ISA Deviation Calculator

Every pilot knows the moment: you need ISA deviation for a performance calculation, the freezing level for icing risk, or a cold-temperature altitude correction — and you're doing it by hand at 0300 local time. Stop doing ISA deviation math on the back of a nav log. Set the ISA deviation in your FMC the right way. In under 10 seconds. On any day of the year. WHY IT MATTERS The FMC assumes standard atmosphere unless told otherwise. On an ISA+20 day, that assumption puts your TOD kilometres off and your descent path several hundred feet from where idle thrust will actually hold it. A correct ISA deviation value fixes that. - Accurate TOD. Start descent where the aircraft actually wants to start, not where a ICAO standard day says it should. - Cleaner idle descent. Less speedbrake, less thrust on descent, less energy-management on the way down. - Stable non-precision approaches. FMCs that apply cold/hot temperature compensation use this value to correct the vertical path at every fix — improving altitude accuracy and reducing the risk of go-arounds and runway excursions. Some even correct after the FAF! This app gets that number to you in seconds. Straight into the box. No paper tables, no mental math, no forgotten column. Just plain accuracy. BUILT TO THE STANDARD THAT MATTERS ISA (International Standard Atmosphere) is a standardized model defining how temperature, pressure, and density change with altitude under fixed conditions. It is used in aviation, aerospace and engineering for performance calculations and calibration. The lapse rates, pressure altitudes, and temperature gradients inside this app trace directly to the official documents your aircraft performance manuals reference. Coverage spans from ground level to 80 km (ICAO) and 86 km (US-1976), including the tropopause transition — so whether you're operating a turboprop at FL260, a Boeing 737 at FL370 or a Gulfstream at FL510, the math is correct. ISO-ZERO AND FREEZING LEVEL, INSTANTLY Enter OAT or SAT at any flight level and the app computes the ISO-zero (isotherm 0°C) freezing level in both feet and meters, corrected for your departure field elevation. No interpolation tables. No mental arithmetic. Just the number you need, when you need it. ALTITUDE-WEIGHTED AVERAGING — THE RIGHT WAY A single-level ISA deviation misses the full picture. The trapezoidal altitude-weighted average integrates your OAT inputs across the actual altitude band flown, reflecting the physics of how temperature changes with height. Classic arithmetic averaging is also available when that's what your operation demands. FOR EVERY AVIATION PROFESSIONAL Whether you're an airline pilot, a flight dispatcher building a flight release, an ATPL student preparing for the official exams, a cargo crew planning a cold-soak departure, a GA pilot checking icing levels, a glider pilot reading a thermal gradient, or a flight-sim enthusiast in MSFS or X-Plane chasing accurate performance numbers — this app was built for your workflow. It's equally at home in collegiate aerospace programs, meteorology coursework, drone and HAPS mission planning, and skydiving weather briefings. KEY CAPABILITIES Enter temperatures in Celsius, Fahrenheit, or Kelvin. Smart auto-signing applies the correct negative sign at cold levels so you never mis-enter a sub-zero OAT. Color-coded deviation alerts match FMC conventions: green for ISA, yellow for ±20°C, amber for ±21–30°C, and red beyond ±30°C. Add unlimited custom flight levels alongside the preset GND, FL100, FL200, and FL310 reference points. ZERO COMPROMISES ON PRIVACY ISA Deviation Calculator is 100% offline. No account required, no data collected, no ads, no subscription. Install it once and it works on the flight deck, in a simulator bay, in a classroom, or anywhere your device goes — including on final approach with the cabin door closed. Download ISA Deviation Calculator and get the standard atmosphere answer in the time it takes to read this sentence.