A millimeter of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high and now defined as precisely 133.322387415 pascals. It is denoted by the symbol "mmHg Although not an SI unit, the millimeter of mercury is still routinely used in medicine, meteorology and many other scientific fields. One millimeter of mercury is approximately 1 Torr, or 1/760 of standard atmospheric pressure. The two units are not exactly equal; however, the difference (less than ) is negligible for most practical uses.