To convert psi to bar, multiply by 0.068948. 1 psi equals 0.068948 bar, about 6.8% of the atmosphere at sea level.
| psi | bar |
|---|---|
| 1 psi | 0.068948 bar |
| 2 psi | 0.1379 bar |
| 5 psi | 0.34474 bar |
| 10 psi | 0.68948 bar |
| 25 psi | 1.7236893 bar |
| 50 psi | 3.4473786 bar |
| 100 psi | 6.8947573 bar |
| 250 psi | 17.236893 bar |
| 500 psi | 34.473786 bar |
| 1000 psi | 68.947573 bar |
Definition: The pressure created by one pound of force pushing on one square inch of area.
History: PSI grew out of the imperial engineering tradition - especially the steam engine era, when engineers needed a practical way to describe boiler pressure using units they already had (pounds and inches) rather than inventing something new.
Current use: Car and bike tire pressure gauges in the US, home plumbing and HVAC systems, and scuba tank ratings are all commonly quoted in PSI.
Definition: A bar is defined as exactly 100,000 pascals - close enough to Earth's average sea-level atmospheric pressure (1 atm is about 1.01325 bar) that "1 bar" feels roughly like normal outdoor pressure.
History: Norwegian meteorologist Vilhelm Bjerknes - one of the founders of modern weather forecasting - introduced the bar in 1909, named from the Greek "baros" (weight), specifically because meteorologists needed a pressure unit that lined up neatly with everyday atmospheric conditions.
Current use: European tire pressure gauges, espresso machines (a good shot typically pulls at around 9 bar), scuba depth gauges, and industrial pressure equipment.
The factor is 0.0689476, which is derived rather than defined, so any decimal you see has been rounded somewhere.
No simple round multiplier is close enough to be useful, so this is one to look up rather than estimate. The exact factor is 0.0689476.
Going the other way, one bar is 14.504 psi, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 20 psi is about 1.38 bar.