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Most pressure units come from the height of a liquid column. The torr is literally one millimeter of mercury, and standard atmospheric pressure is 760 torr, which is 101,325 Pa, or 1.01325 bar. Psi is pounds of force per square inch, and one psi is 6,894.757 Pa.
Two traps. The first is gauge against absolute: a tire at 32 psi is 32 psi above the atmosphere, so its absolute pressure is about 46.7 psi, and roughly 14.7 psi has to be added to any gauge reading before it goes into a gas law. The second is kgf/cm2, common on industrial labels in Asia at 98,066.5 Pa; the symbol is properly kgf/cm2, though it is widely written as kg/cm2, which is technically a mass divided by an area.
| Pa | kPa | bar | psi | atm | Torr | kgf/cm² | |
|---|---|---|---|---|---|---|---|
| 1 Pa | 1 | 0.001 | 1.0000e-05 | 0.000145 | 9.8692e-06 | 0.007501 | 1.0197e-05 |
| 1 kPa | 1000 | 1 | 0.01 | 0.145038 | 0.009869 | 7.500615 | 0.010197 |
| 1 bar | 100000 | 100 | 1 | 14.503774 | 0.986923 | 750.061505 | 1.019716 |
| 1 psi | 6894.757 | 6.894757 | 0.068948 | 1 | 0.068046 | 51.714918 | 0.070307 |
| 1 atm | 101325 | 101.325 | 1.01325 | 14.695949 | 1 | 759.99982 | 1.033227 |
| 1 Torr | 133.3224 | 0.133322 | 0.001333 | 0.019337 | 0.001316 | 1 | 0.00136 |
| 1 kgf/cm² | 98066.5 | 98.0665 | 0.980665 | 14.223344 | 0.967841 | 735.559066 | 1 |
How many psi are in a bar? 14.5038. Going the other way, one psi is 0.0689476 bar. For tire pressures the shortcut of 14.5 is accurate enough, which is why 2.4 bar and 35 psi describe nearly the same pressure.
What is standard atmospheric pressure? 101,325 Pa, which is 1.01325 bar, 14.696 psi, or 760 torr. It is the single value that ties every unit in this category together, and it defines the atmosphere itself.
What is the difference between psi and psig? Psi can mean absolute or gauge; psig always means gauge, the pressure above the atmosphere. Tire and plumbing gauges read psig, so add about 14.7 psi when you need an absolute figure for a gas law calculation.
What is a torr? One millimeter of mercury, or 133.3224 Pa. Vacuum work uses it because a mercury manometer reads directly in millimeters, and 760 torr is one atmosphere.