself, the division yields a unitless one. When two different units are multiplied or divided, the result is a new unit, referred to by the combination of the units. For instance, in SI, the unit of speed is metres per second (m/s). See
dimensional analysis In engineering and science, dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base quantity, base quantities (such as length, mass, time, and electric current) and units of measure ...
. A unit can be multiplied by itself, creating a unit with an exponent (e.g. m2/s2). Put simply, units obey the laws of indices. (See
Exponentiation Exponentiation is a mathematical operation, written as , involving two numbers, the '' base'' and the ''exponent'' or ''power'' , and pronounced as " raised to the power of ". When is a positive integer An integer (from the Latin wikt: ...
.) * Some units have special names, however these should be treated like their equivalents. For example, one newton (N) is equivalent to 1 kg⋅m/s2. Thus a quantity may have several unit designations, for example: the unit for
surface tension Surface tension and hydrophobicity interact in this attempt to cut a water droplet.">water_droplet.html" ;"title="hydrophobicity interact in this attempt to cut a water droplet">hydrophobicity interact in this attempt to cut a water droplet. ...

can be referred to as either N/m (newton per metre) or kg/s2 (kilogram per second squared).

## Expressing a physical value in terms of another unit

Conversion of units Conversion or convert may refer to: Arts, entertainment, and media * "Conversion" (''Doctor Who'' audio), an episode of the audio drama ''Cyberman'' * "Conversion" (''Stargate Atlantis''), an episode of the television series * The Conversion (Th ...
involves comparison of different standard physical values, either of a single physical quantity or of a physical quantity and a combination of other physical quantities. Starting with: : replace the original unit with its meaning in terms of the desired unit , e.g. if , then: : Now $n_i$ and $c_$ are both numerical values, so just calculate their product. Or, which is just mathematically the same thing, multiply ''Z'' by unity, the product is still ''Z'': : For example, you have an expression for a physical value ''Z'' involving the unit ''feet per second'' () and you want it in terms of the unit ''miles per hour'' (): Or as an example using the metric system, you have a value of fuel economy in the unit ''litres per 100 kilometres'' and you want it in terms of the unit ''microlitres per metre'': :$\mathrm = \mathrm \mathrm \mathrm = \frac \,\mathrm = 90\,\mathrm$

# Real-world implications

One example of the importance of agreed units is the failure of the
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agencies of the United States government, independent agency of the Federal government of the United States, U.S. federal government responsible for the civilian Lis ...

Mars Climate Orbiter The ''Mars Climate Orbiter'' (formerly the Mars Surveyor '98 Orbiter) was a robotic Robotics is an interdisciplinarity, interdisciplinary field that integrates computer science and engineering. Robotics involves design, construction, operati ...
, which was accidentally destroyed on a mission to Mars in September 1999 instead of entering orbit due to miscommunications about the value of forces: different computer programs used different units of measurement (
newton Newton most commonly refers to: * Isaac Newton (1642–1726/1727), English scientist * Newton (unit), SI unit of force named after Isaac Newton Newton may also refer to: Arts and entertainment * Newton (film), ''Newton'' (film), a 2017 Indian fil ...
versus pound force). Considerable amounts of effort, time, and money were wasted. On 15 April 1999,
Korean Air The Korean Air Co., Ltd. (), trade name, operating as Korean Air, is the largest airline and flag carrier of South Korea based on fleet size, international destinations and international flights. The airline's global headquarters is located i ...
cargo flight 6316 from
Shanghai Shanghai (, , Standard Chinese, Standard Mandarin pronunciation: ) is one of the four Direct-administered municipalities of China, direct-administered municipalities of the China, People's Republic of China. The city is located on the sout ...

to
Seoul Seoul (, like ''soul''; ko, 서울 ; ), officially the Seoul Special City, is the Capital city, capital and largest metropolis of South Korea.Before 1972, Seoul was the "de jure" capital of the North Korea, Democratic People's Republic of Ko ...

was lost due to the crew confusing tower instructions (in metres) and altimeter readings (in feet). Three crew and five people on the ground were killed. Thirty-seven were injured. In 1983, a Boeing 767 (which thanks to its pilot's gliding skills landed safely and became known as the
Gimli Glider Air Canada Flight 143 was a Canadian scheduled domestic passenger flight between Montreal Montreal ( ; officially Montréal, , Tio'tia:ke in Mohawk language, Mohawk) is the List of the 100 largest municipalities in Canada by populati ...

) ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of
Air Canada Air Canada is the flag carrier and the List of largest airlines in North America, largest airline of Canada by fleet size and passengers carried. It is headquartered in the borough of Saint-Laurent, Quebec, Saint-Laurent in Montreal, Quebec, Can ...

's first aircraft to use metric measurements. This accident was the result of both confusion due to the simultaneous use of metric and Imperial measures and confusion of mass and volume measures. When planning his journey across the Atlantic Ocean in the 1480s,
Columbus Columbus is a Latinized version of the Italian surname "''Colombo Colombo ( si, කොළඹ, translit=Kolamba, ; ta, கொழும்பு, translit=Kozhumpu, ) is the commercial capital and largest city of Sri Lanka Sri Lanka (, ...

mistakenly assumed that the
mile The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a imperial unit, British imperial unit and US customary unit of distance; both are based on the older English unit of Unit of length, length eq ...
referred to in the Arabic estimate of 56⅔ miles for the size of a
degree Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
was the same as the actually much shorter Italian mile of 1,480 meters. His estimate for the size of the degree and for the circumference of the Earth was therefore about 25% too small.

*
GNU Units GNU Units is a cross-platform In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer ...
*
List of humorous units of measurement Many people have made use of, or invented, units of measurement A unit of measurement is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measureme ...
*
List of obsolete units of measurementThis is a list of obsolete units of measurement, organized by type. These units of measurement are typically no longer used, though some may be in limited use in various regions. For units of measurement that are unusual but not necessarily obsolete, ...
*
List of unusual units of measurement An unusual unit of measurement is a unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathe ...
* Measure words *
Metric units Metric units are units Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece ...
* Numerical value equation *
Scottish units Scottish or Scots units of measurement are the weights and measures peculiar to Scotland Scotland ( sco, Scotland, gd, Alba ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of ...
*
Seconds pendulum A seconds pendulum is a pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity ...
* Space (punctuation)#Unit symbols and numbers *
System of measurement 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 ...
*
Unified Code for Units of MeasureThe Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans. The code set includes all units def ...
*
United States customary units United States customary units (U.S. customary units) are a system of measurements commonly used in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous ...
*
Unit of account In economics Economics () is the social science that studies how people interact with value; in particular, the Production (economics), production, distribution (economics), distribution, and Consumption (economics), consumption of goods a ...
*
Units of information In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. In information theory, units of information are ...

# Notes

* Rowlett, Russ (2018
How Many? A Dictionary of Units of Measurement

NIST Handbook 44
''Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices''
Official SI website

Quantity System Framework
– Quantity System Library and Calculator for Units Conversions and Quantities predictions

;Historical
"Arithmetic Conventions for Conversion Between Roman [i.e. Ottoman] and Egyptian Measurement"
is a manuscript from 1642, in Arabic, which is about units of measurement. * ;Legal
Ireland – Metrology Act 1996
* ;Metric information
BIPM
(official site)
The Unified Code for Units of Measure
(UCUM) {{DEFAULTSORT:Units of Measurement * * Measurement * *