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.

The standard atmosphere (atm) is a pressure unit defined as exactly 101,325 pascals, representing the average atmospheric pressure at sea level on Earth. It serves as a reference point in chemistry for gas law calculations and in engineering for rating pressure vessel capacities. One atmosphere equals approximately 1.01325 bar, 14.696 psi, or 760 mmHg.