Altimeter Setting Calculator

Enter your station pressure and field elevation to calculate the altimeter setting (QNH) in hPa and inHg, along with pressure altitude, density altitude, and ISA deviation.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Station Pressure (hPa)
Input the measured atmospheric pressure at the airfield in hectopascals (hPa or millibars).
2
Enter Field Elevation (ft)
Provide the elevation of the airfield above mean sea level (MSL) in feet.
3
Review Your Results
The calculator will display QNH in hPa and inHg, pressure altitude, and density altitude, crucial for flight planning.

Example Calculation

A pilot needs to determine altimeter settings for an airfield with a station pressure of 950 hPa and a field elevation of 1,500 ft.

Station Pressure (hPa)

950

Field Elevation (ft)

1,500

Results

1002.82 hPa

Tips

Always Verify ATIS/AWOS Information

Before flight, always obtain the current altimeter setting (QNH) from ATIS (Automatic Terminal Information Service) or AWOS (Automated Weather Observing System) for the departure and destination airports. This ensures your altimeter is calibrated to local conditions, preventing altitude errors.

Understand Pressure Altitude for Performance

Pressure altitude is critical for calculating aircraft performance (takeoff/landing distance, climb rate). Higher pressure altitude than field elevation means thinner air, requiring longer runways and reducing climb performance. Always use pressure altitude when consulting performance charts.

Know the Transition Altitude

In most countries, pilots transition from local QNH to standard pressure (1013.25 hPa or 29.92 inHg) above a certain 'transition altitude' (e.g., 18,000 ft in the US). This ensures all aircraft are on a common altimeter setting for vertical separation in the upper airspace.

Deciphering Airfield Pressure: The Altimeter Setting Calculator

The Altimeter Setting Calculator is an indispensable tool for pilots and aviation enthusiasts, providing critical atmospheric data needed for safe flight. It computes QNH (sea-level pressure) in both hPa and inHg, pressure altitude, and density altitude from local station pressure and field elevation. These figures are fundamental for accurate altimeter calibration and aircraft performance calculations, ensuring compliance with aviation standards in 2025.

The Physics of Atmospheric Pressure and Altitude

Aircraft altimeters rely on atmospheric pressure to determine altitude. Air pressure naturally decreases with increasing altitude, and altimeters are essentially barometers calibrated to display height. However, atmospheric pressure also varies with local weather conditions. To account for this, pilots use specific altimeter settings. QNH is the pressure setting that, when dialed into an altimeter, will cause it to read the airfield's elevation when on the ground. Pressure altitude is the altitude indicated when the altimeter is set to a standard 29.92 inHg (1013.25 hPa), which is crucial for performance calculations as it relates directly to air density.

Elevation (m) = Field Elevation (ft) × 0.3048
QNH (hPa) = Station Pressure × (1 + (0.0065 × Elevation (m)) / 288.15)^5.255
inHg = QNH / 33.8639
Pressure Altitude (ft) = (1 - (Station Pressure / 1013.25)^0.190284) × 145366.45

These formulas are derived from the International Standard Atmosphere (ISA) model.

💡 Understanding actual air speed is just as vital as altitude. Our True Airspeed (TAS) Calculator helps determine your aircraft's speed relative to the airmass.

Calculating Altimeter Settings for a Mountain Airfield

Imagine a pilot preparing for takeoff from a mountain airfield with a field elevation of 1,500 ft. The local station pressure, obtained from an Automated Weather Observing System (AWOS), is 950 hPa.

Input Station Pressure: 950 hPa.
Input Field Elevation: 1,500 ft.
Calculate Elevation in Meters: 1,500 ft × 0.3048 = 457.2 m.
Calculate QNH (hPa): 950 × (1 + (0.0065 × 457.2) / 288.15)^5.255 ≈ 1002.82 hPa.
Calculate QNH (inHg): 1002.82 hPa / 33.8639 ≈ 29.62 inHg.
Calculate Pressure Altitude: (1 - (950 / 1013.25)^0.190284) × 145366.45 ≈ 1,847 ft.
Calculate Density Altitude: (This involves temperature, which is not an input here, so the calculator uses a standard approximation based on ISA. The formula output shows densityAltFinal as 145366 * (1 - Math.pow(qnh / 1013.25, 0.234969)), yielding approximately 1,601 ft).

The primary result, QNH, is 1002.82 hPa.

💡 For precise navigation and bearing, our VOR Radial to Heading Calculator can help you translate navigational aid readings into usable headings.

Decoding Airfield Pressure for Safe Flights

Accurate altimeter settings are unequivocally critical in aviation for maintaining safe separation from terrain and other aircraft. The QNH setting, derived from local atmospheric pressure, ensures that an aircraft's altimeter reads the correct field elevation when on the ground. This is fundamental for pilots during takeoff and landing phases, where precise vertical awareness is paramount. Deviations from standard pressure (1013.25 hPa or 29.92 inHg) necessitate these adjustments, particularly when flying below 18,000 feet, where local pressure variations significantly impact altimeter readings. Failing to use the correct QNH can lead to dangerous altitude errors, potentially resulting in controlled flight into terrain (CFIT) or mid-air collisions. Aviation safety guidelines consistently emphasize the importance of verifying and setting the correct QNH before every flight.

Pilot's Use of Altimeter Settings in Flight Operations

Professional pilots continuously utilize and interpret altimeter settings as a fundamental aspect of flight safety and navigation. Before departure, they meticulously set their altimeter to the local QNH obtained from Air Traffic Control (ATC) or an Automated Terminal Information Service (ATIS). This ensures the altimeter displays the precise airfield elevation on the ground, a critical check for instrument accuracy. As the aircraft climbs and passes the transition altitude (typically 18,000 feet in the United States), pilots switch their altimeters to the standard pressure setting of 29.92 inHg (1013.25 hPa). This universal setting ensures all aircraft in the high-altitude airspace are operating on a common reference, preventing vertical separation errors. Conversely, during descent into an airport, the pilot again requests and sets the local QNH to accurately determine their height above terrain for approach and landing. This diligent process is a cornerstone of preventing altitude-related incidents.

Frequently Asked Questions

What is QNH in aviation?

QNH is an altimeter setting that, when set in an aircraft's altimeter, causes the altimeter to read the elevation of the airfield above mean sea level (MSL) when the aircraft is on the ground. It is the atmospheric pressure at mean sea level (or a nearby datum) obtained by adjusting the station pressure to sea level using the standard lapse rate. Pilots use QNH to ensure accurate altitude readings during takeoff, landing, and flight in the lower airspace, maintaining safe separation from terrain and other aircraft.

What is pressure altitude and why is it important?

Pressure altitude is the altitude indicated when an altimeter is set to the standard datum plane of 29.92 inches of mercury (1013.25 hPa). It is the theoretical altitude in a standard atmosphere corresponding to a given pressure. Pressure altitude is crucial for calculating aircraft performance, as engine power, lift, and drag are all affected by air density, which is directly related to pressure. Pilots use pressure altitude to determine takeoff and landing distances, climb rates, and true airspeed.

How does density altitude differ from pressure altitude?

Density altitude is pressure altitude corrected for non-standard temperature. It is the altitude in the standard atmosphere at which the air density would be equal to the actual air density at your location. While pressure altitude accounts for pressure variations, density altitude further adjusts for temperature, which also significantly impacts air density. Higher density altitude (hotter, higher, or more humid conditions) means thinner air, leading to reduced aircraft performance, including longer takeoff rolls and decreased climb rates.

What is the standard atmospheric pressure for altimeter settings?

The standard atmospheric pressure used for altimeter settings is 1013.25 hectopascals (hPa) or 29.92 inches of mercury (inHg). This value represents the average atmospheric pressure at mean sea level in a standard atmosphere. Pilots use this standard setting when flying above the 'transition altitude' to ensure vertical separation from other aircraft, as all aircraft in that airspace are operating on the same altimeter reference, regardless of local pressure variations.