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The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. They are not SI units, nor are they cgs units, but they are accepted for use with the SI. The bar is still widely used in descriptions of pressure because it is about the same as atmospheric pressure.

The bar, decibar and millibar are defined as:

• 1 bar = 100 kPa (kilopascals) = 1,000,000 dynes per square centimeter (baryes)
• 1 dbar = 0.1 bar = 10 kPa = 100,000 dyn/cm²
• 1 mbar = 0.001 bar = 0.1 kPa = 1,000 dyn/cm²

(A pascal is one newton per square meter.)

The word bar has its origin in the Greek word βάρος (baros), meaning weight. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially in "mb" rather than the proper "mbar" for millibars.

The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929.

Atmospheric air pressure is often given in millibars where "standard" sea level pressure is defined as 1013.25 mbar (hPa), equal to 1.01325 bar. Despite millibars not being an SI unit, they are still used locally in meteorology in some countries to describe atmospheric pressure. The SI unit is the pascal (Pa), with 1 mbar = 100 Pa = 1 hPa = 0.1 kPa. Meteorologists worldwide have long measured air pressure in millibars. After the introduction of SI units, many preferred to preserve the customary pressure figures. Therefore, some continue to use millibars under their own name, while others use hectopascals (which are equivalent to millibars) so they could stick to the same numeric scale. Similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. In particular, Canadian weather reports use kilopascals (which could also be called centibars).

Americans are familiar with the millibar in US reports of hurricanes and other cyclonic storms, where lower central pressure generally means higher winds and a stronger storm.

In some countries, pressure is measured with reference to atmospheric pressure. This is gauge pressure and denoted by barg, often written with no spaces, spoken "bar gauge", and sometimes using symbols such as 'bar(g)'. For example, if someone says that their car tyres are pressurised to 2.3 bar they actually mean bar gauge: the pressure in the tyre is really 3.3 bar, but only 2.3 bar above atmospheric, which is the scale a tyre gauge would read. When absolute pressure is desired, it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of units of measure for this purpose is now deprecated, with qualification of the physical property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute pressure is 3.3 bar".

In water, a measurement in decibars is approximately equal to the depth in meters; 1 decibar occurs at a depth of 1.019716 m. As a result, decibars are commonly used in oceanography.

Unicode has a character for "mb": (㏔), but exists only for compatibility with legacy Asian encodings; it should not be used in new texts.

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 10.197×10⁻⁶ at  = 9.8692×10⁻⁶ atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.

• Official SI website: Table 8. Non-SI units accepted for use with the SI
• Conversion factors from bar to various pressure units

• Conversion of units