Atmosphere (unit)

The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa (1.01325 bar). It is sometimes used as a reference or standard pressure.

Unit ofPressure
1 atm in ...... is equal to ...
   SI units   101.325 kPa
   U.S. customary units   14.69595 psi


It was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (g = 9.80665 m/s2).[1] It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Centigrade (later Celsius) scale of temperature by defining 100 °C as being the boiling point of water at this pressure. In 1954, the 10th Conférence Générale des Poids et Mesures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1013250 dynes per square centimetre (101325 Pa).[2] This defined both temperature and pressure independent of the properties of particular substance. In addition (the CGPM noted) there had been some misapprehension that it "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."[2]

In chemistry and in various industries, the reference pressure referred to in “Standard Temperature and Pressure” (STP) was commonly 1 atm (101.325 kPa) but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “standard pressure” should be precisely 100 kPa (1 bar).[3]

Pressure units and equivalencies

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (lbf/in2)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 0.000 145 037 737 730
1 bar 105 ≡ 100 kPa

≡ 106 dyn/cm2

1.0197 0.98692 750.06 14.503 773 773 022
1 at 98066.5 0.980665 ≡ 1 kgf/cm2 0.967 841 105 354 1 735.559 240 1 14.223 343 307 120 3
1 atm 101325 1.01325 1.0332 1 760 14.695 948 775 514 2
1 Torr 133.322 368 421 0.001 333 224 0.001 359 51 1/760 ≈ 0.001 315 789 1 Torr

≈ 1 mmHg

0.019 336 775
1 lbf/in2 6894.757 293 168 0.068 947 573 0.070 306 958 0.068 045 964 51.714 932 572 ≡ 1 lbf/in2

A pressure of 1 atm can also be stated as:

1.01325 bar
101325 pascals (Pa) or 101.325 kilopascals (kPa)
1013.25 millibars (mbar or mb)
760 torr (Torr)[n 1]
760.001 mmHg, 0 °C, subject to revision as more precise measurements of mercury’s density become available[n 2][n 3]
29.9213 inHg, 0 °C, subject to revision as more precise measurements of mercury’s density become available[n 3]
1.033227452799886 kgf/cm²
1.033227452799886 technical atmosphere
1033.227452799886 cm H2O, 4 °C[n 2]
406.7824617322385 in H2O, 4 °C[n 2]
14.6959487755134 pounds-force per square inch (psi)
2116.21662367394 pounds-force per square foot (psf)
= 1 ata (atmosphere absolute). The ata unit is used in place of atm to indicate that the pressure shown is the total ambient pressure, compared to vacuum, of the system being calculated or measured.[4] For example, for underwater pressures, a pressure of 3.1 ata would mean that the 1 atm of the air above water is included in this value and the pressure due to water would total 2.1 atm.
  1. ^ Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
  2. ^ 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.
  3. ^ a b NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C

Other applications

Scuba divers and others use the word atmosphere and "atm" in relation to pressures that are relative to mean atmospheric pressure at sea level (1.013 bar). For example, a partial pressure of oxygen is calibrated typically using air at sea level, so is expressed in units of atm.

The old European unit technical atmosphere (at) is equal to 1 kilogram-force per square centimetre (kgf/cm2), 98066.5 Pa.

See also


  1. ^ Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
  2. ^ a b BIPM Definition of the standard atmosphere
  3. ^, Gold Book, Standard Pressure
  4. ^ "The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.

ATM or atm may refer to:

Atmosphere (unit) or atm, a unit of atmospheric pressure

Automated teller machine, a cash dispenser or cash machine


Atmos may refer to:

An abbreviation for the Earth's atmosphere

The atmosphere (unit), a unit of pressureIt may also refer to:

ATMOS (festival) The Technical Festival of BITS Pilani Hyderabad Campus

Atmos Energy Corporation, a U.S. energy company

Atmos clock, a clock manufactured by Jaeger-LeCoultre, which runs on changes in ambient temperature

Oric Atmos, a home computer first marketed in 1984

Ambience (sound recording)

Dolby Atmos surround sound format

ATMOS Software, an Australian computer and video game company

Atmos (comics), a minor character in the 30th century DC Universe

Atmos Heating Systems, a British supplier of industrial and domestic heaters, including the Drainfree Condensing Boiler, and solar heating systems

ATMOS (ATMospheric Omission System), fictional technology that appears in the 2008 Doctor Who two-part serial "The Sontaran Stratagem" / "The Poison Sky"

ATMOS 2000, a self-propelled Israeli artillery system produced by Soltam Systems Ltd

EMC Atmos, a clustered computer storage system from EMC Corporation

Workshop on Algorithmic Approaches for Transportation Modeling, Optimization, and Systems (ATMOS)

Atmos, a summonable minion in the Playstation game Final Fantasy IX.

Atmosphere (disambiguation)

An atmosphere is a gas layer around a celestial body.

Atmosphere may also refer to:

Atmosphere (unit), a unit of pressure

Atmosphere of Earth

Extraterrestrial atmospheres

Stellar atmosphere

Bottled gas

Bottled gas is a term used for substances which are gaseous at standard temperature and pressure (STP) and have been compressed and stored in carbon steel, stainless steel, aluminum, or composite bottles known as gas cylinders.

CSIRO Oceans and Atmosphere

CSIRO Oceans and Atmosphere (O&A) is one of the current 8 Business Units (formerly: Flagships) of the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's largest government-supported science research agency.

Cabin pressurization

Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.

Standard atmosphere

Standard atmosphere may refer to:

A standard reference value for air pressure:

Atmosphere (unit), an approximation of the value at sea level

Atmospheric pressure, other reference values

One of various static atmospheric models of how atmospheric pressure, density, and temperature vary with altitude, such as:

The U.S. Standard Atmosphere, a series of models that give values for pressure, density, and temperature over a range of altitudes

The International Standard Atmosphere (ISA), an international standard model, defining typical atmospheric properties with altitude, at mid-latitude

Thermochemical cycle

Thermochemical cycles combine solely heat sources (thermo) with chemical reactions to split water into its hydrogen and oxygen components. The term cycle is used because aside of water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled.

If work is partially used as an input, the resulting thermochemical cycle is defined as a hybrid one.


The torr (symbol: Torr) is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).

Historically, one torr was intended to be the same as one "millimeter of mercury". However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.

The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.

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