The torr (symbol: Torr) is a traditional unit of pressure, now defined as exactly 1/760 of a standard atmosphere, which in turn is defined as exactly 101325 pascals. Thus one torr is about 133.322368421053 pascals (the exact value being an infinitely repeating decimal fraction.)
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 (about 0.133322368421053 pascal).
The unit's name "torr" is written in lower case, while its symbol ("Torr") is always written with upper-case initial; including in combinations with prefixes and other units, as in "mTorr" (millitorr) or "Torr·L/s" (torr-liters per second). 
The torr is sometimes incorrectly denoted by the symbol "T", which is the SI symbol for the tesla.
Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born.
Over time, 760 millimeters of mercury 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 millimeter of mercury.
In 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM) to the currently accepted definition: one atmosphere is equal to 101325 pascals. The torr was then redefined as 1⁄760 of one atmosphere. This was necessary in place of the definition of the torr as one millimetre of mercury, because the height of mercury changes at different temperatures and gravities.
Manometric units of pressure 
Manometric units are units such as millimeters of mercury or centimeters of water that depend on an assumed density of a fluid and an assumed acceleration of gravity. The use of these units is discouraged. Nevertheless, manometric units are routinely used in medicine and physiology, and they continue to be used in areas as diverse as weather reporting and scuba diving.
Manometric results in medicine are sometimes given in torr. This is usually incorrect, since the torr and the millimeter of mercury are not the same thing. Pressures obtained with a manometer (or its transducer equivalent) should be reported in millimeters of mercury.
Conversion factors 
The millimeter of mercury by definition is 133.322387415 Pa (13.5951 g/cm3 × 9.80665 m/s2 × 1 mm ), which is approximated with known accuracies of density of mercury and gravitational acceleration.
The torr is defined as 1/760 of one standard atmosphere, while the atmosphere is defined as 101,325 pascals. Therefore, 1 Torr is equal to 101325⁄760 Pa. The decimal form of this fraction (133.322368421052...) is an infinitely long, periodically repeating decimal, as is its reciprocal.
The relationship between the torr and the millimeter of mercury is:
- 1 Torr = 0.999999857533699... mmHg
- 1 mmHg = 1.000000142466321... Torr
The difference between one millimeter of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (101.3250144354 kPa), is less than one part in seven million (or less than 0.000015%). This small difference is negligible for most applications outside metrology.
Other units of pressure include:
- The bar (symbol: bar), defined as 100 kPa exactly.
- The atmosphere (symbol: atm), defined as 101.325 kPa exactly.
- The torr (symbol: Torr), defined as 1⁄760 atm exactly.
These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures. The torr is used in high-vacuum physics and engineering.
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pounds per square inch|
|1 Pa||≡ 1 N/m2||10−5||1.0197×10−5||9.8692×10−6||7.5006×10−3||1.450377×10−4|
|1 bar||105||≡ 106 dyn/cm2||1.0197||0.98692||750.06||14.50377|
|1 at||0.980665 ×105||0.980665||≡ 1 kp/cm2||0.9678411||735.5592||14.22334|
|1 atm||1.01325 ×105||1.01325||1.0332||≡ p0||≡ 760||14.69595|
|1 Torr||133.3224||1.333224×10−3||1.359551×10−3||1.315789×10−3||≈ 1 mmHg||1.933678×10−2|
|1 psi||6.8948×103||6.8948×10−2||7.03069×10−2||6.8046×10−2||51.71493||≡ 1 lbF/in2|
See also 
- Devices similar to the modern barometer, using water instead of mercury, were studied by a number of scientists in the early 1640s (see History of the Barometer). Torricelli’s explanation of the principle of the barometer appears in a letter to Michelangelo Ricci dated 11 June 1644.
- "Rules and style conventions". NIST. Retrieved 2012-09-29.
- BIPM – Resolution 4 of the 10th CGPM
- National Physical Laboratory: Pressure units
- Note that a pressure of 1 bar (100000 Pa) is slightly less than a pressure of 1 atmosphere (101325 Pa).
- Cohen E.R. et al. Quantities, Units and Symbols in Physical Chemistry, 3rd ed. Royal Society of Chemistry, 2007 ISBN 0-85404-433-7 (IUPAC pdf copy)
- DeVoe H. Thermodynamics and Chemistry. Prentice-Hall, Inc. 2001 ISBN 0-02-328741-1