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Standard atmosphere (unit)

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Unit ofPressure
1 atm in ...... is equal to ...
   SI units   101.325 kPa
   US customary units   14.69595 psi
   other metric units   1.013250 bar
Aneroid barometer for household use from c. 1925

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 (gn = 9.80665 m/s2).[2] It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Celsius temperature scale, which defined 100 °C (212 °F) 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]

Pressure units and equivalencies[edit]

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch
(Pa) (bar) (at) (atm) (Torr) (lbf/in2)
1 Pa 1 Pa = 10−5 bar 1 Pa = 1.0197×10−5 at 1 Pa = 9.8692×10−6 atm 1 Pa = 7.5006×10−3 Torr 1 Pa = 0.000145037737730 lbf/in2
1 bar 105 = 1.0197 = 0.98692 = 750.06 = 14.503773773022
1 at 98066.5 0.980665 0.9678411053541 735.5592401 14.2233433071203
1 atm 101325 1.01325 1.0332 760 14.6959487755142
1 Torr 133.322368421 0.001333224 0.00135951 1/7600.001315789 0.019336775
1 lbf/in2 6894.757293168 0.068947573 0.070306958 0.068045964 51.714932572

A pressure of 1 atm can also be stated as:

101325 pascals (Pa)
1.01325 bar
1.033 kgf/cm2
1.033 technical atmosphere
10.33 m H2O, 4 °C[n 1]
760 mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[n 1][n 2]
760 torr (Torr)[n 3]
29.92 inHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[n 2]
406.782 in H2O, 4 °C[n 1]
14.6959 pounds-force per square inch (lbf/in2)
2116.22 pounds-force per square foot (lbf/ft2)
= 1 ata (atmosphere absolute).

The ata unit is used in place of atm to indicate the total pressure of the system, compared to a vacuum.[5] For example, an underwater pressure of 3 ata would mean that this pressure includes 1 atm of air pressure and thus 2 atm due to the water.[citation needed]


  1. ^ a b c This is the customarily accepted value for cm–H2O, 4 °C. It is precisely the product of 1 kg-force per square centimeter (one technical atmosphere) times 1.013 25 (bar/atmosphere) divided by 0.980 665 (one gram-force). It is not accepted practice to define the value for water column based on a true physical realization of water (which would be 99.997 495% of this value because the true maximum density of Vienna Standard Mean Ocean Water is 0.999 974 95 kg/L at 3.984 °C). Also, this "physical realization" would still ignore the 8.285 cm–H2O reduction that would actually occur in a true physical realization due to the vapor pressure over water at 3.984 °C.
  2. ^ a b NIST value of 13.595 078(5) g/mL assumed for the density of Hg at 0 °C
  3. ^ Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.

See also[edit]


  1. ^ "Water Pressures at Ocean Depths". NOAA Pacific Marine Environmental Laboratory. Retrieved 11 October 2022.
  2. ^ Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
  3. ^ a b "BIPM - Resolution 4 of the 10th CGPM". www.bipm.org.
  4. ^ IUPAC.org, Gold Book, Standard Pressure
  5. ^ "The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.