How this instrument works
Barometric pressure is reported in two units almost nowhere else agree on: inches of mercury (inHg), the traditional US aviation and weather-service unit, and hectopascals (hPa) — numerically identical to the older millibar — used by the World Meteorological Organization and by every METAR and synoptic chart issued outside North America. Both trace back to the mercury barometer: a column of mercury held up by the weight of the air above it, first read directly in inches or millimetres of how high that column stood.
The factor this converter applies, 33.86388640341, is not a rounded approximation — it falls out of three exact physical definitions stacked together: mercury's conventional density (13,595.1 kg/m³, the value fixed for barometric calculations), standard gravity (9.80665 m/s², the internationally agreed acceleration used to turn a mercury column's height into a pressure), and the exact inch (0.0254 m by international agreement since 1959). Multiply those together correctly and an inHg reading converts to a hPa reading with no measurement uncertainty of its own, only whatever precision the original reading carried.
There is one honest wrinkle worth knowing: US aviation's familiar 'standard' altimeter setting, 29.92 inHg, is itself a rounded figure. The true equivalent of one standard atmosphere, 1013.25 hPa, is 29.9213 inHg to four decimal places — so entering the rounded 29.92 into this converter returns 1013.21 hPa, not 1013.25. The gap is tiny and never matters operationally, but it explains why a precise recalculation of the 'standard' figure doesn't land on the exact textbook number.
- Enter your reading into the Inches of mercury (inHg) field — it opens at 29.92, the standard aviation altimeter setting.
- Read the converted value in Hectopascals / millibars (hPa) beneath it; it recalculates the instant the inHg figure changes.
- Compare against a METAR or synoptic chart quoted in hPa to see how a US-reported pressure lines up with an international one.
- Watch the hPa figure rise above 1013 for high pressure (fair weather) and fall below it for low pressure (storms approaching).
- Negative inHg readings are rejected — barometric pressure cannot go below zero, so the field only accepts zero or positive values.
Worked example — 29.92 inHg, the standard altimeter setting
A pilot reads 29.92 inHg off a US airport's altimeter setting — the figure quoted in every US aviation weather briefing. Enter 29.92 into Inches of mercury (inHg) and Hectopascals / millibars (hPa) reads 1013.21, the number that would appear on a European METAR for essentially the same atmospheric condition, since 1013.21 hPa and 29.92 inHg both describe air pressure very close to one standard atmosphere.
The gap between 29.92 inHg and the textbook 1013.25 hPa figure — 1013.21 versus 1013.25 — exists because 29.92 is itself a two-decimal rounding of the true standard-atmosphere equivalent, 29.9213 inHg. A high-pressure system reading 30.50 inHg converts to 1032.85 hPa, and a storm-associated low of 28.50 inHg converts to 965.12 hPa — both large enough swings that the rounding at the third decimal of inHg never changes which side of 'fair' or 'stormy' the number falls on.
Questions
Why does the US report barometric pressure in inHg while other countries use hPa?
It's a matter of historical convention rather than physics. US weather services and aviation adopted inches of mercury early on and never switched, while the World Meteorological Organization standardized on hectopascals (numerically the same as the older millibar) for its international exchange of weather data. Both describe the identical physical quantity — the weight of the atmosphere pressing down at a point — just scaled differently.
Is a hectopascal the same thing as a millibar?
Yes, exactly — 1 hectopascal equals 1 millibar, with no conversion needed between them. Meteorology largely shifted its terminology from millibar to hectopascal to align with SI prefixes, but older charts, some textbooks, and older barometers still print 'mb' for numbers that are numerically identical to the hPa figures on a modern forecast.
Why doesn't 29.92 inHg convert to exactly 1013.25 hPa?
Because 29.92 is a rounded, commonly quoted figure, not the precise equivalent of one standard atmosphere. The exact match for 1013.25 hPa is 29.9213 inHg to four decimal places; converting the rounded 29.92 with the full-precision factor gives 1013.20748119 hPa, which this converter displays as 1013.21. The difference is about 0.04 hPa — invisible in practice, but worth knowing if you're checking a calculation against the textbook number.
What does a barometric reading of 30.50 inHg indicate?
It converts to about 1032.85 hPa, a notably high reading associated with a strong high-pressure system and generally settled, fair weather. Aviation altimeter settings above roughly 30.20 inHg (about 1023 hPa) are considered on the high side of normal.
What does a barometric reading of 28.50 inHg indicate?
It converts to about 965.12 hPa, a low reading often associated with an approaching storm system or a deep low-pressure center — the kind of pressure drop that shows up on a forecast ahead of significant wind and precipitation. Many home barometers mark this range with a 'stormy' label.
Where does the 33.86388640341 conversion factor come from?
It is built from three internationally fixed physical constants: mercury's conventional density (13,595.1 kg/m³, the value standardized for barometric use), standard gravity (9.80665 m/s²), and the exact inch (0.0254 m). Multiplying pressure = density × gravity × height for a one-inch mercury column and converting the result to hectopascals yields 33.86388640341 exactly, per NIST's guide to SI unit conversions.