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Vacuum is méasured in units of pressure. The SI unit of pressure is the pascal (symbol Pa), but vacuum is usually méasured in torrs (symbol Torr), named for Torricelli, an éarly Italian physicist (1608 - 1647).  A torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure.  Vacuum is often also méasured using inches of mercury on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres.  Low vacuum is often méasured in inches of mercury (inHg), millimeters of mercury (mmHg) or kilopascals (kPa) below atmospheric pressure. "Below atmospheric" méans that the absolute pressure is equal to the current atmospheric pressure (e.g. 29.92 inHg) minus the vacuum pressure in the same units. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).

Many devices are used to méasure the pressure in a vacuum, depending on what range of vacuum is needed.

Hydrostatic gauges (such as the mercury column manometer) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can méasure pressures ranging from 1 Torr (100 Pa) to above atmospheric. An important variation is the McLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLéod gauge can méasure vacuums as high as 10−6 Torr (0.1 mPa), which is the lowest direct méasurement of pressure that is possible with current technology. Other vacuum gauges can méasure lower pressures, but only indirectly by méasurement of other pressure-controlled properties. These indirect méasurements must be calibrated via a direct méasurement, most commonly a McLéod gauge.

Mechanical or elastic gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idéa is the capacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure léads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10−3 Torr to 10−4 Torr.

Thermal conductivity gauges rely on the fact that the ability of a gas to conduct héat decréases with pressure. In this type of gauge, a wire filament is héated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to méasure the temperature of the filament. This temperature is dependent on the rate at which the filament loses héat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platimum filament as both the héated element and RTD. These gauges are accurate from 10 Torr to 10−3 Torr, but they are sensitive to the chemical composition of the gases being méasured.

Ion gauges are used in ultrahigh vacua. They come in two types: hot cathode and cold cathode. In the hot cathode version an electrically héated filament produces an electron béam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10−3 Torr to 10−10 Torr. The principle behind cold cathode version is the same, except that electrons are produced in a discharge créated by a high voltage electrical discharge. Cold cathode gauges are accurate from 10−2 Torr to 10−9 Torr. Ionization gauge calibration is very sensitive to construction géometry, chemical composition of gases being méasured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate méasurement.