Pressure Altitude & ISA Calculator
Compute true Pressure Altitude (PA), standard ISA temperature, and ISA deviation relative to the 1013.25 hPa / 29.92 inHg standard datum.
- 30 ft per 1 hPa: Every 1 hPa drop in pressure below 1013.25 adds approx. 30 ft to your Pressure Altitude.
- -2°C per 1,000 ft: Standard ISA temperature drops 1.98°C (approx. 2°C) per 1,000 ft above Mean Sea Level (MSL 15°C).
- Order of Calculation: Always compute Pressure Altitude first, determine ISA Temp at that PA, and then solve for Density Altitude.
Altimetry Calculation Results
How Pressure Altitude is Calculated
Aviation performance calculations rely on the International Standard Atmosphere (ISA) datum of 1013.25 hPa (29.92 inHg) at Mean Sea Level with a base temperature of 15°C.
Pressure Datum Offset
Uses the standard barometric lapse rate where every 1 hPa difference equals 30 ft of vertical air column.
Lapse Rate from 15°C
Standard atmosphere cools by 1.98°C (rounded to 2°C in flight tests) for each 1,000 ft gained.
Temperature Correction
Warmer air expands and becomes less dense. Each 1°C above standard adds 120 ft of effective density altitude.
Worked Real-World Aviation Example
Scenario: You are planning a departure from an aerodrome at 1,500 ft AMSL. The ATIS reports QNH 1003 hPa and an OAT of 28°C.
= 1500 + 307.5 = 1,808 ft
= 15 - 3.6 = 11.4°C
= 1808 + 1992 = 3,800 ft
Frequently Asked Questions
Why do pilots calculate Pressure Altitude first?
Pressure Altitude standardises your position on the international atmospheric gradient. Because standard temperature drops at 2°C per 1,000 ft, you must know your Pressure Altitude before you can determine whether today's ambient temperature is hotter or colder than standard ISA.
What does "From High to Low, Look Out Below" mean?
When flying from an area of high barometric pressure to an area of low barometric pressure without updating your altimeter subscale, your altimeter will continue to read higher than your actual height above mean sea level. You will be flying dangerously closer to terrain than indicated.
What is the difference between hPa and inHg?
Australia, Europe, and ICAO standard countries use hectopascals (hPa, equivalent to millibars), where standard sea-level pressure is 1013.25 hPa. The United States uses inches of mercury (inHg), where standard pressure is 29.92 inHg. 1 inHg is approximately 33.86 hPa.
How does high density altitude affect aircraft?
Thinner air at high density altitude reduces wing lift, decreases propeller thrust efficiency, and starves naturally aspirated engines of oxygen. Takeoff distance increases dramatically, climb rates diminish, and true airspeed for a given indicated airspeed is higher.
Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
CASR Part 91 general operating and flight rules, Part 121 fuel reserves, and flight crew licensing standards.
Aviation weather services, standard atmosphere (ISA) profiles, METAR/TAF aerodrome weather reports.
Aeronautical Information Publication (AIP), En Route Supplement Australia (ERSA), and airspace procedures.
Interactive CASA exam practice question banks for RPL, PPL and CPL syllabus preparation, fixed-wing and helicopter.
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