Convert Inch of Mercury to Millimeter of Mercury (inHg → mmHg)
The inch of mercury is the American aviation standard for altimeter settings and historic barometer readings.
Inch of Mercury to Millimeter of Mercury Conversion Table
10 common values| Inch of Mercury | Millimeter of Mercury |
|---|---|
| 1 inHg | 25.400003 mmHg |
| 5 inHg | 127.00001 mmHg |
| 10 inHg | 254.00003 mmHg |
| 25 inHg | 635.00007 mmHg |
| 50 inHg | 1,270.0001 mmHg |
| 100 inHg | 2,540.0003 mmHg |
| 200 inHg | 5,080.0005 mmHg |
| 500 inHg | 12,700.001 mmHg |
| 1,000 inHg | 25,400.003 mmHg |
| 5,000 inHg | 127,000.01 mmHg |
How to Convert Inch of Mercury to Millimeter of Mercury Manually
Step by StepConverting inches of mercury to millimeters of mercury is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.
- 1Take your value in inches of mercuryStart with the number of inches of mercury (inHg) you want to convert.
- 2Multiply by 25.400003The conversion factor from inHg to mmHg is 25.400003. Multiply your value by this number.
- 3Read the result in millimeters of mercuryThe result is your value in millimeters of mercury (mmHg).
Formula
Multiply the value in inches of mercury by 25.400003. For the reverse direction, multiply by 0.03937.
mmHg = inHg × 25.400003inHg = mmHg × 0.03937Tips
Use these in everyday conversions- 1 inHg ≈ 3386 Pa ≈ 33.86 mbar.
- 29.92 inHg = 1013.25 mbar = 1 atm.
- US pilots use inHg; European pilots use hPa (mbar).
Common Mistakes
Avoid these- 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.
About Inch of Mercury and Millimeter of Mercury
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)
Sea-level standard: 29.92 inHg. Mountain pass at 3000 m: ~21 inHg. US aviation QNH given in inHg.
What is the Millimeter of Mercury?
The millimeter of mercury equals approximately 133.322 pascals and is the universal medical unit for blood pressure measurements worldwide. Normal blood pressure is around 120/80 mmHg (systolic/diastolic). The unit's origin is the manometer: a column of mercury rises 1 mm for every 133 Pa of pressure difference. Mercury barometers historically measured atmospheric pressure (760 mmHg = 1 atm at sea level). Beyond medicine, mmHg appears in vacuum-system specifications, certain laboratory contexts, and altimeter settings (in inHg in the US, in mmHg or hPa elsewhere). The mmHg relates to the torr (1 mmHg ≈ 1.0000003 torr — essentially identical), the pascal (1 mmHg ≈ 133.3 Pa), the atmosphere (760 mmHg = 1 atm), and inches of mercury (1 inHg = 25.4 mmHg). The unit's persistence in medicine reflects historical inertia and the precision of mercury manometers.
- Blood pressure measurement globally
- Vacuum-system specifications
- Historical scientific texts
Normal blood pressure: 120/80 mmHg. Atmospheric: 760 mmHg. Good vacuum: <1 mmHg.