mmHg stands for millimeters of mercury. mmHg is a manometric unit of pressure, which is almost identical to 1 Torr. In SI units, 1 mmgH = 133.3 Pa. The standard atmospheric pressure is 760 mmHg.
Manometric units are units that measure pressure using the change in height change of a liquid like mercury. The mercury in a tube would rise or fall depending on the pressure difference, so you would literally be measuring millimeters of mercury (mmHg).
Manometric units are imprecise because they can vary from location to location, depending on the density and temperature of the liquid. Even the local gravity may affect the measurement slightly (this is more problematic than you’d think because both gravity and air pressure are affected by altitude).
In addition to the most-popular manometric unit millimeters of mercury (mmHg), other manometric units include centimeters of mercury (cmHg), inches of mercury (inHg or “Hg), millimeters of water (mmH2O), centimeters of water (cmH2O), and inches of water (inH2O, “H2O, or iwc). Each of these units changes depending on the temperature.
Although mercury is toxic, it was very useful for making early pressure gauges for the same reasons mercury is the liquid of choice in thermometers, which you can read here.
Because of the effort to standardize measurements of pressure, torr was actually redefined to be different than mmHg. Technically, the standard mmHg is now 1.00000014247 Torr.
| 1 mmHg = | 133.3 Pa | 1.316*10-3 atm | 1 Torr |
This article was a definition of mmHg. If you are interested in all the different, strange units that are used to measure pressure, I wrote an entire article explaining what each unit is, and how to convert between them. Or, if there is a unit you are especially interested in, you can check out one of the other unit definitions I’ve written.
Other Pressure Measurements:
Pa
mbar or bar
Torr
atm
at
psi or ksi
mmHg
Ba
pz
