Bar (unit of pressure)

Definition and simple conversions

By definition, one bar equals 100,000 pascals (Pa). Because the pascal is the SI unit of pressure, conversions between bars and SI units are exact. Several commonly used multiples and subdivisions are:

• 1 bar = 100,000 Pa
• 1 millibar (mbar or mb) = 0.001 bar = 100 Pa
• 1 decibar (dbar) = 0.1 bar = 10,000 Pa
• Standard atmosphere = 101,325 Pa ≈ 1.01325 bar

Symbols, prefixes and usage

The unit name is written as "bar" and the accepted symbols include "bar" for the unit itself and standard SI prefixes such as "mbar" for millibar and "dbar" for decibar. Though not an SI base unit, the bar is commonly used alongside SI units in practical fields where its size is convenient.

History and applications

The bar arose as a practical unit for atmospheric and engineering pressures because its magnitude is comparable to ambient air pressure. It became popular in meteorology for reporting weather pressure (often in millibars), and in oceanography the decibar is convenient for expressing pressures related to depth. Engineers use bars for hydraulic and pneumatic systems because the numbers are compact and easy to interpret.

Notable distinctions and practical notes

Although convenient, the bar is not an SI base unit; users who require strict SI conformity convert to pascals. For weather maps and forecasters, millibars remain common in many countries, while other places use hectopascals (hPa), which are numerically identical to millibars (1 hPa = 1 mbar = 100 Pa). The decibar is particularly useful in oceanography because a pressure increase of about 1 dbar corresponds roughly to a one-meter increase of depth in seawater.

For further technical definitions and official conventions, see reference materials on units of pressure.