What is the ISA lapse rate?

Under the International Standard Atmosphere (ISA), sea-level temperature is 15 °C at 1013.25 hPa and decreases by 2 °C for every 1,000 ft of pressure altitude.

How is pressure altitude calculated from QNH?

Pressure altitude equals field elevation plus (1013.25 − QNH) × 30 ft per hPa. When QNH is below standard, pressure altitude is higher than field elevation.

How is density altitude calculated?

Density altitude = pressure altitude + 120 × (OAT − ISA temperature at that pressure altitude). Hotter-than-ISA air reduces density and raises density altitude, reducing aircraft climb and take-off performance.

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Density & Pressure Altitude Calculator

Enter field elevation and QNH (1013.25 hPa ISA), or pressure altitude directly. OAT in °C or °F. ISA lapse rate: 15 °C at MSL, −2 °C per 1,000 ft.

Field Elevation AMSL (ft)

≈ 609.6 m AMSL

QNH (hPa)1013.25 hPa

Low 980ISA 1013.25High 1040

Outside Air Temperature (OAT)28 °C (82.4 °F)

Formula: Pressure Alt = Elev + (1013.25 − QNH) × 30 ft · Density Alt = PA + 120 × (OAT − ISA temp). ISA temp = 15 − 2 × (PA / 1000) °C.

Results

Pressure Altitude

2,000 ft

From 2,000 ft elev @ 1013.25 hPa QNH

ISA Temperature @ PA

11 °C (51.8 °F)

OAT deviation: +17 °C from ISA

Density Altitude

4,040 ft

+2040 ft above pressure altitude due to temperature

High density altitude — expect noticeably reduced climb and take-off performance.

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How it works

Pressure altitude

When you set your altimeter to 1013.25 hPa (29.92 inHg), the reading is pressure altitude. From field elevation, we derive it as: elevation + (1013.25 − QNH) × 30 ft per hPa.

ISA standard atmosphere

At mean sea level under ISA, temperature is 15 °C at 1013.25 hPa. Temperature decreases 2 °C for every 1,000 ft of pressure altitude — not every 100 ft. At 5,000 ft PA, ISA temp is 5 °C.

Density altitude

Hot air is less dense, so aircraft perform as if they are at a higher altitude. The rule-of-thumb used by pilots and CASA training materials: add 120 ft to pressure altitude for every 1 °C that OAT exceeds ISA at that level.