How pressure units relate
Pressure is force spread over an area, so every unit is some force divided by some area. The SI unit is the pascal, one newton per square meter, which is tiny: a sheet of paper lying on a desk presses down with about one pascal. Practical units are multiples of it. A kilopascal is 1,000 Pa, a megapascal is a million, a bar is exactly 100,000 Pa, and a millibar is 100 Pa, identical to the hectopascal that weather maps now use. The US customary unit is the pound per square inch, and one psi works out to 6,894.757 Pa, so a bar is 14.5038 psi.
Two older units survive because they describe a column of mercury. One millimeter of mercury, also called a Torr, is 133.322 Pa, and one inch of mercury is 3,386.39 Pa. Blood pressure and vacuum work stay in mmHg; American barometers and aircraft altimeters read in inHg. The standard atmosphere ties them together: 1 atm = 101,325 Pa = 14.696 psi = 760 mmHg = 29.9213 inHg = 1,013.25 mbar. Machinery from Europe and Asia often uses kgf/cm2, which is 98.0665 kPa, close enough to a bar that the two are frequently swapped by mistake.
Worked examples
Tires: a car door placard that says 32 psi means 32 x 6.894757 = 220.6 kPa, or 2.21 bar. A European manual asking for 2.5 bar wants 36 psi. Bicycle tires run much higher, 100 psi being 6.9 bar, while a truck tire at 110 psi is 758 kPa. Remember that these are cold pressures; driving warms the air and can add 4 to 6 psi.
Weather: a barometer reading of 30.12 inHg is 1,020 mbar, a fair-weather high. A hurricane with a central pressure of 940 mbar is 27.76 inHg and 13.63 psi. Altitude matters too: Denver sits near 24.9 inHg of actual station pressure, which is why airliners reset altimeters constantly. Around the house, municipal water arrives at 40 to 80 psi (276 to 552 kPa), an espresso machine pulls a shot at 9 bar (130 psi), and a pressure washer rated 2,000 psi is 13.8 MPa.
Gauge versus absolute, and other cautions
Almost every gauge you will touch reads zero when it is sitting in open air, which means it measures the difference from atmospheric pressure. That is gauge pressure, written psig. Absolute pressure counts up from a perfect vacuum and is written psia; it equals gauge plus 14.696 psi at sea level. A tire at 32 psig holds 46.7 psia. Science, refrigeration charts and vacuum specifications usually mean absolute, so check before you plug a number into an equation such as the ideal gas law, which only works in absolute units.
This converter treats your number as a plain quantity, so the same factors apply whether you are working in gauge or absolute; the result line adds the absolute equivalent as a convenience. Vacuum levels are a special case, usually quoted as inches of mercury below atmosphere or in Torr, where 1 Torr is roughly 1 mmHg and a rough industrial vacuum is a few Torr.