The International Standard Atmosphere (ISA) is a model representing an average of atmospheric conditions around the globe. In aviation, it is used as a reference for determining aircraft performance at different altitudes and for calibrating avionics.

The ISA assumes the following conditions at mean sea level:

• Temperature is 15° C
• Air pressure is 1013.25 hPa (29.92 inHg)
• Air density is 1.225 kg/m³

Conditions at other altitudes can be described using formulas or tables, e.g:

Source: getting to grips with aircraft performance, Airbus Customer Services, January 2002