托（符号为Torr），与毫米汞柱（符号为mmHg）近乎等价，为压强、压力的单位，但并非国际单位制单位 （SI unit） 的成员之一。

原本的 1 mmHg 是指「将幼细直管内的水银顶高一毫米之压力」，而正常之大气压力可以将水银升高 760 mm ，故此将一毫米水银定为大气压力的 1/760 倍。

不过，由于标准大气压在后来才被严谨定义，与之前所定义的正 760 mm 有轻微差异，现在两者皆被重新定义为 101325/760 Pa 。

The torr (symbol: Torr) is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 7005101325000000000♠101325/760 pascals (~133.3 Pa).

Historically, one torr was intended to be the same as one "millimetre of mercury". However, subsequent redefinitions of the two units made them slightly different (by less than 6993150000000000000♠0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.