Millimetre of mercury

A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322387415 pascals or exactly 133.322 pascals. It is denoted mmHg or mm Hg.

Although not an SI unit, the millimetre of mercury is still often encountered in some fields; for example, it is still widely used in medicine, as demonstrated for example in the medical literature indexed in PubMed. For example, the U.S. and European guidelines on hypertension, in using millimeters of mercury for blood pressure, are reflecting the fact (common basic knowledge among health care professionals) that this is the usual unit of blood pressure in clinical medicine.

One millimetre of mercury is approximately 1 torr, which is 1/760 of standard atmospheric pressure (101325/760 ≈ 133.322368 pascals). Although the two units are not equal, the relative difference (less than 0.000015%) is negligible for most practical uses.