One torr is 133.3223684210526 pascals, defined as exactly 1/760 of a standard atmosphere. It is very nearly but not exactly a millimetre of mercury, and in vacuum work that distinction occasionally matters.
About the units
The torr is named after Evangelista Torricelli, who built the first mercury barometer in 1643 and demonstrated that the atmosphere has weight. It was defined in 1954, alongside the standard atmosphere, as exactly 101,325 ÷ 760 pascals. The millimetre of mercury is defined differently — from the density of mercury and standard gravity — and comes to 133.322387415 Pa. The two differ by about one part in seven million. That is irrelevant in medicine and meteorology, where the units are treated as identical, and it is detectable in precision vacuum metrology.
The exact factor
The exact value is 101,325 ÷ 760 = 133.32236842105263... Pa. The scale of vacuum work is where the unit earns its place: atmospheric pressure is 760 torr, a rough vacuum is 1 to 25 torr, a high vacuum is 10⁻³ to 10⁻⁹ torr, and ultra-high vacuum below that. Expressed in pascals those become awkward decimals, which is why the torr and the millitorr persist despite SI recommending the pascal. One millitorr is 0.133 Pa.
Where you meet this conversion
Vacuum technology throughout: semiconductor fabrication, electron microscopy, thin film deposition, freeze drying and mass spectrometry. Vacuum pump specifications state ultimate pressure in torr or millibar depending on the manufacturer's origin, and comparing two pumps requires the conversion. Laboratory rotary evaporation uses millibar in European equipment and torr in American. In physics, the torr appears in older gas discharge and plasma literature where much of the foundational work was done.