When expressed as a measurement, an atmosphere or standard atmosphere is a unit of pressure roughly equal to the average atmospheric pressure at sea level on the earth. It is defined as the pressure under 760 mm of mercury.

1 atm

= 101325 Pa = 101325 kg/m·s² = 101325 N/m²

= 1013.25 hPa = 1013.25 mbar

= 101.325 kPa

= 1.01325 bar

= 760 torr = 760 mm of mercury (mmHg)

= ca. 29.921 259 842 519 685 039 363 inches of mercury

= ca. 14.695949 pounds per square inch (PSI)

In Britain, scuba divers and others often use the word atmosphere loosely to mean 1 bar (1000 millibars or 100000 Pa).

The unit technical atmosphere (at) is defined as the pressure under 10 m of water, 1 at = 98066.5 Pa.