Bar to PSI

The bar is defined as exactly 100,000 pascals, deliberately close to average sea-level air pressure, which is 1.01325 bar. That makes it intuitive: a tire at 2.3 bar holds a bit more than twice the pressure of the atmosphere above it. The bar is not an SI unit, but it is used everywhere in European engineering, car manuals and diving.

Weather maps use its thousandth, the millibar, which equals one hectopascal. A strong high-pressure system around 1,040 mbar is 15.08 psi absolute; a deep storm near 960 mbar is 13.92 psi.

Espresso machines advertise 15 or 19 bar pumps, but good extraction happens at about 9 bar at the coffee puck, which is 130.5 psi. European car manuals list tire pressures like 2.3 bar front and 2.5 bar rear, which are 33.4 and 36.3 psi on an American gauge.

Scuba is where bar figures get large. A European steel cylinder filled to 200 bar holds 2,901 psi, and a 232-bar fill is 3,365 psi. The common US aluminum 80 cylinder is rated at 3,000 psi, about 207 bar, so divers traveling between regions convert these numbers constantly. Pressure rises about 1 bar for every 10 meters of seawater, which the atm to bar page relates to the standard atmosphere.

Multiply by 15 and subtract 3%: 2.4 bar × 15 = 36, minus 1.1 gives 34.9 psi, against the exact 34.8. Small errors matter more at high pressure, so for cylinders and hydraulic systems use the full factor. For psi readings going the other way, use psi to bar.