The bar is a metric unit of pressure, equal to ten Newtons per square centimeter. The word bar is of greek origin báros meaning heavy.

The preferred SI unit of pressure is the Pascal (Pa), because the Pascal is based on square meters instead of square centimeters. 1 bar is equal to 100,000 Pa. The bar is still widely used by the general public.

Atmospheric air pressure is often given in millibars, where "standard" pressure is equal to 1013.2 millibars. If the measure is more correctly given in SI units, the hectopascal gives the same number as the millibar measure, i.e. 1000 mb = 1000 hPa. Thus the millibar and the hectopascal are directly interchangeable. This follows from the definition of the units.