The inch of mercury, unit of pressure
The inch of mercury is exactly 3 386.388640341 pascals, that is 33.86 hectopascals. It is the unit in which American aviation sets its altimeters and American weather reports its barometric pressure.
It measures a mercury that does not exist. The value rests on a column of mercury of agreed density, 13 595.1 kilograms per cubic meter at zero degrees, under a gravity that is also agreed, 9.806 65 meters per second squared, the standard value fixed by the third General Conference on Weights and Measures in 1901. Real mercury and real gravity both vary; the unit does not. The standard sea level pressure, exactly 1 013.25 hectopascals, therefore comes to 29.9213 inches, and the 29.92 that pilots dial is that figure rounded to the hundredth.
The inch of mercury in other units
| 1 inHg | 0.0338639 bar |
| 1 inHg | 0.491154 psi |
| 1 inHg | 3,386 Pa |
| 1 inHg | 3.38639 kPa |
| 1 inHg | 33.8639 hPa |
| 1 inHg | 0.0334211 atm |
| 1 inHg | 25.4 mmHg |
Frequently asked questions
How many hectopascals are in an inch of mercury?
33.863886 exactly. The standard sea level pressure of 1 013.25 hectopascals is 29.9213 inches, which is why the setting is read as 29.92: the familiar number is itself a rounding, by about one and a half pascals.
Why does American aviation not use hectopascals?
Because the altimeter setting is a number read aloud on the radio dozens of times an hour, and changing it changes every chart, every procedure and every habit at once. Canada made the change, the United States did not, so a flight crossing the border changes units in mid-air.
Is the inch of mercury related to the millimeter of mercury?
It is exactly 25.4 of them, since an inch is 25.4 millimeters by definition. Both rest on the same agreed mercury, which is why the conversion is exact rather than physical.