|Named after||Evangelista Torricelli|
|1 Torr in ...||... is equal to ...|
|SI derived units||133.3224 Pa|
|British Gravitational System||0.01933677 psi|
Historically, one torr was intended to be the same as one " millimeter of mercury", but 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). It is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.
The unit 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 unit symbols, as in "mTorr" (millitorr) or "Torr⋅L/s" (torr-litres per second).  The symbol (uppercase) should be used with prefix symbols (thus, mTorr and millitorr are correct, but mtorr and milliTorr are not).
The torr is sometimes incorrectly denoted by the symbol "T", which is the SI symbol for the tesla, the unit measuring the strength of a magnetic field. Although frequently encountered, the alternative spelling "Tor" is incorrect.
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 at 0 °C came to be regarded as the standard atmospheric pressure. In honour of Torricelli, the torr was defined as a unit of pressure equal to one millimeter of mercury at 0 °C. However, since the acceleration due to gravity – and thus the weight of a column of mercury – is a function of elevation and latitude (due to the rotation and non- sphericity of the Earth), this definition is imprecise and varies by location.
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 yields a precise definition that is unambiguous and independent of measurements of the density of mercury or the acceleration due to gravity on Earth.
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 due to 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.
The torr is defined as 1/760 of one standard atmosphere, while the atmosphere is defined as 101325 pascals. Therefore, 1 Torr is equal to 101325/760 Pa. The decimal form of this fraction (133.322368421052631578947) is an infinitely long, periodically repeating decimal ( repetend length: 18).
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||Pound per square inch|
|1 Pa||1 Pa ≡ 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 = 0.000 145 037 737 730 lbf/in2|
|1 bar||105||≡ 100
≡ 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|
- 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 29 September 2012.
- BIPM – Resolution 4 of the 10th CGPM.
- National Physical Laboratory: Pressure units.
- BS 350: Part 1: 1974 – Conversion factors and tables. British Standards Institution. 1974. p. 49.
- 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.