Inches of Mercury to PSI Converter
Convert inHg to psi quickly and accurately for barometric pressure, aviation, and weather applications
Quick Conversions
Conversion Formula
PSI = inHg × 0.491154
inHg = PSI × 2.03602
The conversion factor between inHg and psi is based on the standard definition of each unit. One inch of mercury at 0°C equals approximately 0.491154 pounds per square inch. This relationship derives from the physical properties of mercury and the standardized definitions used in pressure measurement.
Conversion Examples
Example 1: Standard Atmospheric Pressure
Convert 29.92 inHg to psi:
PSI = 29.92 × 0.491154 = 14.696 psi
This represents standard atmospheric pressure at sea level, commonly used as a reference in aviation and meteorology.
Example 2: Low Pressure System
Convert 28.5 inHg to psi:
PSI = 28.5 × 0.491154 = 13.998 psi
Lower barometric readings like this often indicate stormy weather or low-pressure systems.
Example 3: High Pressure Condition
Convert 31.0 inHg to psi:
PSI = 31.0 × 0.491154 = 15.226 psi
High pressure readings typically correlate with fair weather and stable atmospheric conditions.
Detailed Conversion Table
| Inches of Mercury (inHg) | Pounds per Square Inch (psi) |
|---|---|
| 0.1 | 0.0491 |
| 0.5 | 0.2456 |
| 1 | 0.4912 |
| 2 | 0.9823 |
| 3 | 1.4735 |
| 4 | 1.9646 |
| 5 | 2.4558 |
| 10 | 4.9115 |
| 15 | 7.3673 |
| 20 | 9.8231 |
| 25 | 12.2789 |
| 29.92 | 14.696 |
| 30 | 14.7346 |
| 35 | 17.1904 |
| 40 | 19.6462 |
| 50 | 24.5577 |
| 75 | 36.8366 |
| 100 | 49.1154 |
What is Inches of Mercury (inHg)?
Inches of mercury (inHg) is a unit of pressure measurement representing the height of a mercury column in a barometer. This non-SI unit remains widely used in the United States for barometric pressure readings in weather forecasts and aviation altimeter settings.
The measurement is based on the pressure exerted by a column of mercury exactly one inch tall at 0°C under standard gravity. At this reference temperature, 1 inHg equals 3,386.389 pascals. The unit has historical significance dating back to early barometer designs using mercury-filled tubes.
- Standard atmospheric pressure: 29.92 inHg at sea level
- Commonly used in aviation for altimeter calibration
- Weather reports in the US typically cite barometric pressure in inHg
- Aircraft manifold pressure gauges often display readings in inHg
What is PSI (Pounds per Square Inch)?
PSI, or pounds per square inch, measures pressure as the force of one pound applied over an area of one square inch. This imperial unit is extensively used in North America for tire pressure, hydraulic systems, pneumatic equipment, and various industrial applications.
The precise term is pound-force per square inch (lbf/in²), distinguishing it from pound-mass. In the SI system, 1 psi equals 6,894.76 pascals. PSI provides an intuitive pressure measurement for many practical applications in engineering and everyday use.
- Tire pressure specifications typically range from 30-35 psi for passenger vehicles
- Standard atmospheric pressure equals approximately 14.7 psi
- Used extensively in plumbing, HVAC, and compressed air systems
- Industrial pressure gauges commonly display measurements in psi
Applications in Aviation
The inHg to psi conversion plays a critical role in aviation, where both units serve distinct purposes. Altimeters in aircraft use inHg for barometric pressure settings, while manifold pressure indicators on piston engines display readings in inHg to show engine power output.
Pilots flying above 18,000 feet set their altimeters to the standard pressure of 29.92 inHg (14.696 psi) regardless of actual sea level pressure. This standardization creates “flight levels” that allow safe separation between aircraft. Below 18,000 feet, pilots adjust altimeters to local barometric pressure for accurate altitude readings.
Engine manifold pressure, measured in inHg, indicates the air pressure in the intake manifold. Higher manifold pressure generally means more power output. Pilots monitor this gauge alongside RPM to properly manage engine performance and fuel efficiency during different phases of flight.
Weather Forecasting Applications
Meteorologists rely on barometric pressure measurements to predict weather patterns and track atmospheric changes. In the United States, weather reports typically express barometric pressure in inHg, while some scientific applications prefer psi or millibars.
Rising barometric pressure generally indicates improving weather conditions with clearer skies, while falling pressure often signals approaching storms or precipitation. Rapid pressure changes can indicate severe weather development, making accurate pressure monitoring essential for forecasting.
- Pressure above 30.20 inHg (14.83 psi): High pressure, typically fair weather
- Pressure 29.80-30.20 inHg (14.63-14.83 psi): Normal range
- Pressure below 29.80 inHg (14.63 psi): Low pressure, possible storms
- Hurricane pressure can drop below 27.00 inHg (13.26 psi)
Common Pressure Conversions
Beyond inHg and psi, pressure can be expressed in numerous units depending on the application and geographical region. Converting between these units requires specific conversion factors.
| From | To | Multiply By |
|---|---|---|
| inHg | psi | 0.491154 |
| inHg | Pascal (Pa) | 3,386.39 |
| inHg | kPa | 3.38639 |
| inHg | bar | 0.0338639 |
| inHg | mmHg (torr) | 25.4 |
| inHg | atm | 0.0334211 |
| psi | inHg | 2.03602 |
| psi | Pascal (Pa) | 6,894.76 |
| psi | kPa | 6.89476 |
| psi | bar | 0.0689476 |
Frequently Asked Questions
How do I convert inHg to psi?
Multiply the inHg value by 0.491154 to get psi. For example, 25 inHg × 0.491154 = 12.279 psi. You can also use our calculator above for instant conversion.
What is the standard atmospheric pressure in both units?
Standard atmospheric pressure at sea level is 29.92 inHg or 14.696 psi. This value serves as the reference point for many pressure calculations and is used in aviation as the standard altimeter setting above 18,000 feet.
Why does aviation use inHg instead of psi?
Aviation adopted inHg because early altimeters were based on mercury barometers. The tradition continues in the US and Canada, while many other countries use hectopascals (hPa) or millibars (mb). The inHg scale provides appropriate resolution for altimeter settings and manifold pressure readings.
Is the conversion factor different at different temperatures?
Yes, technically the conversion varies slightly with temperature. The standard conversion (1 inHg = 0.491154 psi) assumes 0°C (32°F). At 60°F, the conversion is 1 inHg = 0.48977 psi. For most practical applications, the difference is negligible and the standard conversion factor is used.
Can I use this conversion for vacuum measurements?
Yes, the conversion works for vacuum measurements. Vacuum is often expressed as negative gauge pressure or absolute pressure below atmospheric. For example, a vacuum of 15 inHg equals approximately 7.37 psi below atmospheric pressure.
What pressure reading indicates a strong hurricane?
Major hurricanes can have central pressures below 27.0 inHg (13.26 psi). The lower the pressure, the stronger the hurricane. For reference, Hurricane Wilma (2005) reached 25.69 inHg (12.62 psi), one of the lowest pressures ever recorded in the Atlantic.
How accurate does my conversion need to be?
For aviation and weather applications, rounding to 2-3 decimal places is typically sufficient. Industrial applications may require more precision depending on the specific use case. Our calculator provides results to 4 decimal places for maximum accuracy.
What other units can I convert inHg to?
Besides psi, inHg can be converted to pascals (Pa), kilopascals (kPa), bar, millibar (mb), atmospheres (atm), millimeters of mercury (mmHg or torr), and other pressure units. Each conversion uses a specific multiplication factor.