or the acceleration due to gravity on Earth. 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 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. Conversion factors The millimeter of mercury by definition is ( × × ), which is approximated with known accuracies of density of mercury and standard gravity. The torr is defined as of one standard atmosphere, while the atmosphere is defined as pascals. Therefore, 1 Torr is equal to Pa. The decimal form of this fraction () is an infinitely long, periodically repeating decimal (repetend length: 18). The relationship between the torr and the millimeter of mercury is: 1 Torr = mmHg 1 mmHg = Torr The difference between one millimeter of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (), is less than one part in seven million (or less than ). 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 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. See also Atmosphere (unit) Centimetre of water Conversion of units Inch of mercury Outline of the metric system Pascal (unit) Pressure head Pressure References External links NPL – pressure units Non-SI metric units Units of pressure Mercury (element)