What is Pressure Altitude?
Pressure altitude is your altimeter reading when set to the standard reference of 29.92 inHg (1013.25 hPa) — the universal baseline used in aviation performance calculations.
It removes local pressure variations from the equation, allowing pilots everywhere to share a common altitude reference. Pressure altitude underpins density altitude calculations, aircraft performance charts, and high-altitude IFR operations.
How to Calculate Pressure Altitude
To calculate pressure altitude, apply the standard aviation approximation:
PA = Field Elevation + (29.92 − QNH) × 1,000
Where QNH is the current altimeter setting in inHg, and the result is in feet. Each 0.01 inHg difference from standard is worth approximately 10 ft of pressure altitude correction. If QNH is below 29.92, pressure altitude is higher than field elevation; if QNH is above 29.92, pressure altitude is lower. For metric inputs, the equivalent is: PA = Field Elevation + (1013.25 − QNH_hPa) × 27.
Step-by-Step Example
Enter your field elevation and QNH into the pressure altitude calculator above to get an instant result, or work through it manually:
Suppose your field elevation is 1,500 ft and the current altimeter setting (QNH) is 29.72 inHg.
- Difference from standard: 29.92 − 29.72 = 0.20 inHg
- Correction: 0.20 × 1,000 = 200 ft
- Pressure Altitude: 1,500 + 200 = 1,700 ft
Because the local pressure is below standard (29.72 vs. 29.92), the air is less dense than standard sea level, and the pressure altitude is 200 ft higher than the field elevation. This value then feeds directly into your density altitude calculation.
When Pressure Altitude Matters
Pressure altitude is used in several critical contexts. On the ground, pilots check that the altimeter reads field elevation ±75 ft when set to QNH — a larger discrepancy indicates a faulty instrument. In the flight levels (above the transition altitude, e.g., 18,000 ft MSL in the US), all aircraft set 29.92 so everyone shares the same reference, preventing collisions. Pressure altitude is also the required input for aircraft performance charts: takeoff distance, climb gradient, and fuel burn tables are all computed at a given pressure altitude. Finally, it is the first step in calculating density altitude, which governs actual aircraft performance. Use the pressure altitude calculator at the top of this page to check your value before consulting your aircraft's performance charts.
Frequently Asked Questions
- What is the difference between pressure altitude and indicated altitude?
- Indicated altitude is what your altimeter reads when set to the local QNH — it approximates your height above mean sea level under current conditions. Pressure altitude is what the altimeter reads when set to 29.92 inHg. The two values are equal only when local QNH happens to equal 29.92. On a low-pressure day, pressure altitude will be higher than indicated altitude; on a high-pressure day, lower.
- When should I set 29.92 on my altimeter?
- In the US, set 29.92 when climbing above 18,000 ft MSL (the base of Class A airspace) and when operating in the flight levels. You also use 29.92 temporarily to read pressure altitude during a preflight altimeter check and when entering performance data into your E6B or flight computer. Return to QNH any time you are operating below the transition altitude in controlled or uncontrolled airspace.