The bar is a metric pressure unit equal to 100,000 pascals, widely used in meteorology, oceanography, and industrial applications because it closely approximates atmospheric pressure at sea level. Tire pressure gauges in Europe, scuba diving depth ratings, and weather maps all commonly reference bars or millibars. One bar equals approximately 0.9869 atmospheres or 14.504 psi.

PSI (pounds per square inch) is an imperial pressure unit equal to about 6,894.76 pascals, the standard measurement for tire pressure, compressed air systems, and hydraulic equipment in the United States. Almost every American gas station air pump and pressure washer displays ratings in PSI, making it one of the most recognized pressure units worldwide. One PSI equals approximately 0.06895 bar or 6.895 kPa.