Convert Inch of Mercury to Torr (inHg → Torr)

Converting inches of mercury to torr 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 25.400006

   The conversion factor from inHg to Torr is 25.400006. Multiply your value by this number.
3. 3
   Read the result in torr

   The result is your value in torr (Torr).

Multiply the value in inches of mercury by 25.400006. For the reverse direction, multiply by 0.03937.

• ✓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 Torr?

The torr equals approximately 1.0000003 mmHg (essentially identical, 133.322 Pa) and is named after Italian physicist Evangelista Torricelli (1608–1647), who invented the mercury barometer. The torr is widely used in vacuum-system specifications, laboratory instruments, and high-vacuum physics. Vacuum levels are categorized: 'low vacuum' is 760 to 1 torr, 'medium vacuum' 1 to 10⁻³ torr, 'high vacuum' below 10⁻³ torr. Mass spectrometers, electron microscopes, and semiconductor manufacturing systems require high vacuum (10⁻⁶ to 10⁻⁹ torr). The torr relates to the mmHg (essentially equal), the pascal (1 torr ≈ 133.3 Pa), the millibar (1 torr ≈ 1.333 mbar), and the atmosphere (760 torr = 1 atm). Modern scientific publications increasingly use the pascal, but the torr remains entrenched in vacuum technology.

• Vacuum-chamber pressure readings
• Semiconductor manufacturing
• Older physics literature