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Standard atmosphere is a pressure defined as 101 325 Pa and used as unit of pressure (symbol: atm). Standard atmosphere is a non-SI unit that is internationally recognized. The 10th Conférence Générale des Poids et Mesures (10th CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1,013,250 dynes per square centimeter (101,325 Pa). This value is intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France, and as a practical matter, truly reflects the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris).

A pressure of 1 atm can also be stated as:

≡1.013 25 bar

≡101.325 kPa

≡1013.25 millibars (mbar, also mb) or hectopascals (hPa)

≡760 torr ^[B]

≈760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^{[B, C]}

≈29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^[C]

≈1.033 227 452 799 886 kgf/cm²

≈1.033 227 452 799 886 technical atmosphere

≈1033.227 452 799 886 cm–H₂O, 4 °C ^[A]

≈406.782 461 732 2385 in–H₂O, 4 °C ^[A]

≈14.695 948 775 5134 pounds-force per square inch (psi)

≈2116.216 623 673 94 pounds-force per square foot (psf)

^A 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.

^B Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.

^C NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C

In chemistry, the original definition of “Standard Temperature and Pressure” (STP) was a reference temperature of 0 °C (273.15 K) and pressure of 101.325 kPa (1 atm). However, in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.062 Torr).^[1]

In the United Kingdom, scuba divers and others often use the word atmosphere loosely (the correct term is "ambient pressure") to mean 1 bar (1000 millibars, or 100 000 Pa).

The unit technical atmosphere (at) is roughly equal to the gauge pressure under 10 m of water; 1 at = 98066.5 Pa.

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 10.197×10⁻⁶ at  = 9.8692×10⁻⁶ atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.

1. ^ IUPAC.org, Gold Book, Standard Pressure