Bar (unit)

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar).[1][2] By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.

A pressure of 700 bar flattened this length of aluminium tubing, which had a wall thickness of 5 millimetres (0.20 in).
General information
Unit systemMetric system
Unit ofPressure
1 bar in ...... is equal to ...
   SI units   100 kPa
   CGS units   1.0×106 Ba
   US customary units   14.50377 psi
   Atmospheres   0.99 atm

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]

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]

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