inHg to hPa Converter

How the inhg to hpa conversion works

One inch of mercury equals 33.8639 hectopascals: hPa = inHg × 33.8639, and back again inHg = hPa ÷ 33.8639. A hectopascal is 100 pascals, and it is numerically identical to the millibar you may remember from older forecasts — 1 hPa = 1 mbar, exactly.

Why two units for the same sky: American forecasters inherited the mercury barometer, which literally measured how high atmospheric pressure pushed a column of mercury — hence inches of mercury. Most of the rest of the world moved to the SI-derived hectopascal. The standard sea-level pressure both systems agree on is 29.92 inHg = 1,013.25 hPa, the reference every altimeter setting and weather map is built around.

Reading a forecast: a falling barometer — say from 30.10 inHg (1,019.3 hPa) down through 29.80 inHg (1,009.1 hPa) — usually means unsettled weather on the way, while a steady reading near 30.20 inHg (1,022.7 hPa) means settled, clear conditions. The absolute number matters less than the direction and speed of the change over a few hours.

Handy benchmarks: 28.00 inHg = 948.19 hPa (deep low), 29.53 inHg = 1,000 hPa, 29.92 inHg = 1,013.25 hPa (standard sea level), 30.20 inHg = 1,022.69 hPa and 31.00 inHg = 1,049.78 hPa (strong high). Remember that station pressure at altitude reads lower than sea-level pressure until corrected — forecasts quote the sea-level figure.