Whether you’re a student pilot prepping for a checkride or a seasoned flyer heading into high-density altitude conditions, understanding Pressure Altitude (PA) is critical for safety. It isn’t just a theoretical number; it’s the altitude your aircraft thinks it’s at when the altimeter is set to the standard datum of 29.92 “Hg.

Why It Matters

Aircraft performance charts for takeoff distance and climb rate are based on standard atmospheric conditions. Since the atmosphere is rarely “standard,” you must convert your indicated altitude to pressure altitude to use these charts accurately.

The “Quick & Dirty” Formula

To calculate PA without a flight computer, use this reliable rule of thumb:

1. Find the Difference: Subtract the current local altimeter setting from the standard pressure (29.92).
2. Multiply by 1,000: Every 1-inch change in mercury roughly equals 1,000 feet of altitude.
3. Adjust Elevation: Add (or subtract) this result to your field elevation.

The Math:

$$PA = (\text{29.92} – \text{Current Altimeter}) \times 1,000 + \text{Field Elevation}$$

Quick Reference Table

Based on a sea-level airport (0 ft elevation)

Frequently Asked Questions

What is the difference between Pressure Altitude and Density Altitude?

Pressure Altitude corrects for non-standard pressure. Density Altitude takes PA and further corrects it for non-standard temperature. High heat makes the air “thinner,” increasing your density altitude.

When is Pressure Altitude equal to True Altitude?

Only when atmospheric conditions perfectly match the International Standard Atmosphere (ISA)—specifically when the pressure at sea level is exactly 29.92 “Hg.

Can I find PA just by looking at my altimeter?

Yes! If you are sitting on the ramp, temporarily turn your altimeter setting knob to 29.92. The needle will move to indicate your current Pressure Altitude. (Just remember to set it back to the local setting before takeoff!)