mTorr

mTorr, short for millitorr, is a unit of pressure used in vacuum technology. It represents one thousandth of a Torr, so 1 mTorr equals 0.001 Torr. Since 1 Torr is defined as 133.322368 Pascals, 1 mTorr is about 0.133 pascals. The Torr is a non-SI unit named after Evangelista Torricelli.

In practice, vacuum engineers express very low pressures using Torr and its submultiples, including mTorr, µTorr (microscopic Torr), and nTorr (nanotorr). For example, 1 mTorr equals 1000 µTorr and 1,000,000 nTorr. These units help distinguish different regions of vacuum performance and measurement sensitivity.

mTorr commonly appears in discussions of high-vacuum systems, such as those used in thin-film deposition, surface science, electron microscopy, and semiconductor fabrication. Measurement in this range is achieved with instruments like capacitance manometers, Pirani gauges, and ionization gauges, each with different operating ranges and calibration references. The choice of gauge depends on the expected pressure regime and the required accuracy.

Because Torr and its subunits are not SI units, conversions to pascals (and, by extension, to other SI-derived units) are standard in technical documentation. While the exact applicability of mTorr can vary by instrument and context, it remains a common, practical unit for describing pressures in the upper end of the vacuum spectrum and the transition toward ultra-high vacuum.