Pressure Altitude
Altitude referenced to standard pressure; correct field elevation for the altimeter setting.
Formula
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Example
QNH 29.42, field 1000 ft: PA = 1500 ft
How it works
Pressure altitude is the altitude an aircraft would indicate if the altimeter were set to standard pressure, 29.92 inHg. It matters because air density and aircraft performance depend on pressure, so this converts a local field elevation into a standard reference. The difference between 29.92 and the current QNH is turned into feet using the rough rule of 1000 ft per 1.00 inHg, then added to the field elevation.
When to use it
Use this when calculating takeoff performance, density altitude inputs, or comparing airport elevations under different pressure settings. It is also useful anytime aviation charts or procedures require altitude based on standard pressure rather than local sea-level pressure.
Worked example, step by step
Given QNH = 30.12 inHg and field elevation h = 750 ft Step 1: Find the pressure difference: 29.92 - 30.12 = -0.20 Step 2: Convert to feet: -0.20 × 1000 = -200 ft Step 3: Add field elevation: PA = -200 + 750 = 550 ft Final answer: Pressure altitude = 550 ft
Common mistakes to avoid
- Using QFE or actual altimeter reading instead of QNH
- Forgetting that a higher QNH gives a lower pressure altitude
- Multiplying by 100 instead of 1000 when converting inches of mercury to feet
Did You Know?
Set the altimeter to 29.92 inHg and it reads pressure altitude — the common reference for all high-flying traffic.
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