Bar (unit)

Last updated
Aluminium cylinder.jpg
A pressure of 700 bar flattened this length of aluminium tubing, which had a wall thickness of 5 millimetres (0.197 in).
General information
Unit system Metric system
Unit of Pressure
1 bar in ...... is equal to ...
    SI units    100  kPa
    CGS units    1.0×106  Ba
    US customary units    14.50377  psi

The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000  Pa, which is slightly less than the current average atmospheric pressure on Earth at sea level. [1] [2]

Unit of measurement real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (International vocabulary of metrology)

A unit of measurement is a definite 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 quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.

Pressure Force distributed continuously over an area

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

International System of Units a system of units of measurement for base and derived physical quantities

The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.


The bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather forecasting. [3]

Vilhelm Bjerknes Norwegian physicist and meteorologist

Vilhelm Friman Koren Bjerknes was a Norwegian physicist and meteorologist who did much to found the modern practice of weather forecasting.

Weather forecasting application of science and technology to predict the conditions of the atmosphere for a given location and time

Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th century. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere at a given place and using meteorology to project how the atmosphere will change.

The International Bureau of Weights and Measures (BIPM) lists the bar as one of the "non-SI units [that authors] should have the freedom to use", but has declined to include it among the "Non-SI units accepted for use with the SI". [1] The bar has been legally recognised in countries of the European Union since 2004. [2] The US National Institute of Standards and Technology (NIST) deprecates its use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used". [4] The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated". [5]

International Bureau of Weights and Measures an intergovernmental organization established by the Metre Convention, through which Member States act together on matters related to measurement science and measurement standards (BIPM)

The International Bureau of Weights and Measures is an intergovernmental organization that was established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. The organisation is usually referred to by its French initialism, BIPM. The BIPM's headquarters is based at Sèvres, France. It has custody of the International Prototype of the Kilogram and houses the secretariat for this organization as well as hosting its formal meetings.

This is a list of units that are not defined as part of the International System of Units (SI), but are otherwise mentioned in the SI, because either the General Conference on Weights and Measures (CGPM) accepts their use as being multiples or submultiples of SI-units, they have important contemporary application worldwide, or are otherwise commonly encountered worldwide.

European Union Economic and poitical union of states located in Europe

The European Union (EU) is a political and economic union of 28 member states that are located primarily in Europe. It has an area of 4,475,757 km2 (1,728,099 sq mi) and an estimated population of about 513 million. The EU has developed an internal single market through a standardised system of laws that apply in all member states in those matters, and only those matters, where members have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market, enact legislation in justice and home affairs and maintain common policies on trade, agriculture, fisheries and regional development. For travel within the Schengen Area, passport controls have been abolished. A monetary union was established in 1999 and came into full force in 2002 and is composed of 19 EU member states which use the euro currency.

Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). The notation bar(g), though deprecated by various bodies, [2] represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.

Definition and conversion

The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100,000 Pa ≡ 100,000 N/m2.

SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation.

Pascal (unit) SI unit of pressure

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre. It is named after the French polymath Blaise Pascal.

Thus, 1 bar is equal to:

and 1 bar is approximately equal to:

The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference or standard pressure.

Pounds per square inch unit of pressure or stress

The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.

A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322387415 pascals. It is denoted by the symbol mmHg or mm Hg.



The word bar has its origin in the Greek word βάρος (baros), meaning weight. The unit's official symbol is bar; the earlier symbol b is now deprecated and conflicts with the use of b denoting the unit barn, but it is still encountered, especially as mb (rather than the proper mbar) to denote the millibar. Between 1793 and 1795, the word bar was used for a unit of weight in an early version of the metric system. [6]


Map showing atmospheric pressure in mbar or hPa Day5pressureforecast.png
Map showing atmospheric pressure in mbar or hPa
A tire pressure gauge displaying bar (outside) and pounds per square inch (inside) ReifendruckPruefen.jpg
A tire pressure gauge displaying bar (outside) and pounds per square inch (inside)

Atmospheric air pressure is often given in millibars, where standard atmospheric pressure at sea level is defined as 1013.25 mbar, 101.325  kPa, 1.01325 bar, which is about 14.7 pounds per square inch. Despite the millibar not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps. [7] [8] In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms. [9] [10]

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase in depth. In sea water with respect to the gravity variation, the latitude and the geopotential anomaly the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25). [11] As a result, decibars are commonly used in oceanography.

Many engineers worldwide use the bar as a unit of pressure because, in much of their work, using pascals would involve using very large numbers. In measurement of vacuum and in vacuum engineering, residual pressures are typically given in millibar, although torr or millimeter of mercury (mmHg) were historically common.

Engineers that specialize in technical safety for offshore petrochemical facilities would be expected to exclusively refer to explosion loads in units of bar or bars. A bar is a convenient unit of measure for pressures generated by low frequency vapor cloud explosions that are commonly considered as part of accidental loading risk studies.

In the automotive field, turbocharger boost is often described in bars outside the USA. Tire pressure is often specified in bar.

Unicode has characters for "mb" (㏔, U+33D4) and "bar" (㍴, U+3374), but they exist only for compatibility with legacy Asian encodings and are not intended to be used in new documents.

The kilobar, equivalent to 100 MPa, is commonly used in geological systems, particularly in experimental petrology.

"Bar(a)" and "bara" are sometimes used to indicate absolute pressures and "bar(g)" and "barg" for gauge pressures. This usage is deprecated and fuller descriptions such as "gauge pressure of 2 bar" or "2-bar gauge" are recommended. [2] [12]

See also

Related Research Articles

Centimetre unit of length equal to 1/100 of a metre

A centimetre or centimeter is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1/100. The centimetre was the base unit of length in the now deprecated centimetre–gram–second (CGS) system of units.

Litre non-SI unit of volume

The litre or liter is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm3), 1,000 cubic centimetres (cm3) or 1/1,000 cubic metre. A cubic decimetre occupies a volume of 10 cm×10 cm×10 cm and is thus equal to one-thousandth of a cubic metre.

Pressure measurement technique to measure pressure

Pressure measurement is the analysis of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure gauges or vacuum gauges. A manometer is a good example, as it uses a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

Standard conditions for temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

The torr is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).

Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 1013.25 mbar (101.325 kPa), equivalent to 760 mmHg (torr), 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

Hecto- is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekaton, meaning "hundred". In 19th century English it was sometimes spelled hecato, in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.

The humidex is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term humidex is a Canadian innovation coined in 1965. The humidex is a dimensionless quantity based on the dew point.

A decametre or dekameter is a very rarely used unit of length in the metric system equal to ten metres.

The North American High is an impermanent high-pressure area or anticyclone created by a formative process that occurs when cool or cold dry air settles over North America. In summer it is replaced with an Arctic Low, or if it moves to continental land, a North American Low.

Inch of mercury is a unit of measurement for pressure. It is still used for barometric pressure in weather reports, refrigeration and aviation in the United States.

Kilogram-force per square centimetre unit of pressure

A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square is a deprecated unit of pressure using metric units. It is not a part of the International System of Units (SI), the modern metric system. 1 kgf/cm2 equals 98.0665 kPa (kilopascals).

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.

Metre sea water Unit of pressure equal to one tenth of a bar

The metresea water (msw) is a unit of pressure used in underwater diving. It is defined as one tenth of a bar.


This article incorporates material from the Citizendium article "Bar (unit)", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.
  1. 1 2 International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 127, ISBN   92-822-2213-6, archived (PDF) from the original on 2017-08-14.
  2. 1 2 3 4 British Standard BS 350:2004 Conversion Factors for Units.
  3. "Nomenclature of the unit of absolut pressure, Charles F. Marvin, 1918" (PDF). Archived (PDF) from the original on 29 April 2017. Retrieved 6 May 2018.
  4. NIST Special Publication 1038 Archived 2016-03-19 at the Wayback Machine , Sec. 4.3.2; NIST Special Publication 811, 2008 edition Archived 2016-06-03 at the Wayback Machine , Sec. 5.2
  5. International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6
  6. Grave (unit)
  7. Canada, Environment (2013-04-16). "Canadian Weather at a Glance - Environment Canada". Archived from the original on 2 January 2018. Retrieved 6 May 2018.
  8. Canada, Environment (2013-04-16). "Canadian Weather - Environment Canada". Archived from the original on 2 January 2018. Retrieved 6 May 2018.
  9. US government atmospheric pressure map
  10. The Weather Channel
  11. Scientific Committee on Oceanic Research (1983). "Algorithms for computation of fundamental properties of seawater" (PDF). Archived (PDF) from the original on 2015-04-12. Retrieved 2014-05-11.
  12. "What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure) : FAQs : Reference : National Physical Laboratory". Archived from the original on 7 April 2016. Retrieved 7 February 2016.
Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
1 Pa≡ 1 N/m210−51.0197×10−59.8692×10−67.5006×10−30.000 145 037 737 730
1 bar105≡ 100 kPa

≡ 106  dyn/cm2

1.01970.98692750.0614.503 773 773 022
1 at98066.50.980665≡ 1 kgf/cm20.967 841 105 354 1735.559 240 114.223 343 307 120 3
1 atm1013251.013251.0332176014.695 948 775 514 2
1 Torr133.322 368 4210.001 333 2240.001 359 511/760 ≈ 0.001 315 7891 Torr

≈ 1  mmHg

0.019 336 775
1 lbf/in26894.757 293 1680.068 947 5730.070 306 9580.068 045 96451.714 932 572≡ 1 lbf/in2