ISO 2533 / ICAO Standard Atmosphere (ISA) Calculator & Table
Lookup international standard atmosphere temperature, pressure, density, and speed of sound.
Last reviewed by the myCalculator Editorial Team
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Formula & How ISO 2533 / ICAO Standard Atmosphere (ISA) Calculator & Table Works
The International Standard Atmosphere (ISO 2533 and ICAO Doc 7488) defines a reference atmosphere with a sea level temperature of 15°C and pressure of 1,013.25 hPa, and temperature falling 6.5°C per kilometre up to 11 km. At 10,000 ft (3,048 m) it gives a temperature of −4.8°C, a pressure of about 697 hPa, an air density of 0.905 kg/m³ and a speed of sound of about 328 m/s, or 638 knots. Pilots use ISA to calculate density altitude, to compare actual temperature with the standard (ISA deviation) and to apply performance charts. Real conditions differ from ISA every day, so treat the table as a reference.
- Enter Altitude (ft) (ft): Input your target parameter value.
- Apply Formula: Calculate using
T = 288.15 - 0.0065h(m) | P = 1013.25 × (T/288.15)^5.25588 | ρ = (P×100) / (287.058×T) | a = 20.05√T. - Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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Frequently Asked Questions (FAQs)
What is the standard sea level temperature and pressure?
Standard sea level conditions are 15°C (59°F / 288.15 K) temperature, 1013.25 hPa (29.92 inHg / 14.696 psi) pressure, and 1.225 kg/m³ air density.
What is the standard temperature lapse rate in aviation?
In the troposphere (surface up to 36,089 ft / 11,000 m), temperature decreases at standard 1.98°C per 1,000 ft (approx 2°C per 1,000 ft) or 6.5°C per 1,000 metres.
What happens to temperature above the tropopause?
Above 36,089 ft (11,000 m) in the lower stratosphere, the temperature remains isothermal at -56.5°C (216.65 K) up to 65,617 ft (20,000 m).
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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.
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Operational Scenario
Operational Scenario: Coastal VFR Passage Approaching Deteriorating Stratus
You are planning a day VFR flight from Moorabbin (YMMB) to Latrobe Valley (YLTV) via the coastal visual corridor. The GAF indicates broken stratocumulus at 2,500 ft with localised coastal drizzle lowering the cloud base to 1,000 ft AMSL. The forecast freezing level is 4,500 ft with moderate mountain waves over the Strzelecki Ranges.
Practical Decisions
- •Compare the GAF area by area against your personal minimums, and decide before departure the cloud base at which you will turn back.
- •Check whether an alternate is required (CASR Part 91 MOS 8.04; AIP ENR 1.1 para 10.7.2): for an aeroplane, if you will arrive during, or up to 30 minutes before, forecast cloud of more than SCT below 1,500 ft or visibility below 8 km at the destination, you must provide for a suitable alternate — unless flying day VFR within 50 NM of the departure point.
- •Know what CAVOK means (AIP GEN 3.5): visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no cumulonimbus or towering cumulus, and no significant weather.
- •Set a fixed turn-back point before terrain traps or coastal cloud banks, and brief it as a decision already made.
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