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Almost every performance chart, altimeter and exam question in aviation refers back to one model: the International Standard Atmosphere, or ISA. It describes an “average” day so that aircraft and instruments can be compared on the same basis. Real days are rarely standard, and the difference between the real air and ISA is what changes your takeoff distance, climb rate and true airspeed. This guide explains the ISA values you need to know and how to use them.
Look up standard values at any altitude with the ISA Standard Atmosphere Calculator.
The ISA values to memorise
| Quantity | ISA sea-level value |
|---|---|
| Temperature | 15°C (59°F) |
| Pressure | 1013.25 hPa (29.92 inHg) |
| Density | 1.225 kg/m³ |
| Temperature lapse rate | 1.98°C per 1,000 ft (about 2°C) |
| Tropopause | 36,089 ft, where temperature stays at −56.5°C |
| Pressure change near sea level | About 1 hPa per 27–30 ft |
ISA at common altitudes
| Pressure altitude | ISA temp | Pressure | Density ratio |
|---|---|---|---|
| 0 ft | 15.0°C | 1013 hPa | 1.000 |
| 2,000 ft | 11.0°C | 942 hPa | 0.943 |
| 5,000 ft | 5.1°C | 843 hPa | 0.862 |
| 10,000 ft | −4.8°C | 697 hPa | 0.738 |
| 18,000 ft | −20.7°C | 506 hPa | 0.570 |
| FL350 | −54.3°C | 238 hPa | 0.310 |
A quick mental method for ISA temperature: double the altitude in thousands of feet and subtract from 15. At 8,000 ft, 15 − 16 = −1°C.
ISA deviation
ISA deviation is the difference between the actual temperature and the ISA temperature at that pressure altitude. It is written as ISA +10, ISA −5 and so on.
ISA deviation = actual OAT − ISA temperature
Example: at 3,000 ft pressure altitude, ISA is 15 − 6 = 9°C. If the OAT is 25°C, conditions are ISA +16.
Many jet and turboprop performance tables are entered with ISA deviation rather than absolute temperature, so this is a sum worth practising.
Why it matters in hot climates
In tropical countries such as the Philippines, Thailand or the Gulf states, it is common to see ISA +15 to +20 at airfield level for much of the year. Warmer air is less dense, so:
- the engine produces less power, especially a non-turbocharged piston engine;
- the propeller and wings have less air to work with, so takeoff roll increases and climb rate falls;
- true airspeed is higher than indicated, so ground speed on approach and landing is higher.
Pilots combine these effects into one number, density altitude. A rule of thumb is that density altitude rises about 120 ft for every 1°C above ISA. Our Density Altitude Calculator and the guide Density Altitude Explained cover it in detail.
ISA and your altimeter
An altimeter is a barometer calibrated to ISA. It assumes the standard pressure lapse with height. When the air is colder than ISA, the pressure levels are closer together and the aircraft is lower than the altimeter shows. That is the reason for the saying “from high to low, look out below”, and for cold-temperature corrections on instrument approaches. Setting 1013 hPa or 29.92 inHg gives pressure altitude, which is what flight levels and performance charts use; see QNH, QFE and QNE Explained.
Speed of sound
The speed of sound depends only on temperature: about 661 kt at ISA sea level, falling to about 573 kt in the stratosphere. That is why the same Mach number gives a lower true airspeed at cruise altitude than near the ground.
Exam-style practice
- What is the ISA temperature at FL100? 15 − 19.8 = −4.8°C.
- OAT at 6,000 ft pressure altitude is 20°C. What is the ISA deviation? ISA = 3.1°C, so ISA +16.9.
- Approximate density altitude for question 2? 6,000 + 120 × 16.9 ≈ 8,000 ft.
Related tools and guides
- ISA Standard Atmosphere Calculator
- Pressure Altitude Calculator
- True Airspeed Calculator
- Student Pilot Ground School Toolkit
For training and general information. Values follow the ICAO Standard Atmosphere (Doc 7488). Use approved performance data for flight planning.
Sources and further reading
- ICAO Doc 7488, Manual of the ICAO Standard Atmosphere
- FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25) (aerodynamics and weather theory chapters)
Sources checked November 2026. Rules differ between countries and change over time: for real flights, use the current publications of the authority that applies to you, your aircraft flight manual and your operator’s procedures.