Bar (unit)

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is exactly equal to 7005100000000000000♠100000 Pa and is slightly less than the average atmospheric pressure on Earth at sea level.

The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909, while he was the director of the Meteorological Office in London.

Use of the bar is deprecated by various bodies. The BIPM lists it as one of the "non-SI units [that authors] should have the freedom to use" but does not include it among the "Non-SI units accepted for use with the SI", and the NIST includes it in the list of units to avoid and recommends the use of kilopascals (kPa) and megapascals (MPa) instead. The IAU also lists it under "Non-SI units and symbols whose continued use is deprecated." As of 2004, the bar is legally recognized in countries of the European Union.

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, represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.