An altimeter setting is the atmospheric pressure value used to calibrate an aircraft's altimeter so that it accurately displays altitude above mean sea level (MSL). It represents the barometric pressure adjusted to sea level for a specific location and time. Pilots obtain this value from air traffic control (ATC) or automated weather stations (ATIS/AWOS) before takeoff and during flight to ensure accurate altitude readings.

To convert inches of mercury (inHg) to hectopascals (hPa), multiply the inHg value by 33.8639. The precise conversion factor is 33.863886666667. For example: 29.92 inHg × 33.8639 = 1013.25 hPa. Conversely, to convert hPa to inHg, divide the hPa value by 33.8639 (or multiply by approximately 0.02953). This tool performs these calculations instantly with high precision.

The standard altimeter setting is 29.92 inHg (1013.25 hPa), which represents the International Standard Atmosphere (ISA) mean sea-level pressure. This value was established based on extensive meteorological observations as the average atmospheric pressure at sea level. At or above the transition altitude (typically 18,000 feet in the US, known as FL180), all aircraft set their altimeters to this standard value to ensure consistent altitude separation regardless of local weather conditions.

These are Q-codes used in aviation for altimeter settings: QNH is the pressure setting that causes the altimeter to read the airport's elevation above sea level when on the ground (most commonly used). QFE is the pressure setting that causes the altimeter to read zero at the airport reference point (height above the airfield). QNE refers to the standard setting of 29.92 inHg / 1013.25 hPa used above the transition altitude. Most civilian aviation uses QNH for departure and arrival, switching to QNE (standard) at higher flight levels.

Inches of mercury (inHg) is primarily used in the United States, Canada, and Japan for aviation altimeter settings. Hectopascals (hPa) —which are numerically identical to millibars (mb)—are used in most other countries worldwide, including all of Europe, Asia (except Japan), Australia, Africa, and South America. This regional difference means international pilots must be proficient at converting between these units, especially when crossing between different flight information regions (FIRs).

Conversion accuracy is critical in aviation. An error of just 1 hPa translates to approximately 28 feet (8.5 meters) of altitude error at sea level, and this error increases at higher altitudes. For precision approaches, terrain clearance, and aircraft separation, accurate altimeter settings are essential. This calculator uses the full precision conversion factor (33.863886666667) to ensure accuracy well beyond operational requirements, typically displaying results to 2 decimal places for inHg and 1 decimal place for hPa.

Using an incorrect altimeter setting can lead to significant altitude errors. The rule of thumb is: "From high to low, look out below." If you fly from an area of higher pressure to lower pressure without updating your altimeter setting, the altimeter will read higher than your actual altitude, potentially causing you to fly dangerously low. An error of 0.10 inHg (~3.4 hPa) results in roughly 100 feet of altitude error. Always obtain the current altimeter setting for your location and update it regularly during flight.

The transition altitude is the altitude at which pilots switch from using local altimeter settings (QNH) to the standard setting of 29.92 inHg / 1013.25 hPa. In the United States, this is 18,000 feet MSL (FL180). In Europe, transition altitudes vary by country, typically ranging from 3,000 to 6,000 feet. Above this altitude, all aircraft use the same standard setting, and altitudes are expressed as flight levels (FL)—for example, FL350 represents 35,000 feet under standard conditions. This standardization ensures safe vertical separation between aircraft regardless of local pressure variations.