Torr

The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly ⁠1/760⁠ of a standard atmosphere (101325 Pa). Thus one torr is exactly ⁠101325/760⁠ pascals (≈ 133.32 Pa).

Historically, one torr was intended to be the same as one "millimetre of mercury", but subsequent redefinitions of the two units made the torr marginally lower (by less than 0.000015%).

The torr is not part of the International System of Units (SI). Even so, it is often combined with the metric prefix milli to name one millitorr (mTorr), equal to 0.001 Torr.

The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.