Convert Inch of Mercury to Pascal (inHg → Pa)

Converting inches of mercury to pascals is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.

1. 1
   Take your value in inches of mercury

   Start with the number of inches of mercury (inHg) you want to convert.
2. 2
   Multiply by 3,386.389

   The conversion factor from inHg to Pa is 3,386.389. Multiply your value by this number.
3. 3
   Read the result in pascals

   The result is your value in pascals (Pa).

Multiply the value in inches of mercury by 3,386.389. For the reverse direction, multiply by 0.000295.

• ✓1 inHg ≈ 3386 Pa ≈ 33.86 mbar.
• ✓29.92 inHg = 1013.25 mbar = 1 atm.
• ✓US pilots use inHg; European pilots use hPa (mbar).

• ✗Confusing inHg with mmHg — factor of 25.4.
• ✗Mixing US and European altimeter units on a single flight plan.
• ✗Using inHg on scientific papers — prefer Pa.

What is the Inch of Mercury?

The inch of mercury equals exactly 25.4 mmHg (or about 3,386.4 pascals) and is the American aviation standard for altimeter settings and historic barometer readings. Pilots set their altimeters to the local atmospheric pressure in inHg before takeoff and landing — standard 'sea-level pressure' is 29.92 inHg. American weather reports historically used inHg before transitioning to millibars/hPa, and home barometers in the US still display inHg. The unit relates to the mmHg (25.4 mmHg = 1 inHg), the millibar (1 inHg ≈ 33.864 mbar), the kilopascal (1 inHg ≈ 3.386 kPa), and the psi (1 inHg ≈ 0.491 psi). Aviation's continued use of inHg in the United States reflects regulatory tradition rather than scientific advantage; international ATC outside the US uses hectopascals.

• US aviation altimeter QNH settings
• US residential barometers
• US weather forecasts (older)

What is the Pascal?

The pascal is the SI unit of pressure, equal to 1 newton per square meter (1 N/m²). Named after French mathematician and physicist Blaise Pascal (1623–1662), it became the SI standard in 1971. One pascal is a very small pressure: atmospheric pressure at sea level is about 101,325 Pa, the human breath exerts roughly 100 Pa above ambient, and tire pressure is in the hundreds of thousands of pascals. Because the pascal is small, the kilopascal (1 kPa = 1,000 Pa), megapascal (1 MPa = 10⁶ Pa), and gigapascal (1 GPa = 10⁹ Pa) are more commonly used in practice. Modern weather forecasts use hectopascals (1 hPa = 100 Pa = 1 millibar). The pascal relates to the bar (1 bar = 100,000 Pa), the psi (1 psi ≈ 6,895 Pa), the atmosphere (1 atm = 101,325 Pa), and mmHg (1 mmHg ≈ 133.3 Pa). Scientific and engineering publications use the pascal universally.

• Scientific and engineering calculations
• Ventilation and HVAC airflow pressures
• Acoustic pressure (sound) measurements