Pressure Altitude Calculator

Pressure altitude is the altitude indicated when an altimeter is set to the standard pressure setting of 29.92 inHg (1013.25 hPa). This reference level is known as the Standard Datum Plane (SDP) — a theoretical level at which atmospheric pressure equals the international standard. Pressure altitude therefore represents height above this standard reference, not true elevation above mean sea level.

It is a standardized reference altitude used in aviation to eliminate the effect of changing local barometric pressure (QNH). By resetting the altimeter to 29.92 inHg, all aircraft are referenced to the same pressure baseline, allowing consistent performance calculations across different weather conditions and geographic locations.

Pressure altitude is primarily used as the base input for aircraft performance calculations, including takeoff distance, climb performance, cruise performance, and landing distance. It is also the reference altitude used to determine flight levels above the transition altitude, where standard pressure settings are applied globally.

Unlike true altitude, which reflects height above mean sea level, pressure altitude reflects only atmospheric pressure conditions. For this reason, it does not account for temperature or air density variations, which is why density altitude is used when actual aircraft performance must be assessed under non-standard atmospheric conditions.

QNH (altimeter setting) is the regional atmospheric pressure value adjusted to mean sea level, and it directly affects pressure altitude by determining how the aircraft’s indicated altitude relates to the standard pressure reference of 29.92 inHg (1013.25 hPa).

When an altimeter is set to QNH, it displays altitude above mean sea level based on local atmospheric pressure conditions at that time and location. Pressure altitude, however, is obtained by resetting the altimeter to the standard pressure setting of 29.92 inHg (1013.25 hPa), which removes local weather variations and establishes a uniform reference used in aviation performance calculations and flight level operations.

If QNH is lower than standard pressure, pressure altitude becomes higher than true elevation because the atmosphere contains less pressure than standard. If QNH is higher than standard pressure, pressure altitude becomes lower than true elevation because the atmosphere contains more pressure than standard.

QNH therefore serves as the key link between real-time atmospheric conditions and the standardized pressure system used in aviation, making it essential for converting field elevation into pressure altitude and ensuring accurate performance, navigation, and flight level consistency.

The standard pressure datum of 29.92 inHg (1013.25 hPa) is the internationally agreed reference pressure used in aviation to define the standard atmosphere and provide a uniform baseline for altitude measurement, flight levels, and aircraft performance calculations.

This value represents mean sea level pressure in the International Standard Atmosphere (ISA) model and is used when pilots set the altimeter to “standard pressure.” When this setting is applied, the altimeter no longer reflects local weather conditions but instead indicates pressure altitude, allowing all aircraft to reference the same vertical datum regardless of location or atmospheric variation.

Above the transition altitude, all aircraft operate using this standard pressure setting to ensure vertical separation consistency between aircraft. This eliminates discrepancies caused by differing local QNH values and ensures that flight levels are globally standardized.

The 29.92 inHg / 1013.25 hPa reference is therefore the foundation of the flight level system and a core reference point for all pressure-based aviation calculations.

To determine pressure altitude, set the aircraft altimeter to the standard pressure setting of 29.92 inHg (1013.25 hPa); the altitude indicated on the altimeter is the pressure altitude.

Alternatively, pressure altitude can be derived from field elevation and QNH using the standard formula:

Pressure Altitude (ft) = Field Elevation (ft) + [(1013.25 − QNH in hPa) × 30]

If the altimeter setting is in inHg, use the following formula to calculate pressure altitude:

Pressure Altitude (ft) = Field Elevation (ft) + [(29.92 − QNH in inHg) × 1,000]

Both formulas produce the same result — they are unit conversions of the same underlying relationship. This method converts local atmospheric pressure conditions into a standardized altitude reference by correcting field elevation to ISA standard pressure. The constant 30 is derived from the ISA pressure-altitude relationship near sea level, where atmospheric pressure decreases at approximately 1 hPa per 30 ft of altitude gain.

A practical rule of thumb is that a 1 inHg (approximately 34 hPa) decrease in QNH increases pressure altitude by about 1,000 feet, while a 1 inHg increase decreases pressure altitude by a similar amount.

Precise formula (NOAA)

For applications requiring greater precision, the National Oceanic and Atmospheric Administration (NOAA) publishes the following formula for converting atmospheric pressure directly to pressure altitude:

This formula accounts for the non-linear relationship between pressure and altitude across the full troposphere. The aviation approximation (30 ft/hPa) is accurate to within a few feet for pressures near standard, but the NOAA formula is used when precise conversion is required.

Worked example

For a field elevation of 2,000 ft and a QNH of 1000 hPa, pressure altitude is calculated as follows:

Pressure Altitude = Field Elevation + [(1013.25 − QNH) × 30]
= 2,000 + [(1013.25 − 1000) × 30]
= 2,000 + [13.25 × 30]
= 2,000 + 397.5
= 2,397 ft

Hence, for a field elevation of 2,000 ft and a QNH of 1000 hPa, the pressure altitude is 2,397 ft.

The pressure altitude formula is highly accurate for operational aviation use and provides results that are sufficiently precise for all standard flight planning and aircraft performance calculations.

In practical terms, the standard approximation used in aviation — where 1 hPa is treated as approximately 30 feet — introduces only a small margin of error, typically negligible for cockpit and POH performance applications. This level of accuracy is consistent with how pressure altitude is used in real-world aviation, where it serves as a standardized reference rather than a precision measurement.

Most minor differences arise from rounding and local atmospheric variability rather than the formula itself. For example, slight variations in the conversion factor (true atmospheric gradient vs 30 ft per hPa approximation) may result in differences of only a few tens of feet, which has no operational impact on performance chart interpretation.

In operational use, the largest source of inaccuracy is not the formula, but input data quality — particularly incorrect QNH settings or outdated altimeter information. When current METAR or ATIS QNH values are used correctly, pressure altitude remains a reliable and standardized input for density altitude and performance calculations.

Because of this, the pressure altitude formula is considered accurate enough for all aviation planning purposes, including takeoff, climb, landing performance, and flight level determination.

Pilots set pressure altitude in the cockpit by adjusting the altimeter to the standard pressure setting of 29.92 inHg (1013.25 hPa), which causes the altimeter to indicate pressure altitude instead of local altitude.

This is done by rotating the altimeter’s Kollsman window (barometric subscale) until 29.92 inHg or 1013 hPa is displayed. Once set, the altimeter no longer reflects local QNH-based altitude; instead, it shows altitude relative to the standard pressure datum used for flight levels and performance calculations.

This setting is typically used when transitioning from local altitude reference (QNH) to standard altitude reference during climb through the transition altitude, or when operating at flight levels in controlled airspace.

Pressure altitude is not a separate cockpit instrument reading; it is a configuration of the altimeter that standardizes altitude reference across all aircraft, ensuring consistent vertical separation and accurate performance chart usage.

Pressure altitude is a standardized altitude based only on atmospheric pressure, while density altitude is the pressure altitude corrected for non-standard temperature to reflect actual air density and aircraft performance conditions.

Pressure altitude is the altitude an aircraft would indicate when the altimeter is set to the standard pressure setting of 29.92 inHg (1013.25 hPa). It represents a purely pressure-based reference and is used for flight levels, separation standards, and as the baseline input for performance calculations.

Density altitude builds on pressure altitude by incorporating temperature effects using ISA deviation. Warmer-than-standard air is less dense, which increases density altitude above pressure altitude, while colder air reduces it below pressure altitude. As a result, density altitude represents the “effective performance altitude” of the aircraft.

The key operational difference is that pressure altitude is a fixed atmospheric reference, while density altitude directly reflects aircraft performance. Two airports at the same pressure altitude can have very different density altitudes depending on temperature, leading to significantly different takeoff, climb, and landing performance.

Pressure altitude defines “where you are in the standard atmosphere,” while density altitude defines “how the aircraft will perform in those conditions.”

Flight levels are expressed as pressure altitudes divided by 100, making pressure altitude the direct numerical foundation of the flight level system used in all controlled airspace above the transition altitude.

A flight level is a three-digit number representing pressure altitude in hundreds of feet. FL100 corresponds to a pressure altitude of 10,000 ft, FL250 to 25,000 ft, and FL350 to 35,000 ft. The conversion is:

The flight level system uses pressure altitude — not indicated altitude or true altitude — because it provides a uniform vertical reference independent of local atmospheric pressure. When all aircraft above the transition altitude reference the same datum of 29.92 inHg (1013.25 hPa), vertical separation is consistent and calculable regardless of geographic location or QNH differences between regions.

Below the transition altitude, pilots use QNH and report altitude in feet. At the transition altitude, the altimeter is reset from QNH to the standard pressure setting, and altitude is then expressed as a flight level. This changeover point defines the boundary between local altimetry and the standard pressure system.

Because flight levels are pressure altitudes in standardized form, any error introduced before the QNH-to-standard pressure transition — such as an incorrect QNH setting — carries directly into the flight level reading and can affect vertical separation accuracy.

The table below shows common flight levels and their equivalent pressure altitudes, along with typical operational context.

The transition altitude is the altitude during climb at which pilots switch from local QNH altimeter settings to the standard pressure setting of 29.92 inHg (1013.25 hPa), while the transition level is the corresponding altitude during descent where pilots switch back to QNH.

Below the transition altitude, aircraft altimeters are set to local QNH so that altitude reflects height above mean sea level for terrain clearance and traffic separation. Above the transition altitude, all aircraft use the standard pressure setting (29.92 inHg) so that vertical spacing is referenced to flight levels rather than local atmospheric pressure.

The transition level is not fixed and varies depending on atmospheric pressure; it is always the lowest available flight level above the transition altitude that ensures vertical separation from aircraft using QNH below.

The transition layer is the airspace between the transition altitude and transition level where aircraft may be operating on different pressure references, and it is kept as small as possible by air traffic control procedures.

Pressure altitude reduces aircraft performance by lowering air density as altitude increases, which simultaneously decreases engine power, propeller efficiency, and wing lift.

As pressure altitude increases, atmospheric pressure decreases, meaning each engine intake stroke contains less oxygen. This reduces combustion efficiency and results in lower available engine power, especially in naturally aspirated piston engines. At the same time, propellers accelerate less air mass per revolution, reducing thrust output.

Wings are also affected because lower air density requires a higher true airspeed to generate the same amount of lift at a given indicated airspeed. This increases takeoff distance, reduces climb rate, and raises the true airspeed required for all phases of flight.

Overall aircraft performance degrades progressively with increasing pressure altitude, with noticeable effects on acceleration, climb gradient, and runway performance. This is why pressure altitude is a primary input in aircraft performance charts and is always used as the baseline for takeoff, climb, and landing calculations.

Pressure altitude increases takeoff distance and reduces climb performance by lowering air density, which reduces engine power, thrust, and wing lift simultaneously.

During takeoff, reduced air density means the engine produces less power due to lower oxygen availability, and the propeller generates less thrust per revolution. At the same time, the wings require a higher true airspeed to generate sufficient lift, which increases both ground speed at rotation and total runway distance required.

After liftoff, climb performance is reduced because excess power available to gain altitude is lower while aerodynamic efficiency is degraded. This results in a lower rate of climb, reduced climb gradient, and decreased obstacle clearance capability.

As pressure altitude increases, these effects become more pronounced, which is why high-elevation airports require longer runways, careful weight management, and strict use of POH performance charts based on pressure altitude rather than field elevation.

Pressure altitude increases landing distance by increasing the true airspeed and groundspeed corresponding to a given indicated airspeed, resulting in higher touchdown energy and a longer landing roll.

Although the indicated approach speed and indicated stall speed remain unchanged, lower air density means the aircraft must travel faster through the air to generate the same lift. Consequently, the aircraft crosses the threshold and touches down at a higher true airspeed and groundspeed than it would at sea level.

The higher touchdown speed increases kinetic energy, which requires more runway distance to dissipate during braking. Reduced aerodynamic drag at higher pressure altitudes also causes the aircraft to decelerate more slowly and can lead to a longer float during the flare.

The table below summarises the main effects of increasing pressure altitude on landing performance:

For this reason, landing performance charts in the POH or AFM use pressure altitude as a primary input, and pilots should always determine landing distance using pressure altitude rather than field elevation alone.

Aircraft performance charts use pressure altitude as a primary input to determine takeoff distance, climb performance, cruise performance, service ceiling, and landing distance under standardized atmospheric conditions.

Pressure altitude provides a common pressure reference that allows performance data to be applied consistently regardless of local barometric pressure. Most POH and AFM charts combine pressure altitude with outside air temperature to account for the effects of both pressure and temperature on air density.

To use a performance chart, pilots locate the pressure altitude on one axis and the outside air temperature on the other. The intersection of these values provides the expected performance for a specific aircraft configuration, weight, and runway condition.

The table below summarises how pressure altitude is used in the most common aircraft performance charts:

Performance charts are based on manufacturer-approved test conditions and procedures. Pilots should always use pressure altitude rather than field elevation when consulting these charts, because field elevation alone does not account for variations in atmospheric pressure. When temperature is also considered, the resulting performance corresponds to the actual density altitude conditions.

Pressure altitude is fundamental to IFR (Instrument Flight Rules) operations because it provides the standardized altitude reference used for flight levels, vertical separation, and aircraft performance planning.

Above the transition altitude, all aircraft set their altimeters to the standard pressure setting of 29.92 inHg (1013.25 hPa) and operate using flight levels rather than local QNH. This ensures consistent vertical separation between aircraft regardless of regional pressure differences.

Pressure altitude is also used in IFR departure planning because climb performance must be assessed against published instrument procedures such as SIDs (Standard Instrument Departures) and ODPs (Obstacle Departure Procedures), which specify required climb gradients for safe terrain and obstacle clearance under IFR conditions.

During cruise, IFR altitude assignments are based on flight levels, which are pressure altitude references rather than true altitude above mean sea level. For example, FL180 corresponds to a pressure altitude of approximately 18,000 feet with the altimeter set to 29.92 inHg.

Because obstacle clearance and route safety depend on climb capability, pressure altitude must always be combined with temperature to evaluate density altitude and confirm that the aircraft can meet required IFR performance criteria throughout departure, climb, and en-route operations.

Pressure altitude affects oxygen requirements because atmospheric pressure decreases with increasing altitude. Lower atmospheric pressure reduces the partial pressure of oxygen available to the body. Although oxygen still makes up approximately 21% of the atmosphere, the body absorbs less oxygen at higher pressure altitudes.

As pressure altitude increases, oxygen saturation in the blood decreases. This can lead to hypoxia, which impairs judgment, vision, reaction time, and decision-making.

Under FAA regulations, flight crew members must use supplemental oxygen when operating above a cabin pressure altitude of 12,500 ft MSL for more than 30 minutes and continuously above 14,000 ft MSL. Passengers must be provided with supplemental oxygen above 15,000 ft MSL. In unpressurised aircraft, cabin pressure altitude equals the actual flight altitude, so these thresholds apply directly to pressure altitude.

The table below summarises the FAA oxygen requirements for unpressurised aircraft:

These requirements are regulatory minimums rather than physiological optimums. Many pilots begin using supplemental oxygen above 10,000 ft MSL, especially during night operations, because visual acuity and cognitive performance can begin to degrade before the legal thresholds are reached.

The table below shows the relationship between pressure altitude and atmospheric pressure under International Standard Atmosphere (ISA) conditions. These values are derived from the ISA model and represent the standard atmospheric pressure at each altitude level.

Pilots can use this table to cross-check observed pressure readings, understand the pressure environment at cruise altitude, and verify pressure altitude calculations.

Pressure altitude affects cold weather operations because cold air is denser than standard air, which improves aircraft performance but can create altitude indication and terrain-clearance hazards.

Cold temperatures increase air density. Increased air density improves engine power, propeller thrust, and wing lift. As a result, takeoff distance decreases and climb performance improves compared with standard atmospheric conditions.

However, extremely cold temperatures also cause true altitude to be lower than indicated altitude. The altimeter may indicate the correct pressure altitude, but the aircraft will be physically closer to the terrain than indicated. This effect becomes significant during IFR approaches, departures, and operations in mountainous areas.

The table below summarises the main effects of cold weather on aircraft operations:

Because pressure altitude does not account for temperature, pilots operating in very cold conditions must apply published cold temperature corrections to minimum altitudes when required. This ensures adequate obstacle and terrain clearance during instrument departures, approaches, and missed approaches.