What is inch of mercury?
Inch of mercury (inHg) is a pressure unit representing the pressure exerted by a one-inch column of mercury. It is used in aviation altimetry, barometric pressure readings, and vacuum measurements in the United States.
Real-world uses
Inches of mercury are used in US aviation for altimeter settings (standard is 29.92 inHg), weather reporting on American TV broadcasts, and HVAC system vacuum measurements. American barometers are traditionally scaled in inHg.
History
Inches of mercury originate from the mercury barometer invented by Torricelli in 1644. The use of inches (rather than millimetres) reflects the Imperial measurement tradition. US aviation adopted inHg for altimeter settings, while most other countries use hPa.
Common mistakes
Confusing inches of mercury with millimetres of mercury—29.92 inHg = 760 mmHg = 1 atm. Also, assuming the conversion from inHg to psi is simple; 1 inHg ≈ 0.491 psi, not a round number.
What is millibar?
A millibar is a pressure unit equal to one thousandth of a bar. It is the standard unit for atmospheric pressure in meteorology and weather forecasting, particularly in aviation and marine contexts.
Real-world uses
Millibars are the traditional unit in meteorology for reporting atmospheric pressure, particularly in weather maps and hurricane tracking. Standard atmospheric pressure is 1013.25 mbar, and tropical cyclones can have central pressures below 900 mbar.
History
The millibar was introduced by Sir Napier Shaw in 1909 for meteorological use. It dominated weather reporting throughout the 20th century. Since 1982, the WMO has officially recommended hectopascals instead, though both represent exactly 100 Pa.
Common mistakes
Not realizing that 1 millibar = 1 hectopascal exactly. Meteorological services have largely switched to hPa, but the numerical values are identical. Also, assuming lower millibar values mean calmer weather—lower pressure often indicates storms.
When is this conversion used?
Converting between inch of mercury and millibar is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region.
Worked examples
1 inch of mercury = 33.86389 millibar
1 millibar = 100 pascal
How to convert inch of mercury to millibar
To convert inch of mercury to millibar, multiply the value by 33.86389.
To convert millibar back to inch of mercury, multiply by 0.02952998.
Measurement standards
The pascal (Pa) is the SI derived unit of pressure, defined as one newton per square metre. The World Meteorological Organization mandates the hectopascal (hPa) for atmospheric pressure reporting in aviation and weather services.
Did you know?
The pressure at the bottom of the Mariana Trench, nearly 11 km deep, exceeds 1,086 bar — over a thousand times atmospheric pressure. At this depth, water is compressed by about 5%, making it measurably denser than at the surface.
Quick reference: inch of mercury to millibar
| inch of mercury | millibar |
|---|---|
| 0.1 | 3.386389 |
| 0.5 | 16.931945 |
| 1 | 33.86389 |
| 2 | 67.72778 |
| 5 | 169.31945 |
| 10 | 338.6389 |
| 25 | 846.59725 |
| 50 | 1,693.1945 |
| 100 | 3,386.389 |
| 250 | 8,465.9725 |
| 500 | 16,931.945 |
| 1,000 | 33,863.89 |
Common values
| inch of mercury | millibar | |
|---|---|---|
| Car tyre | 64.96595636 inch of mercury | 2,200 millibar |
| Standard atmosphere | 29.9212524 inch of mercury | 1,013.25 millibar |
| Blood pressure (systolic) | 4.72479683 inch of mercury | 160 millibar |
| Deep-sea submersible | 32,482.97818118 inch of mercury | 1,100,000 millibar |
| Bicycle tyre | 183.08587702 inch of mercury | 6,200 millibar |
Available Pressure units
More inch of mercury conversions
- Convert inch of mercury to pascal
- Convert inch of mercury to kilopascal
- Convert inch of mercury to bar
- Convert inch of mercury to pound per square inch
- Convert inch of mercury to atmosphere
- Convert inch of mercury to millibar
- Convert inch of mercury to hectopascal
- Convert inch of mercury to torr
- Convert inch of mercury to megapascal