Torr Definition in Science

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A torr is a unit of pressure defined to be exactly 1/760 of one standard atmosphere. One torr is approximately 133.32 Pa. Prior to the redefinition of the unit, one torr was equal to one mm Hg. While this is close to 1/760 of the standard atmospheric pressure, the two definitions differed by about 0.000015%.

1 Torr = 133.322 Pa = 1.3158 x 10-3 atm.


The torr is named for Italian physicist and mathematician Evangelista Torricelli. In 1644, Torricelli described the principle of the barometer and atmospheric pressure. He demonstrated the first mercury barometer.


The name of the unit (torr) is always written in lower case letters. However, the symbol is always written using a capital "T" (Torr). For example, mTorr and millitorr are correct. Although the symbol "T" is sometimes used to refer to torr, this is incorrect and may cause confusion with the symbol for magnetic field strength (tesla or T).


  • BS 350: Part 1: 1974 – Conversion factors and tables. (1974). British Standards Institution. p. 49.
  • Cohen E. R. et al. (2007). Quantities, Units and Symbols in Physical Chemistry (3rd ed.). Royal Society of Chemistry. ISBN 0-85404-433-7.
  • DeVoe, H. (2001). Thermodynamics and Chemistry. Prentice-Hall, Inc. ISBN 0-02-328741-1.
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Helmenstine, Anne Marie, Ph.D. "Torr Definition in Science." ThoughtCo, Aug. 28, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 28). Torr Definition in Science. Retrieved from Helmenstine, Anne Marie, Ph.D. "Torr Definition in Science." ThoughtCo. (accessed March 24, 2023).