# What is the difference between the units 'torr' and 'mm of Hg'?

According to present definition torr and mm of Hg differ though slightly. I am still confused over it. Can someone write an explicit answer differentiating between them and the reason for differentiation as earlier they were considered same. The explanation is probably related to the changes is pressure at different heights so I want the exact definitions of these units that was used earlier and what modifications have been made?

• In addition to the confusion of the two different units, the unit names and unit symbols are sometimes mixed up. The name of one unit is “torr” and its symbol is “Torr”. The name of the other unit is “conventional millimetre of mercury” and its symbol is “mmHg”.
– user7951
Commented Oct 23, 2015 at 19:21

The two units torr, and mm of Hg were the same until they were redefined.

The torr was named after the Italian Evangelista Torricelli. 1 atmosphere is $101325\ \mathrm{Pa}$. The torr is defined as $1/760$ of an atmosphere. This is equal to $133.322\overline{368421052631578947}~\mathrm{Pa}$, which periodically infinitely repeats.

The mm of Hg was defined as the pressure generated by a column of mercury one millimeter high. Since the pressure of mercury depends on temperature and gravity, it is now redefined as

$$\mathrm{0.001\ mL \cdot 13595.1~mg \cdot 9.80665\ m/s^2 = 133.322387415 ~Pa}$$

This has caused a difference between the units of $0.000015~\%$, so unless you require extremely high precision, you can assume that $1\ \mathrm{torr} = 1\ \mathrm{mmHg}$

• that means 760 mm Hg is not exactly equal to 1 atm Commented Apr 9, 2015 at 16:33
• @DSinghvi: Yes to both questions.
– user467
Commented Apr 10, 2015 at 0:19
• As I said before, the difference between the values is so negligible, you can just assume that 1 torr = 1 mm Hg
– user3735
Commented Apr 10, 2015 at 0:20
• Very very few chemistry experiments are ever done to 0.000015% precision. Something like trying to measure Avogadro's Constant would be an exception.
– MaxW
Commented Jan 25, 2017 at 18:11
• Note that the error in assuming 1 torr = 1 mm Hg is probably spectacularly smaller than the local variation of g depending on altitude, latitude and longitude, or the error introduced by local atmospheric densities.
– user41033
Commented Oct 5, 2017 at 13:32