What is pressure altitude?

Pressure altitude is the altitude indicated when the altimeter is set to the standard datum of 29.92 inHg, or 1013.25 hPa. It is the height in the standard atmosphere corresponding to the ambient pressure, and is the basis for flight levels and most performance charts.

What is the correction formula?

Pressure altitude equals indicated altitude plus 1,000 feet for every inch of mercury the altimeter setting is below 29.92, and minus 1,000 feet for every inch above. In hectopascals the factor is about 27 feet per hPa from 1013.25.

Why not just set 29.92 on the altimeter?

You can, and at and above the transition altitude pilots do exactly that to read pressure altitude directly as a flight level. This calculator lets you compute it on the ground from the local setting without re-baroing the altimeter.

When do I need pressure altitude?

Performance charts for takeoff, climb, and cruise are indexed by pressure altitude, and it feeds the density altitude calculation. Oxygen and cabin-altitude rules are also defined in terms of pressure altitude rather than indicated altitude.

Does temperature affect pressure altitude?

No. Pressure altitude depends only on pressure and the altimeter setting. Temperature does affect true altitude and density altitude, but the pressure-altitude conversion itself is purely a pressure correction from the standard datum.

Pressure Altitude Calculator

Performance charts, flight levels, and the density-altitude calculation all start from pressure altitude rather than the indicated altitude on your altimeter. This calculator makes the conversion in one step, applying the standard correction from the 29.92 inHg datum to whatever altimeter setting you have dialled in.

How it works

Pressure altitude shifts the indicated altitude by the difference between your altimeter setting and the standard datum:

inHg:  pressure altitude = indicated + (29.92 − setting) × 1000
hPa:   pressure altitude = indicated + (1013.25 − setting) × 27.3

When the local setting is below standard the air is low-pressure, so pressure altitude is higher than indicated; when the setting is above standard, pressure altitude is lower. The hPa factor of about 27.3 feet per hectopascal is the metric equivalent of the 1,000 feet per inch of mercury rule.

Example and notes

At an indicated altitude of 3,000 feet with an altimeter setting of 29.42 inHg, the setting is 0.50 inHg below standard, so pressure altitude is 3,000 + 0.50 × 1000 = 3,500 feet. With a 1003 hPa setting at the same indicated altitude, the correction is (1013.25 − 1003) × 27.3 ≈ 280 feet, giving about 3,280 feet. Use this pressure altitude, not the field elevation, when reading performance charts or feeding the density altitude tool.