What is the ISA standard temperature formula?

Below the tropopause, ISA temperature is 15 degrees Celsius minus 1.98 degrees per 1,000 ft of pressure altitude. At 10,000 ft that is 15 minus 19.8, or about minus 4.8 degrees Celsius. Above the standard tropopause it stays constant at minus 56.5 degrees.

What is ISA deviation?

ISA deviation is the actual outside air temperature minus the ISA standard temperature at the same altitude, written as ISA plus or minus a number. ISA plus 15 means the air is 15 degrees warmer than standard, which thins the air and reduces engine and wing performance.

Where is the tropopause in the ISA model?

The standard tropopause sits at 36,089 ft, where the temperature reaches minus 56.5 degrees Celsius. Above that, up to about 65,000 ft, the standard atmosphere is isothermal, so the temperature stops falling and holds at minus 56.5 degrees.

Why does ISA deviation matter for performance?

Air density depends on temperature, and most performance charts are built around standard conditions. A positive ISA deviation means warmer, thinner air: longer takeoff and landing rolls, reduced climb, and a higher true airspeed for a given indicated airspeed. Charts correct for it using the deviation.

Is this pressure altitude or true altitude?

The ISA model is defined against pressure altitude, the altitude shown when the altimeter is set to 1013.25 hPa or 29.92 inHg. Use pressure altitude for the most consistent results, especially when feeding the answer into performance and true-airspeed calculations.

ISA Temperature & Deviation Calculator

The International Standard Atmosphere is the reference every performance chart, altimeter, and airspeed system is built around. This calculator gives the standard ISA temperature at any pressure altitude and, if you supply the actual outside air temperature, the ISA deviation that drives performance and true-airspeed corrections.

How it works

Below the tropopause the ISA temperature falls linearly from 15 degrees Celsius at sea level at a lapse rate of 1.98 degrees per 1,000 ft:

ISA temp (C) = 15 - 1.98 x (pressure altitude / 1000)

This holds up to the standard tropopause at 36,089 ft, where the temperature reaches minus 56.5 degrees Celsius. Above that the model is isothermal and the temperature stays at minus 56.5 degrees. The ISA deviation is simply the actual OAT minus this standard value, expressed as ISA plus or minus a number.

Worked example

At 10,000 ft, ISA standard temperature is 15 minus 19.8, about minus 4.8 degrees Celsius. If the measured OAT up there is plus 5 degrees, the deviation is about ISA plus 10 — warmer than standard, meaning thinner air and reduced climb and takeoff performance.

Notes

Feed pressure altitude, not true altitude, for consistency with altimetry and performance charts. A positive ISA deviation always means degraded performance and a higher true airspeed for a given indicated airspeed, which is exactly why hot days call for careful runway and climb planning.