The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's average atmospheric pressure at sea level.^[1]

The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (g_n = 9.80665 m/s²).^[2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10th General Conference on Weights and Measures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1013250 dynes per square centimetre (101325 Pa).^[3] This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."^[3]

In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure was commonly 1 atm (101.325 kPa) prior to 1982, but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry recommended that for the purposes of specifying the physical properties of substances, standard pressure should be precisely 100 kPa (1 bar).^[4]

A pressure of 1 atm can also be stated as:

≈ 1.033 kgf/cm²

≈ 10.33 m H₂O^[5]

≈ 760 mmHg^[6]

≈ 29.92 inHg^[6]

≈ 406.782 in H₂O^[5]

≈ 2116.22 pounds-force per square foot (lbf/ft²)

The notation ata has been used to indicate an absolute pressure measured in either standard atmospheres (atm)^{[7][better source needed]} or technical atmospheres (at).^[8]

• International Standard Atmosphere