The normal air pressure at sea level is 14.7 pounds per square inch. Standard atmospheric pressure can also be expressed using different units, including 1013.25 millibars, 1013.25 hectoPascals, 760 millimeters of mercury and 29.92 inches of mercury.
Atmospheric pressure pertains to the downward force applied by the weight of air against a particular area on Earth's surface. Standard atmospheric pressure is measured by weighing the column of air that occupies 1 square inch on Earth's surface. This column of air starts at sea level and stretches upward to the uppermost layer of the atmosphere. The measured weight of air per 1 square inch of area at sea level is approximately 14.7 pounds.