About this tool
ISA Temperature & Deviation Calculator is a free, in-browser tool that gives the International Standard Atmosphere (ISA) temperature and pressure at any altitude, and — if you enter the actual outside air temperature — how far the real air deviates from standard. Enter the altitude in feet and, optionally, the OAT in °C or °F; everything is computed locally.
Below the tropopause the ISA temperature follows the standard lapse rate: ISA = 15 − 1.98 × (altitude ÷ 1000) °C, holding at about −56.5 °C above roughly 36,089 feet. Standard pressure comes from P = 1013.25 × (1 − 6.8756×10⁻⁶ × altitude)^5.2559 hectopascals, also shown in inches of mercury. The ISA deviation is simply your OAT minus the ISA temperature, the value that drives density-altitude and true-airspeed corrections.
Use it to reference standard conditions at a cruise level, to sanity-check temperatures aloft, or to feed the ISA-deviation figure into performance planning. It is a standard-atmosphere estimate for study and planning — not for real-world navigation — so verify against actual soundings and official sources.
Frequently asked questions
What is ISA temperature?
The International Standard Atmosphere defines 15 °C at sea level and a lapse of about 1.98 °C per 1,000 feet: ISA = 15 − 1.98 × (altitude ÷ 1000). Above the tropopause (~36,089 ft) it stays near −56.5 °C. It is the reference against which real temperatures are compared.
How is the standard pressure computed?
With the standard-atmosphere relation P = 1013.25 × (1 − 6.8756e-6 × altitude)^5.2559 hectopascals, which the tool also converts to inches of mercury. It is most accurate within the troposphere, up to about 36,000 feet.
What does ISA deviation mean?
It is your actual outside air temperature minus the ISA temperature at that altitude — for example 'ISA +10'. Warm deviations increase density altitude and true airspeed and reduce performance; cold deviations do the opposite.
Is the entered temperature required?
No. Leave the OAT blank to get just the standard temperature and pressure at your altitude. Enter it to also see the ISA deviation. As with all these tools, treat the output as an estimate for study, not for real-world navigation, and confirm with official sources.
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