Aircraft Altimeter Calculator: QNH, QFE, Pressure & Density Altitude

Convert barometric pressure values between QNH and QFE baselines, and calculate Pressure Altitude and Density Altitude profiles to support line maintenance and flight operations.

Barometric Pressure Settings in Aviation

Altimetry tracking requires absolute precision to maintain vertical separation and verify instrumentation health during line checks. Because atmospheric pressure shifts with weather and altitude, systems must be referenced to specific barometric baselines:

• QNH (MSL Reference): Atmospheric pressure corrected to Mean Sea Level. When set to the local QNH, the altimeter indicates altitude above sea level. On the ground at an airport, it reads the official airfield field elevation.
• QFE (Field Reference): The actual barometric pressure measured at the airfield ground datum. When an altimeter is referenced to QFE, it indicates height above that specific field. On the ground at that airport, the display reads zero.
• QNE (Standard Datum Baseline): The standard international atmospheric pressure baseline fixed at 1013.25 hPa (29.92 inHg). It provides a universal baseline for vertical separation above the transition altitude.

Performance Altitudes

• Pressure Altitude (PA): The vertical distance relative to the standard 1013.25 hPa pressure plane. It represents the atmospheric altitude calculated from the physical weight of the air column above the station ground reference.
• Density Altitude (DA): Pressure Altitude corrected for non-standard Outside Air Temperature (OAT). High ambient temperatures lower air density, which elevates the density altitude value and directly reduces engine thrust and wing aerodynamic performance.

How It’s Calculated

The calculator runs dynamic live updates as input changes occur, processing values through these steps:

1. Pressure Baseline Standardization

The tool checks your selected system units. If you enter a baseline using inches of mercury (inHg), the tool converts the value to hectopascals (hPa) internally:

Input Pressure in hPa = Input Pressure * 33.863886666667

2. QNH and QFE Conversion

To transition pressure between sea level and the airport ground level, the tool calculates the standard International Standard Atmosphere (ISA) pressure ratio factor based on the field elevation converted into meters:

Elevation in Meters = Airfield Field Elevation * 0.3048

Pressure Ratio Factor = (1 – (0.0065 * Elevation in Meters / 288.15)) ^ 5.25588

If QNH (MSL) is the known input type, the airfield pressure is derived by:

Calculated QFE = Input Pressure * Pressure Ratio Factor

If QFE (Ground) is the known input type, the sea level pressure is derived by:

Calculated QNH = Input Pressure / Pressure Ratio Factor

3. Pressure Altitude (PA) Derivation

The tool calculates the Pressure Altitude by evaluating the derived QFE station pressure against the standard datum line of 1013.25 hPa:

Pressure Altitude = 145366.45 * (1 – (Calculated QFE / 1013.25) ^ 0.190284)

4. Density Altitude (DA) Derivation

If you enter a temperature value in the Outside Air Temp (OAT) field, the tool calculates the standard ISA temperature for your airfield elevation and applies a density correction factor of 120 feet per degree Celsius deviation:

Standard ISA Temperature = 15 – (Airfield Field Elevation * 0.0019812)

Density Altitude = Pressure Altitude + (120 * (Outside Air Temp – Standard ISA Temperature))

Scope and Limitations

• Atmospheric Pressure Operational Limits: The tool validates and restricts input pressure parameters between 500 hPa and 1100 hPa (14.76 inHg to 32.48 inHg). Entries outside this range halt calculation loop updates.
• Field Elevation Operational Boundaries: Airfield Field Elevation inputs must fall within standard operational limits of -2,000 feet to 20,000 feet.
• Density Altitude Temperature Dependency: Density Altitude calculation requires an active numeric entry in the Outside Air Temp field. If left blank, the tool displays “Provide OAT” for the density metric while maintaining the validity of the pressure metrics.
• Standard Dry Air Assumption: Calculations use standard linear ISA lapse rates and do not factor in relative humidity, which has a negligible effect on primary altimetry calculations.