Inch of mercury

Inch of mercury
Early American barometer calibrated in inches of mercury
General information
Unit ofPressure
SymbolinHg, ″Hg
Conversions
1 inHg in ...... is equal to ...
   SI units   3.38639 kPa
   United States customary units   0.491154 psi

Inch of mercury (inHg and ″Hg) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity. Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are:[1]

Conditions Pressure
conventional 3386.389 pascals
32 °F (0 °C) 3386.38 pascals
60 °F (16 °C) 3376.85 pascals

In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F (15.6 °C).[1]

1 inHg60 °F = 3,376.85 pascals (33.7685 hPa)

In Imperial units: 1 inHg60 °F = 0.489 771 psi, or 2.041 771 inHg60 °F = 1 psi.

  1. ^ a b Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B [1]