Millimetre of mercury

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.^[5] For example, the U.S. and European guidelines on hypertension, in using millimeters of mercury for blood pressure,^[6] 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.

Relation to the torr[edit]

The precision of modern transducers is often insufficient to show the difference between the torr and the millimetre of mercury. The difference between these two units is about one part in seven million or 0.000015%.^[9] By the same factor, a millitorr is slightly less than a micrometre of mercury.

In medicine, pressure is still generally measured in millimetres of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum.


Routine pressure measurements in medicine include:


In physiology manometric units are used to measure Starling forces.