How this calculator works
One hectopascal and one millibar both equal exactly 100 Pa. The conventional NIST inch of mercury factor used here is 3,386.389 Pa. Changing units preserves the reported pressure type. It does not adjust station pressure to sea level or calculate an aviation altimeter setting.
Formula
Pa = hPa x 100; inHg = hPa x 100 / 3386.389; hPa = inHg x 3386.389 / 100.
Worked example
1,013.25 hPa is 101,325 Pa, or approximately 29.9212524 inHg. A 30 inHg reading is 1,015.9167 hPa.
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Frequently asked questions
Are hPa and millibars interchangeable?
Yes. They have the same numerical value, since each is 100 Pa.
Why does my station differ from a weather map?
A map may show pressure adjusted to sea level while the instrument reports station pressure. Unit conversion does not perform that adjustment.
Does 1,013.25 hPa predict fair weather?
It is the standard-atmosphere reference, not a forecast. Weather interpretation also needs location, pressure trends and other observations.