The bar is a unit of pressure equal to exactly 100,000 pascals, or 100 kilopascals. Its symbol is bar. It is not part of the International System of Units, but it is tolerated alongside it and it is used throughout European engineering, because it happens to sit almost exactly on standard atmospheric pressure: one atmosphere is 1.01325 bar.
That coincidence is the whole reason for its success. A pressure quoted in bar can be read as very nearly a number of atmospheres, so a reading of six bar in a compressed air line means about six times the pressure outside, which is a fact an engineer can use without arithmetic. The name comes from the Greek word for weight, and the unit was proposed by the Norwegian meteorologist Vilhelm Bjerknes around 1909.
Diving uses it because the sea obliges. Every ten metres of seawater adds almost exactly one bar, so a diver at twenty metres is under three bar of absolute pressure, counting the atmosphere above. Decompression tables, gas consumption and cylinder pressures all follow from that one relationship, and the whole practice of diving is easier to teach in bar than in any other unit.
Everyday machinery works at a few bar. Car tyres are inflated to between two and two and a half, workshop compressed air runs at six to eight, mains water arrives at three to six, and an espresso machine extracts at nine, a figure so standard that it appears on the front of the machine. Watches carry water resistance ratings in bar, where ten bar corresponds nominally to a hundred metres of depth.
The distinction between gauge and absolute pressure matters more here than anywhere. A tyre gauge reading 2.2 bar means 2.2 bar above the surrounding air, so the absolute pressure inside is 3.2. Industrial practice writes barg for gauge and bara for absolute, and mixing the two is a common source of error in specifications and in safety calculations.
Standards bodies have discouraged the bar for decades in favour of the pascal, without much effect. The kilopascal is the coherent alternative and is used in Canada, Australia and parts of Asia, but a hundred kilopascals reads less naturally than one bar, and habit has proved stronger than the recommendation.
One bar equals 100,000 pascals, 100 kilopascals, 1000 millibars, or about 14.5 pounds per square inch.
| Unit | Symbol | 1 Bar equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanopascal | nPa | 100000000000000 nPa | 1 × 10-14 bar | Bars to Nanopascals |
| Micropascal | µPa | 100000000000 µPa | 1 × 10-11 bar | Bars to Micropascals |
| Millipascal | mPa | 100000000 mPa | 0.00000001 bar | Bars to Millipascals |
| Centipascal | cPa | 10000000 cPa | 0.0000001 bar | Bars to Centipascals |
| Decipascal | dPa | 1000000 dPa | 0.000001 bar | Bars to Decipascals |
| Pascal | Pa | 100000 Pa | 0.00001 bar | Bars to Pascals |
| Newton per square meter | N/m² | 100000 N/m² | 0.00001 bar | Bars to Newtons per square meter |
| Decapascal | daPa | 10000 daPa | 0.0001 bar | Bars to Decapascals |
| Hectopascal | hPa | 1000 hPa | 0.001 bar | Bars to Hectopascals |
| Kilopascal | kPa | 100 kPa | 0.01 bar | Bars to Kilopascals |
| Megapascal | MPa | 0.1 MPa | 10 bar | Bars to Megapascals |
| Gigapascal | GPa | 0.0001 GPa | 10000 bar | Bars to Gigapascals |
| Terapascal | TPa | 0.0000001 TPa | 10000000 bar | Bars to Terapascals |
| Atmosphere | atm | 0.986923266716 atm | 1.01325 bar | Bars to Atmospheres |
| Pound per square inch | psi | 14.503773773 psi | 0.0689475729317 bar | Bars to Pounds per square inch |
| Kilopound per square inch | ksi | 0.0145037743897 ksi | 68.94757 bar | Bars to Kilopounds per square inch |
| Megapound per square inch | Mpsi | 0.0000145037743897 Mpsi | 68947.57 bar | Bars to Megapounds per square inch |