Thebar is ametric unit ofpressure defined as 100,000 Pa (100 kPa), though not part of theInternational System of Units (SI). A pressure of 1 bar is slightly less than the current averageatmospheric pressure on Earth atsea level (approximately 1.013 bar).^[1][2] By thebarometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.

Bar
An aluminium cylinder of wall thickness 5 millimetres (0.20 in) after being applied with apressure of 700 bar.
General information
Unit system	Metric system
Unit of	pressure
Symbol	bar
Conversions
1 barin ...	... is equal to ...
SI units	100 kPa
CGS units	106 Ba
US customary units	14.50377 psi
Atmospheres	0.986923 atm

The bar and the millibar were introduced by the Norwegian meteorologistVilhelm Bjerknes, who was a founder of the modern practice ofweather forecasting, with the bar defined as onemegadyne per squarecentimeter.^[3]

TheSI brochure, despite previously mentioning the bar,^{[citation needed]} now omits any mention of it.^[1] The bar has been legally recognised in countries of theEuropean Union since 2004.^[2] The USNational Institute of Standards and Technology (NIST)deprecates its use except for "limited use inmeteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used".^[4] TheInternational Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".^[5]

Units derived from the bar include themegabar (symbol:Mbar),kilobar (symbol:kbar),decibar (symbol:dbar),centibar (symbol:cbar), andmillibar (symbol:mbar).

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

Thus,1 bar is equal to:

• 1,000,000 Ba (barye) (incgs units);

and 1 bar is approximately equal to:

• 0.98692327 atm
• 14.503774 psi
• 29.529983 inHg
• 750.06158 mmHg
• 750.06168 Torr
• 1019.716 centimetres of water (cmH₂O) (1 bar approximately corresponds to thegauge pressure of water at a depth of 10 meters).

1millibar (mbar) is equal to:

• 1×10⁻³ bar (0.001 bar)
• 100 Pa.

The wordbar has its origin in theAncient Greek wordβάρος (baros), meaningweight. The unit's official symbol isbar;^{[citation needed]} the earlier symbolb is now deprecated and conflicts with the uses ofb denoting the unitbarn orbit, but it is still encountered, especially asmb (rather than the propermbar) to denote the millibar. Between 1793 and 1795, the wordbar was used for a unit of mass (equal to the moderntonne) in an early version of the metric system.^[6]

Atmospheric air pressure wherestandard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about 14.7pounds per square inch. Despite the millibar not being anSI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibar as the values are convenient. After the advent of SI units, some meteorologists began usinghectopascals (symbol hPa) which are numerically equivalent to millibar; 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 ofEnvironment 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 ofhurricanes and other cyclonic storms.^[9][10]

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibar and the change in depth from the water surface inmetres. 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 thegeopotential anomaly the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25).^[11] As a result, decibar is commonly used inoceanography.

In scuba diving, bar is also the most widely used unit to express pressure, e.g. 200 bar being a full standard scuba tank, and depth increments of 10 metre of seawater being equivalent to 1 bar of pressure.

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 ofvacuum and invacuum engineering, residual pressures are typically given in millibar, althoughtorr ormillimeter of mercury (mmHg) were historically common.

Pressures resulting fromdeflagrations are often expressed in units of bar.^[12]

In the automotive field,turbocharger boost is often described in bar outside the United States.Tire pressure is often specified in bar. Inhydraulic machinery components are rated to the maximum system oil pressure, which is typically in hundreds of bar. For example, 300 bar is common for industrial fixed machinery.

In the maritime ship industries, pressures in piping systems, such as cooling water systems, is often measured in bar.

Unicode has characters for "mb" (U+33D4㏔SQUARE MB SMALL), "bar" (U+3374㍴SQUARE BAR) andミリバール (U+334A㍊SQUARE MIRIBAARU; "millibar" spelt inkatakana), 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 experimentalpetrology.

The abbreviations "bar(a)" and "bara" are sometimes used to indicateabsolute pressures, and "bar(g)" and "barg" forgauge pressures. The usage is deprecated but still prevails in the oil industry (often by capitalized "BarG" and "BarA"). As gauge pressure is relative to the current ambient pressure, which may vary in absolute terms by about 50 mbar, "BarG" and "BarA" are not interconvertible. Fuller descriptions such as "gauge pressure of 2 bars" or "2-bar gauge" are recommended.^[2][13]

• Centimetre or millimetre of water
• Conversion of units § Pressure or mechanical stress
• List of metric units
• Metric prefix
• Orders of magnitude (pressure) § 100kPa

This article incorporates material from theCitizendium article "Bar (unit)", which is licensed under theCreative Commons Attribution-ShareAlike 3.0 Unported License but not under theGFDL.

• Official SI website: Table 8. Non-SI units accepted for use with the SI
• US government atmospheric pressure map showing atmospheric pressure in mbar