Science, Tech, Math › Science Torr Definition in Science Share Flipboard Email Print mevans / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated December 03, 2019 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. History 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. Nomenclature 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). Sources 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.