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The torr is a non-SI unit of pressure (symbol: torr) which is equal to 1/760 of an atmosphere (symbol: atm).[note 1] It was selected to be approximately equal to the fluid pressure exerted by 1 millimetre of liquid mercury (symbol: mmHg) and thus 1 torr ≈ 1 mmHg.

It was named after Evangelista Torricelli, an Italian physicist and mathematician who, in 1644, first explained that a barometer responded to fluctuations in atmospheric pressure.[1][2]

Brief history

Prior to Torricelli, it was thought that the atmosphere was weightless, but Torricelli believed that the atmosphere did have weight and that the water barometers then in use were responding to fluctuations in the weight of the atmospheric. Water barometers required tubes of water that were about 10.5 meters (≈ 35 feet) tall. Knowing that mercury was about 14 times as heavy as water, Torricelli built a manometer that used mercury instead of water and demonstrated that it only required a tube that was about 76 centimeters (≈ 30 inches) tall which was about 1/14 of the height needed for a water barometer. That, in effect, proved that the atmosphere did indeed have weight. He is considered to have provided the first modern explanation of atmospheric pressure.

Over time, 760 millimetres of mercury (abbreviated: mmHg) came to be regarded as the standard atmospheric pressure. In honor of Torricelli, the torr was defined as a unit of pressure equal to one mmHg.

Before 1954, the torr was defined as being equal to 1 mmHg. In 1954, the definition of the atmosphere was revised by the 10th Conférence Générale des Poids et Mesures (10th CGPM)[3] to the currently accepted definition that one atmosphere is equal to 101,325 pascals. Because mercury's specific gravity varies with temperature, the height of mercury equivalent to one atmosphere also varies with temperature. Thus it was necessary to re-define the torr as 1/760 of an atmosphere and as being approximately 1 mmHg.

Various units of pressure

Pressure Units
per square inch

per square centimeter

1 Pa ≡ 1 N/m2 10−5 9.8692×10−6 7.5006×10−3 145.04×10−6 1.01972×10−5
1 bar 100,000 ≡ 106 dyn/cm2 0.98692 750.06 14.504 1.01972
1 atm 101,325 1.01325 ≡ 1 atm 760 14.696 1.03323
1 torr 133.322 1.3332×10−3 1.3158×10−3 ≡ 1 torr
≈ 1 mmHg
19.337×10−3 1.35951×10−3
1 psi 6,894.76 68.948×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2 7.03059×10−2
1 kgf/cm2 98,066.5 0.980665 0.967838 735.5576 14.22357 ≡ 1 kgf/cm2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 9.8692×10−6 atm  = 7.5006×10−3 torr, etc.
Note: mmHg is an abbreviation for millimetre of mercury


  1. There is no consensus in the technical literature about whether the name of the torr should be "Torr" or "torr". Nor is there any consensus about whether the symbol for that unit of pressure should be "Torr" or "torr". Both the United Kingdom's National Physical Laboratory (see Pressure Units) and New Zealand's Measurement Standards Laboratory (see Barometric Pressure Units) use "torr" as the name and as the symbol. An extensive search of the website of the U.S. National Institute of Standards and Technology found no clear-cut definitions or any consensus. Therefore, this article uses "torr" as both the name and the symbol.


  1. Brief History of the Barometer From the website of the Barometer.WS company.
  2. 1644, Torricelli, Evangelista. "Letter to Michelangelo Ricci Concerning the Barometer." From the Classic Chemistry Archive. Carmen Giunti, Editor. Le Moyne College. 21 Jan. 2002. Available online here on the website of Le Moyne College in Syracuse, New York. Torricelli's letter includes a sketch of his barometer and explains his conclusions.
  3. BIPM – Resolution 4 of the 10th CGPM