ISA± Notation Converter
Translate both ways between actual OAT at a flight level and the ISA± shorthand of forecasts, POH columns and turbine charts — instantly, at any altitude.
ISA−18 °C at FL240 sounds frigid and is merely ISA−3 — while −18 °C at 4,000 ft is a remarkable ISA−25. The notation exists because raw OAT without altitude is meteorologically meaningless.
Formula
⚠️ For planning and education only. Weight & balance must be computed from YOUR aircraft's actual empty weight, arm and current equipment list, and verified against the POH/AFM envelope before flight.
Translate both ways between actual OAT at a flight level and the ISA± shorthand of forecasts, POH columns and turbine charts — instantly, at any altitude.
About ISA± Notation Converter
POH columns say 'ISA+20', the forecast says '−18 at FL240', and the turbine chart says 'flat rated to ISA+15' — three dialects of one quantity. This converter translates fluently in both directions at any altitude: OAT to the ISA± shorthand, or the shorthand back to the raw temperature, with the reference ISA value always displayed so the arithmetic teaches itself.
How to use ISA± Notation Converter
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula ISA± = OAT − ISA(alt); OAT = ISA(alt) + (±); ISA(alt) = 15 − 1.98·(alt/1000), floor −56.5 °C substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use ISA± Notation Converter?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula ISA± = OAT − ISA(alt); OAT = ISA(alt) + (±); ISA(alt) = 15 − 1.98·(alt/1000), floor −56.5 °C with sources cited on the page
- ✓ISA−18 °C at FL240 sounds frigid and is merely ISA−3 — while −18 °C at 4,000 ft is a remarkable ISA−25. The notation exists because raw OAT without altitude is meteorologically meaningless.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What does ISA+10 actually mean?+
That the air is 10 °C warmer than the standard atmosphere prescribes at that pressure altitude — at FL180 (ISA −20.7 °C), ISA+10 is an OAT of −10.7 °C. It travels across altitudes the way raw temperature can't: ISA+10 names the same relative density deficit, and the same performance consequence, anywhere.
Why do turbine charts say 'flat rated to ISA+15'?+
The engine can produce rated thrust/torque up to that deviation; hotter, it becomes temperature-limited and output falls each degree. The notation compresses a full altitude-temperature limit surface into three characters — and explains why hot-country operators watch the forecast deviation like a fuel gauge.
How do I read ISA± off a winds-aloft forecast quickly?+
Mental ISA first: 15 minus double the altitude in thousands (FL240 → 15 − 48 = −33). Forecast −18 there is ISA+15. Above FL360, ISA pins at −56.5, so a −51 reading at FL390 is ISA+5.5. With the doubling rule memorized, the conversion is one subtraction — this tool checks your math until it's reflex.
Is ISA± the same as the 'temperature deviation' in my FMS?+
Yes — FMS performance pages, takeoff data and cruise predictions all consume the deviation (some label it 'ΔISA'). Entering forecast ISA± at cruise levels is how the box predicts fuel and optimum altitude; a 10-degree mis-entry shifts predicted fuel several hundred kilograms on a long leg, which is why dispatch and FMS must speak the same deviation.
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