bar
Pressureexact by definition
The bar is defined as exactly 100 000 pascals. The definition is exact and deliberately round, chosen because it is very close to average atmospheric pressure at sea level, which is 1.01325 bar. It is not an SI unit but is accepted for use with the SI and is entrenched in meteorology, hydraulics and diving.
Watch out: A bar is close to an atmosphere but not equal to one: 1 atm is 1.01325 bar, a difference of 1.3 percent. Bar readings on industrial gauges are usually gauge pressure, sometimes written barg, meaning pressure above ambient rather than absolute.
| 1 bar | 100 kPa |
The name comes from the Greek baros, meaning weight, and the unit was introduced for meteorological work in the early twentieth century. The millibar became the standard unit of surface pressure on weather charts and survives there, although modern practice writes the numerically identical hectopascal.
About the bar
The bar earns its place by being round. Working in pascals means carrying six digits for ordinary atmospheric pressure, and working in atmospheres means carrying the awkward constant 101 325. The bar splits the difference: it is exactly 100 kPa, close enough to atmospheric that "2 bar" and "twice atmospheric" are interchangeable in casual speech, yet exact enough that no conversion error creeps in.
Two forms dominate in practice. The millibar, 100 Pa, is the traditional meteorological unit, and a typical sea-level reading of 1013 mb is one you may have seen on a barometer dial. Meteorological services have largely relabelled it the hectopascal, which is exactly the same size, so 1013 mb and 1013 hPa are the identical number. Diving uses the other end: pressure underwater rises by about one bar for every 10 metres of seawater, which makes the bar the natural unit for gas planning and decompression tables.