PSI vs Bar

• 11 bar = 14.5038 psi. A car tyre at 2.2 bar is about 32 psi, which is why European and American tyre placards look so different for the same tyre.
• 2Neither is SI. The SI pressure unit is the pascal, and engineering calculations usually work in kilopascals or megapascals. Both psi and bar persist because they produce convenient numbers at everyday pressures.
• 3The gauge-versus-absolute distinction causes more errors than the conversion does. Gauge pressure (psig, barg) is measured relative to atmosphere; absolute (psia, bara) includes it. They differ by about 14.7 psi or 1.013 bar, which is a large error at low pressures and a small one at high pressures.
• 4Compressor and vacuum work is where this bites hardest. A vacuum quoted as -0.9 bar gauge is 0.113 bar absolute, and the ratio — not the difference — is what determines the work done.
• 5The bar was deliberately defined as a round 100 kPa rather than as exactly one atmosphere, so bar and atmosphere are close but not equal.

The bar was introduced by the meteorologist Vilhelm Bjerknes in the early twentieth century, which is why atmospheric pressure is still reported in millibars (now formally hectopascals, numerically identical). Industry adopted it because 1 bar is close enough to atmospheric pressure that gauge readings are intuitive. The pascal, though the correct SI unit, is inconveniently small — one pascal is roughly the pressure of a sheet of paper resting on a table — so it is almost always used with a prefix.