Engineering Guide
Torr and mmHg: The Engineer's Complete Vacuum Pressure Reference
Published June 27, 2026 · by Industrial Unit Converter Editorial Team
Torr and mmHg: the identical twins of pressure measurement
For all practical engineering purposes, 1 Torr = 1 mmHg. Both represent the pressure exerted by a 1-millimeter column of mercury under standard gravity. Both equal 1/760 of a standard atmosphere. Both convert to 0.01934 psi. The theoretical difference (0.000015%) is smaller than the accuracy of any commercially available pressure gauge.
The Torr is named after Evangelista Torricelli, the Italian physicist who invented the mercury barometer in 1643. The unit was formally adopted in 1954 to replace the slightly ambiguous "mmHg" (which technically depends on mercury density at a specific temperature) with an exact definition: 1 Torr = 1/760 atm.
In practice, vacuum engineers use "Torr" and high-pressure engineers use "mmHg". Same number, same conversion, different convention.
The vacuum pressure ladder: 15 orders of magnitude
Vacuum pressure spans from atmospheric (760 Torr) to ultra-high vacuum (10⁻¹² Torr). Expressing this range in psi would require values from 14.7 to 1.9 × 10⁻¹⁴, which is unwieldy. The Torr scale makes the vacuum world navigable:
| Pressure Range | Torr | psi (absolute) | Technology | Application |
|---|---|---|---|---|
| Atmospheric | 760 | 14.7 | (no gas conversion) | Sea-level air pressure |
| Rough vacuum | 760-1 | 14.7-0.019 | Rotary vane pumps, diaphragm pumps | Vacuum packaging, filtration, vacuum forming |
| Medium vacuum | 1-10⁻³ | 0.019-1.9×10⁻⁵ | Roots blowers, oil-sealed rotary pumps | Vacuum distillation, freeze-drying, degassing |
| High vacuum | 10⁻³-10⁻⁹ | 1.9×10⁻⁵-1.9×10⁻¹¹ | Turbomolecular pumps, diffusion pumps | Semiconductor PVD/CVD, mass spectrometry, electron microscopy |
| Ultra-high vacuum (UHV) | <10⁻⁹ | <1.9×10⁻¹¹ | Ion pumps, cryopumps, getter pumps | Surface science, particle accelerators, gravitational wave detectors |
Each order of magnitude in Torr corresponds to a different vacuum pump technology, a different pressure gauge type, and a different set of engineering challenges. The Torr (and its subdivision, the milliTorr or micron: 1 mTorr = 0.001 Torr = 1 micron of mercury) is the universal language across this 15-decade span.
Converting Torr to psi (and back)
1 Torr = 0.0193368 psi (exact: 14.69595 psi / 760 Torr)
1 psi = 51.7149 Torr
760 Torr = 14.696 psi (standard atmospheric pressure, the most important equivalence in vacuum engineering)
Quick mental shortcuts:
- 10 Torr ≈ 0.19 psi (medium vacuum, thin-film evaporation)
- 1 Torr ≈ 0.019 psi (high vacuum boundary)
- 100 Torr ≈ 1.93 psi (rough vacuum, vacuum distillation)
- 1 psi ≈ 51.7 Torr (compressed air increments)
Where Torr dominates in engineering practice
1. Semiconductor manufacturing
CVD (chemical vapor deposition), PVD (physical vapor deposition), and plasma etch processes operate at 1 mTorr to 10 Torr. A typical PECVD (plasma-enhanced CVD) chamber operates at 1-5 Torr. Converting this to psi: 1 Torr = 0.0193 psi, a tiny absolute pressure. The vacuum pump system (typically a dry pump + turbomolecular pump combination) is specified with an ultimate pressure in Torr (e.g., 10⁻⁶ Torr) and a pumping speed in liters per second (L/s) or cubic feet per minute (CFM).
2. Pharmaceutical freeze-drying (lyophilization)
Lyophilization chamber pressure during primary drying: 0.1-0.5 Torr (0.002-0.010 psi). The ice condenser (cold trap) operates at 0.01-0.1 Torr. These pressures are far below anything a standard pressure gauge can measure. Specialized Pirani, thermocouple, or capacitance manometer gauges calibrated in Torr are required. The conversion to psi is needed for the ASME pressure vessel registration of the chamber, which is stamped in psi even though the operating pressure is in Torr.
3. Vacuum distillation and evaporation
A wiped-film evaporator operating at 10 Torr (0.19 psia) reduces the boiling point of heat-sensitive materials. Converting the operating Torr to psi absolute verifies that the vessel is operated within its ASME nameplate vacuum rating (typically "FV", full vacuum, 15 psi external). A 50-Torr process is 0.97 psia, well within the FV rating, but the verification requires the conversion.
4. Medical vacuum systems
NFPA 99 (Health Care Facilities Code) specifies medical-surgical vacuum systems at 15-19 inHg gauge (approximately 380-480 Torr absolute). The vacuum pump is rated in CFM at the specified vacuum level, and the receiver tank is an ASME pressure vessel rated for full vacuum (15 psi external). Converting between inHg gauge, Torr absolute, and psi is routine in hospital mechanical equipment documentation.
Torr, mmHg, mbar, and Pa: the vacuum unit quartet
| Unit | 1 atm equivalent | Best used for |
|---|---|---|
| Torr | 760 | US vacuum technology, semiconductor |
| mmHg | 760 | Barometric pressure, blood pressure, laboratory |
| mbar (millibar) | 1,013.25 | European vacuum equipment, meteorology |
| Pa (pascal) | 101,325 | SI standard, European vacuum (1 Pa = 0.0075 Torr) |
Key conversions:
- 1 Torr = 1.33322 mbar
- 1 mbar = 0.75006 Torr
- 1 Pa = 0.0075006 Torr
- 1 Torr = 133.322 Pa
European vacuum pump manufacturers (Leybold, Pfeiffer Vacuum, Edwards' European division) typically specify in mbar, while US specifications use Torr. The 1.333 factor is worth memorizing for any engineer working across both markets.
Gauge pressure in vacuum systems: a special trap
Vacuum systems use absolute pressure (psia, Torr absolute) exclusively because the concept of "gauge pressure" breaks down below atmospheric: you cannot have a negative absolute pressure, only a gauge pressure that reads negative relative to atmosphere. A vacuum gauge reading "25 inHg vacuum" means 25 inHg below atmospheric pressure, equivalent to about 125 Torr absolute (760 − 25 × 25.4 = 760 − 635 = 125 Torr). The conversion to absolute psi: 125 / 51.715 = 2.42 psia. The conversion to psig: 2.42 − 14.7 = −12.28 psig.
Always work in absolute pressure for vacuum calculations. Convert to gauge only for comparison with positive-pressure systems.
Frequently asked questions
Q: At what pressure does "vacuum" start?
"Vacuum" is any pressure below atmospheric (760 Torr). But the practical thresholds are: rough vacuum (< 760 Torr, achievable with a simple pump), medium vacuum (< 1 Torr), and high vacuum (< 10⁻³ Torr, requiring specialized pumps and seals). For ASME pressure vessel purposes, "full vacuum" means the vessel can withstand atmospheric external pressure (15 psi) with the interior evacuated. This is a structural rating, not a process condition.
Q: Why do vacuum gauges read in Torr instead of psi?
Because vacuum pressures span 15 orders of magnitude. Expressing 10⁻⁶ Torr as 1.9 × 10⁻⁸ psi is technically correct but thoroughly impractical. The Torr scale gives readable numbers across the entire range: 760 (atmosphere), 1 (medium vacuum), 10⁻³ (high vacuum), 10⁻⁹ (UHV). Using psi for these values would require scientific notation, which is error-prone for operators reading gauges on a production floor.