ISA Deviation Calculator

Enter altitude in metres and outside air temperature to calculate a temperature reference and signed deviation.

Enter altitude from 0 to 11,000 m and outside air temperature from −100 to 60 °C.

ISA_°C = 15 − 6.5 × altitude_m / 1000; deviation_°C = OAT_°C − ISA_°C

ISA temperature = 15 − 6.5 × (altitude / 1,000) °C; deviation = OAT − ISA temperature. Altitude is limited to the tropospheric teaching model, 0–11,000 m (about 36,089 ft). It does not apply stratospheric layers or local weather corrections. A positive deviation means warmer than the reference; negative means colder. This gives no aircraft performance or operational advice and must not be used for flight decisions.

Reference: FAA Pilot’s Handbook of Aeronautical Knowledge