Pressure Altitude Calculator

Pressure Altitude

Pressure altitude is the altitude shown by an altimeter when its Kollsman window is set to the standard sea-level pressure of 29.92 inHg (1013.25 hPa). It removes local weather variations from altitude calculations and is the reference altitude used by all aircraft performance charts.

Formula

PA = field elevation + (29.92 − altimeter setting) × 1000 (inHg version)

PA = field elevation + (1013.25 − altimeter setting hPa) × 29.53 (metric version, ft)

Where 1,000 and 29.53 come from. The physically exact figure near sea level is about 925 ft per inHg, but every FAA text and every E6B uses a round 1,000 ft per inHg, because it is memorable and it errs on the conservative side. Once you accept that, the hPa figure has to follow it: one inHg is 33.8639 hPa, so 1,000 ÷ 33.8639 = 29.53 ft per hPa.

You will also see 27 ft per hPa quoted, which comes from the exact physics rather than the rounded rule. Mixing the two produces a page that gives two different answers for the same weather depending on which unit the ATIS happened to broadcast in, so this calculator uses the rounded rule consistently in both.

One footnote for the pedantic. The two published standard pressures, 29.92 inHg and 1013.25 hPa, are not quite the same pressure: 29.92 inHg is 1013.21 hPa. The 0.04 hPa gap is worth about 1 ft of pressure altitude, so the same weather entered in the two units can differ by a single foot. That is the convention talking, not an error, and no aircraft has ever noticed.

Why It Matters

Aircraft engines, propellers, and wings respond to air density, not the geometric altitude. On a low-pressure day, the air at field elevation behaves like a higher altitude. Performance charts (takeoff distance, climb rate, fuel burn) all assume standard pressure, so pilots must convert local conditions to pressure altitude before reading the chart.

Worked Example: Denver, Low-Pressure Day

• Field elevation: 5430 ft
• Altimeter setting: 29.55 inHg
• PA = 5430 + (29.92 − 29.55) × 1000 = 5430 + 370 = 5800 ft

The aircraft will perform as if it is at 5800 ft on a standard day, even though it is sitting at 5430 ft.

Pressure Altitude vs Density Altitude

For day-of-flight performance, density altitude is the one that matters most. It is calculated from pressure altitude, which is why this conversion is the first step rather than an optional extra.

Common Altimeter Settings

Reading the Result

If your pressure altitude is higher than your field elevation, expect reduced performance: longer takeoff rolls, lower climb rates, smaller useful loads. If it is lower, performance improves and the airplane feels eager. The spread between the two values is the pilot’s “weather margin.” The bigger it is, the more important the conversion becomes for safety.

Going further: density altitude

Pressure altitude is the foundation, but day-of-flight performance ultimately depends on density altitude (DA), which folds in temperature:

DA = PA + 120 × (OAT − ISA temp at that altitude)

where ISA temperature at altitude = 15°C − (2°C per 1,000 ft × altitude in thousands of ft), and OAT is the outside air temperature in °C.

A hot summer day at a mountain airport produces a density altitude far higher than the physical elevation. With a pressure altitude of 5,000 ft on a 35°C day, ISA temp is 5°C (15 − 10), so DA = 5,000 + 120 × (35 − 5) = 5,000 + 3,600 = 8,600 ft. The aircraft performs as if it were nearly 9,000 ft up, even though the wheels are on the ground. Mountain flying accidents disproportionately involve pilots who forgot to convert.