Convert barometric pressure values between European/ICAO standard hectopascals (hPa / mb) and American inches of mercury (inHg).
How This Calculator Works
An aircraft barometric altimeter is essentially a precision aneroid barometer calibrated to display height instead of pressure. Because atmospheric weather systems constantly alter surface barometric pressure, pilots must dial the local station altimeter setting (QNH) into the altimeter subscale (Kollsman window) to ensure accurate terrain separation.
The world is split between two primary measurement units for altimeter settings:
- Inches of Mercury (inHg): Used primarily in the United States, Canada, and parts of the Caribbean and Latin America. Standard sea level pressure is 29.92 inHg.
- Hectopascals (hPa) / Millibars (mb): Standardized by the International Civil Aviation Organization (ICAO) and utilized throughout Europe, Asia, Africa, South America, and Australasia. Standard sea level pressure is 1013.25 hPa. Note that 1 hPa is identically equal to 1 millibar (mb).
An error in altimeter calibration is dangerous: a pressure discrepancy of just 1.00 inHg equates to an altitude display error of approximately 1,000 feet.
Mathematical Formula
Worked Examples
Example 1: Standard Atmosphere Conversion
Convert standard sea level pressure 1013.25 hPa to inches of mercury.
1013.25 × 0.02952998 = 29.921 inHg (standard 29.92 inHg)
Example 2: Severe Low Pressure Storm System
A hurricane METAR reports an intense low pressure center of 960 hPa.
960 × 0.02953 = 28.35 inHg
Frequently Asked Questions
What is the difference between hPa and millibars (mb)?
There is zero physical difference. One hectopascal (1 hPa = 100 Pascals) equals exactly one millibar (1 mb).
What happens if you fly from high pressure to low pressure without resetting your altimeter?
The time-honored aviation warning applies: "High to low, look out below!" The altimeter will read higher than you actually are, placing the aircraft dangerously closer to terrain.
What altitude error corresponds to 0.01 inHg?
A pressure difference of 0.01 inHg corresponds to approximately 10 feet of altitude (or roughly 30 feet per 1 hPa).
What is QNE?
QNE is the standard altimeter setting of 29.92 inHg (1013.25 hPa) dialed when climbing through transition altitude into Flight Levels.
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