Pressure at Altitude Calculator

How Pressure Altitude Works

Pressure altitude is the altitude at which the International Standard Atmosphere (ISA) would have the same atmospheric pressure as currently observed at your location. It is the reference altitude used for aircraft performance calculations and altimeter settings.

Standard atmosphere reference:

• Sea level pressure: 1013.25 hPa (29.92 inHg)
• Sea level temperature: 15°C (59°F)
• Pressure lapse rate: approximately 1 hPa per 27 feet up to 5,000 ft

Pressure altitude formula:

Pressure Altitude (ft) = (1013.25 − Station Pressure in hPa) × 27.3 + Elevation (ft)

Worked example:

• Airport elevation: 2,400 ft
• Current altimeter setting (QNH): 29.78 inHg = 1008 hPa

PA = (1013.25 − 1008) × 27.3 + 2,400 PA = 5.25 × 27.3 + 2,400 PA = 143 + 2,400 = 2,543 ft pressure altitude

Why this matters for pilots:

Aircraft performance (takeoff roll, climb rate, engine power) depends on air density, not your GPS altitude. A hot, low-pressure day at a high-elevation airport can create a density altitude far higher than the physical elevation — effectively making the aircraft perform as if it were thousands of feet higher.

Density altitude (the actual performance reference):

DA = PA + [120 × (Outside Air Temp °C − ISA Temp at that altitude)]

ISA temperature at altitude = 15 − (2°C per 1,000 ft × altitude/1,000)

On a 35°C day at 2,543 ft PA, ISA temp = 15 − 5 = 10°C:

DA = 2,543 + [120 × (35 − 10)] = 2,543 + 3,000 = 5,543 ft density altitude

That airport effectively performs like a 5,500 ft airport on that hot day.