Aircraft Performance
Convert field elevation and QNH into pressure altitude instantly. Compute ISA deviation, density altitude, and equivalent flight level — the foundation of every aircraft performance calculation.
Enter field elevation and QNH — results update instantly
Usage Guide
From raw METAR data to a complete pressure altitude picture in four steps.
Enter the airport elevation in feet or metres. This is published on approach plates, in the AIP aerodrome section, and on the airport information page of your EFB. Choose feet or metres using the dropdown beside the input.
Enter the current QNH from the airport METAR or ATIS. European METARs report QNH in hPa (e.g. Q1023). US METARs report altimeter setting in inHg (e.g. A2992). Toggle the unit button to match your source.
Pressure altitude displays instantly in feet. The equivalent flight level is shown for reference when planning transitions above the transition altitude. Pressure altitude is the value used in all aircraft performance charts.
Optionally enter the outside air temperature to compute density altitude and ISA deviation. Density altitude is the critical value for actual takeoff and landing performance — it accounts for both non-standard pressure and non-standard temperature.
Foundational Knowledge
Pressure altitude is the cornerstone of aircraft performance. Every figure in your aircraft flight manual — takeoff distance, climb rate, cruise fuel flow — is based on it.
Pressure altitude is the altitude in the International Standard Atmosphere (ISA) at which the ambient pressure equals the current atmospheric pressure. It is measured by setting the aircraft altimeter to the standard datum plane pressure of 1013.25 hPa (29.92 inHg) and reading the indicated altitude.
When the atmosphere exactly matches ISA conditions, pressure altitude equals true altitude above mean sea level. On a high-pressure day (QNH above 1013.25), pressure altitude is lower than the true altitude. On a low-pressure day, pressure altitude is higher. This deviation has direct consequences for aircraft performance — lower air pressure means less air mass, less engine power, and longer runway requirements.
Aircraft performance charts use pressure altitude as their reference because it directly represents air density at a given pressure, independent of what the altimeter reads on any given day. The relationship is deterministic: at a given pressure altitude in ISA conditions, air density is always the same.
Every takeoff distance, climb gradient, maximum continuous power setting, and fuel burn figure in the Pilot Operating Handbook uses pressure altitude on its horizontal axis. Using indicated altitude instead of pressure altitude — particularly at airports with QNH significantly above or below standard — introduces a systematic error that can make performance appear better than it actually is.
The Mathematics
The formula derives from the ISA pressure lapse rate. Understanding the derivation helps you know the precision limits and when linear approximation is appropriate.
Altitude Types Explained
These four altitude types are used for different purposes. Confusing them is one of the most common errors in performance planning.
Operational Knowledge
Pressure altitude is not an abstract concept — it has direct go/no-go and performance consequences on every flight.
Every figure in the POH performance section is entered against pressure altitude on the X-axis. If you use field elevation instead of pressure altitude, you are reading the wrong column. On a low-pressure day with QNH 990 hPa, the pressure altitude of a sea-level airport is 630 ft — meaning you must look up takeoff performance for 600 ft, not 0 ft. At a 5,000 ft airport on the same day, the pressure altitude is 5,630 ft.
At or above the transition altitude (TA), pilots set 1013.25 hPa and fly by flight level. FL350 = 35,000 ft pressure altitude. The transition layer is the vertical space between the TA (highest QNH altitude used climbing) and TL (lowest FL used descending). Country-specific TAs range from 3,000 ft (UK) to 18,000 ft (US). Always know the TA and TL for the country you are entering.
US FARs trigger oxygen requirements at pressure altitude thresholds: above 12,500 ft for more than 30 minutes (crew); above 14,000 ft continuously (crew); above 15,000 ft (passengers must be offered oxygen). In Europe, EU-OPS and national CAA rules apply similar thresholds. Remember these apply to pressure altitude — not indicated altitude. On a low-QNH day, you could reach the threshold at a lower indicated altitude.
Mode C and Mode S transponders encode pressure altitude directly from the static system, regardless of the altimeter setting. ATC radar displays show pressure altitude converted to flight levels or altitudes using standard pressure. A 30 hPa low-pressure event adds 819 ft to your transponder-reported altitude compared to your QNH reading. Always report QNH-indicated altitude to ATC unless operating in flight levels.
In cold temperatures, indicated altitude reads higher than true altitude. The ICAO cold temperature correction table provides corrections when the airport OAT is below ISA. This is critical for obstacle clearance during approaches — if the MDA is 1,200 ft indicated and OAT is −30°C, the true altitude at MDA may be hundreds of feet lower. Some approach plates now include cold temperature corrections directly on the plate.
At airports above 5,000 ft elevation — Denver (5,430 ft), Johannesburg (5,558 ft), Kathmandu (4,390 ft), La Paz (13,313 ft) — pressure altitude is already high before any QNH correction. Combine elevation with a low QNH and hot temperature and density altitude can reach 10,000 ft or above at major airports, requiring careful performance calculation, possible load reduction, and early morning operations to use cooler temperatures.
Common Questions
Pressure altitude is the altitude above the standard datum plane — the theoretical level at which atmospheric pressure equals 29.92 inHg (1013.25 hPa). It is obtained by setting the altimeter to 29.92 inHg and reading the indicated altitude. Indicated altitude, by contrast, is the altitude shown when the altimeter is set to the local QNH (altimeter setting for that location). Indicated altitude approximates true altitude above mean sea level when the atmosphere matches ISA conditions. Pressure altitude is used for performance calculations, density altitude computation, flight level assignment above the transition altitude, and transponder encoding. The difference between indicated altitude and pressure altitude equals the correction for non-standard pressure: for every 1 hPa that QNH deviates from 1013.25, pressure altitude differs from field elevation by approximately 27 feet.
The standard aviation formula for pressure altitude is: Pressure Altitude (ft) = Field Elevation (ft) + (1013.25 − QNH in hPa) × 30. In inHg: Pressure Altitude (ft) = Field Elevation (ft) + (29.92 − QNH in inHg) × 1000. The constant 30 ft/hPa is the aviation standard approximation used in EASA and FAA training materials. The inHg equivalent of 1000 ft/inHg is the independently established standard for US altimeter settings. The ICAO standard atmosphere defines the exact pressure-altitude relationship using a more complex hypsometric formula, but these linear approximations are used universally in aviation performance calculations.
Pressure altitude accounts only for the non-standard pressure component — it is the altitude at which the standard atmosphere has the same pressure as the current atmosphere. Density altitude accounts for both non-standard pressure and non-standard temperature. It is the pressure altitude corrected for temperature deviation from ISA. Density altitude = Pressure Altitude + (ISA Temperature Deviation × 120). At ISA conditions (temperature equals ISA standard for that altitude), pressure altitude and density altitude are identical. When temperature is above ISA, density altitude is higher than pressure altitude — the air is less dense than standard, and aircraft performance degrades accordingly. Density altitude is the critical value for takeoff and landing performance calculations, engine performance, and propeller efficiency.
Above the transition altitude (typically FL100 in UK/Europe and FL180 in the US, though it varies by country), all aircraft set their altimeters to 1013.25 hPa (29.92 inHg) and fly at flight levels based on pressure altitude. This eliminates the need to continuously update QNH settings as aircraft cross regional boundaries, and ensures a consistent altitude reference for all aircraft in the same airspace. Below the transition level, each aircraft uses local QNH to maintain terrain and obstacle clearance. The transition from QNH to standard setting occurs at the transition altitude (climbing) and at the transition level (descending). Above the transition, there is no "indicated altitude" — only flight level expressed as pressure altitude in hundreds of feet (FL350 = 35,000 ft pressure altitude).
QNH is the altimeter setting that causes the altimeter to read field elevation when on the ground at the airport. A high QNH (above 1013.25 hPa) means atmospheric pressure is above standard — pressure altitude is lower than indicated altitude. A low QNH (below 1013.25 hPa) means atmospheric pressure is below standard — pressure altitude is higher than indicated altitude. This matters operationally because aircraft performance is based on pressure altitude: on a low-pressure day (QNH 990 hPa), your pressure altitude could be 690 feet higher than the field elevation, significantly degrading takeoff performance. The rule of thumb is that for every 1 hPa below 1013.25, pressure altitude increases by approximately 30 feet.
ISA deviation is the difference between the actual temperature and the ISA standard temperature at a given pressure altitude. The ISA standard temperature at sea level is 15°C, decreasing at 1.98°C per 1,000 ft (the standard temperature lapse rate) up to the tropopause at 36,089 ft. ISA deviation = Actual Temperature − (15 − (Pressure Altitude ÷ 1000 × 1.98)). A positive ISA deviation means the air is warmer than standard, which means the density altitude is higher than pressure altitude — performance will be worse than charts suggest for the pressure altitude alone. Airlines use ISA deviation in their takeoff performance calculations to select the correct assumed temperature for flexible thrust settings. High ISA deviation (+20°C and above) is a significant performance hazard at high-elevation airports.
Oxygen requirements in aviation are triggered by pressure altitude thresholds, not indicated altitude. In the United States under FAR Part 91, flight crew must use supplemental oxygen above a cabin pressure altitude of 12,500 ft for periods of more than 30 minutes, and continuously above 14,000 ft. Passengers must be provided with oxygen above 15,000 ft cabin altitude. At altitudes above 25,000 ft, pressurised aircraft must carry enough oxygen to allow descent to 14,000 ft within 4 minutes following decompression, and one pilot must wear an oxygen mask at all times above FL410. In unpressurised aircraft operating at 10,000 ft pressure altitude or above, pilots should be aware of hypoxia even before the regulatory thresholds — night vision degrades noticeably above 5,000 ft without supplemental oxygen.
Naturally aspirated (non-turbocharged) piston engines lose power as pressure altitude increases because the air entering the engine is less dense, reducing the mass of oxygen available for combustion. The typical power loss is approximately 3% per 1,000 ft of density altitude. At a density altitude of 8,000 ft, a naturally aspirated engine produces roughly 75% of its sea-level power. Turbocharged and turbonormaised engines maintain sea-level manifold pressure up to the critical altitude (typically 15,000–20,000 ft) because the turbocharger compresses the intake air. Turbine engines also lose thrust with increasing pressure altitude, following the relationship: thrust decreases roughly in proportion to air density. Both jet and piston aircraft performance charts are based on pressure altitude corrected for non-standard temperature (density altitude).
Mode C and Mode S transponders encode pressure altitude — not indicated altitude — to report to ATC and TCAS systems. The transponder reads directly from the aircraft's static system and encodes the pressure altitude in 100-foot increments, regardless of the altimeter QNH setting. This is why ATC sometimes asks pilots to verify altitude when there is a discrepancy — the transponder is showing pressure altitude while the pilot is reading indicated altitude on a QNH setting. The reported transponder altitude is always relative to 1013.25 hPa standard pressure. On a low-QNH day, the transponder will report a higher altitude than the altimeter reads, and the difference should equal (1013.25 − QNH) × 30 feet.
VFR altitude limits vary by country and airspace structure. In the United States, VFR flight above FL180 is not permitted — Class A airspace begins at FL180 and requires IFR operations. In ICAO-controlled airspace, the Class A airspace base varies by country but typically begins at FL195, FL245, or FL285 depending on national AIP. Above the transition layer, all VFR flights that are permitted must use pressure altitude for altitude assignment and cannot operate in Class A airspace. In practice, most VFR piston aircraft are limited by service ceiling (typically 10,000–14,000 ft density altitude) and oxygen requirements well below Class A limits. High-performance pressurised piston and turboprop aircraft flying IFR routinely operate at FL200–FL280.