14.693646
psi
101.320759
kPa
101,320.76
Pa
0.999958
atm
14.693646
psi
101.320759
kPa
101,320.76
Pa
0.999958
atm
The Inches of Mercury to PSI Converter converts pressure from inches of mercury (inHg) to pounds per square inch (PSI). The conversion factor is 1 inHg = 0.491154 PSI. Inches of mercury is the traditional pressure unit used in American meteorology, aviation, and HVAC systems.
The inch of mercury derives from the mercury barometer: standard atmospheric pressure supports a column of mercury approximately 29.92 inches (760 mm) high. American weather reports, aviation altimeter settings, and barometer readings use inHg as their primary pressure unit. When a weather forecast reports 'the barometer is at 30.15 inches,' it means the atmospheric pressure is 30.15 inHg, which equals about 14.81 PSI or 102.1 kPa.
In aviation, the altimeter setting (QNH) is given in inches of mercury in the United States and Canada. Pilots adjust their altimeters based on the local barometric pressure in inHg — the standard setting being 29.92 inHg (1013.25 hPa). Accurate inHg-to-PSI conversion is important for aircraft pressurization system specifications and maintenance.
In HVAC and refrigeration, vacuum levels in air conditioning systems are often measured in inches of mercury. A system vacuum of 29 inHg (close to atmospheric) indicates a near-complete vacuum in the system before charging with refrigerant. Understanding the PSI equivalent helps technicians assess system conditions. Our converter also provides kPa output for international specifications.
The formula: PSI = inHg × 0.491154. This derives from: 1 inHg = 3,386.389 Pa and 1 PSI = 6,894.757 Pa, so 1 inHg = 3,386.389 / 6,894.757 ≈ 0.491154 PSI.
For kilopascals: kPa = inHg × 3.38639.
Standard references: 29.9213 inHg = 14.696 PSI = 1 atm, 1 inHg = 0.491 PSI = 3.386 kPa. A barometric reading of 30.00 inHg = 14.73 PSI = 101.59 kPa. HVAC vacuum of 29 inHg = 14.24 PSI.
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Standard atmospheric pressure
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Results
High pressure system
1 inHg = 0.491154 PSI. Standard atmospheric pressure (29.92 inHg) equals 14.696 PSI.
PSI = inHg × 0.491154. For example, 30 inHg × 0.491154 = 14.735 PSI.
29.92 inHg = 14.695 PSI. This is standard atmospheric pressure, used as the default altimeter setting in aviation.
American weather services traditionally use inches of mercury because early barometers measured the height of a mercury column in inches. The tradition persists in US weather forecasting.
High pressure systems typically produce readings above 30.20 inHg (14.84 PSI). Very high readings above 30.50 inHg (14.98 PSI) are associated with clear, calm weather.
Low pressure readings below 29.50 inHg (14.49 PSI) suggest approaching storms. Hurricane central pressures can drop below 27.00 inHg (13.26 PSI).
Pilots set their altimeters using the local barometric pressure in inHg (QNH setting). At FL180 and above, all pilots use 29.92 inHg (standard pressure).
HVAC technicians use inHg to measure vacuum in refrigeration systems. A deep vacuum of 500 microns is about 0.02 inHg.
Multiply by 25.4. Since 1 inch = 25.4 mm, 1 inHg = 25.4 mmHg. Standard atmosphere: 29.92 inHg = 760 mmHg.
inHg can refer to either absolute or gauge pressure. In weather and aviation, it is absolute. In HVAC vacuum measurements, it is typically gauge (relative to atmosphere).
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