inHg

Inches of mercury (inHg) is a non-SI unit of pressure based on the height of a mercury column. It is defined so that pressure can support a column of mercury 1 inch high at 0 °C and standard gravity, roughly 3,386 pascals. At sea level, standard atmospheric pressure is about 29.92 inHg, equal to about 101,325 Pa.

The unit is widely used in the United States, particularly in meteorology and aviation, and for older mercury barometers.

Historically derived from Torricelli's mercury barometer (1643), inHg remains common for weather reports, barometer readings, and altimeter settings. In practice, pressures are typically in the range of about 28 to 31 inHg at Earth's surface; low-pressure systems reduce the reading toward the high 20s, while high-pressure systems push toward the low 30s.

InHg is related to other pressure units by straightforward conversions: 1 inHg ≈ 3,386 Pa ≈ 0.4911 psi; 1 atm ≈ 29.92 inHg ≈ 101,325 Pa. The related unit torr equals 1 mmHg, so 760 torr = 29.92 inHg. Many other countries use hectopascals (hPa) or kilopascals (kPa) instead.

Mercury barometers measuring inHg have largely been supplanted by SI-based instruments in many contexts, but inHg remains in common use for aviation altimeter settings and weather reporting in several regions.