ISA Temperature Calculator
Standard atmosphere temperature at any altitude — the 15 °C / −2 °C-per-thousand reference every deviation, chart and forecast is measured against.
The honest lapse rate is 1.9812 °C per 1,000 ft (6.5 °C/km) — '2 degrees per thousand' is the cockpit rounding. Every 'ISA+10' in a forecast or POH chart is measured from this line.
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.
Standard atmosphere temperature at any altitude — the 15 °C / −2 °C-per-thousand reference every deviation, chart and forecast is measured against.
About ISA Temperature Calculator
The International Standard Atmosphere is aviation's agreed-upon fiction: 15 °C at sea level, cooling 1.9812 °C per thousand feet to −56.5 °C at the tropopause, then holding. Every performance chart column, every 'ISA+15' forecast, every density-altitude formula measures reality against this line. This calculator returns the ISA temperature at any altitude in Celsius, Fahrenheit and kelvin — the reference half of every deviation you'll ever compute.
How to use ISA Temperature Calculator
- 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 = 15 − 1.9812 × (h/1000) °C up to 36,089 ft; −56.5 °C above (tropopause) 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 Temperature Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula ISA = 15 − 1.9812 × (h/1000) °C up to 36,089 ft; −56.5 °C above (tropopause) with sources cited on the page
- ✓The honest lapse rate is 1.9812 °C per 1,000 ft (6.5 °C/km) — '2 degrees per thousand' is the cockpit rounding. Every 'ISA+10' in a forecast or POH chart is measured from this line.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why does aviation need a standard atmosphere at all?+
Comparability: an altimeter calibrated, a performance chart drawn, and an engine rated must all assume the same air, or no two numbers could meet. ISA is that handshake — not a forecast or an average day, but a fixed reference everyone deviates from in a documented way ('ISA+12') rather than privately.
Is the real atmosphere ever actually ISA?+
Momentarily and locally at best. Real lapse rates run anywhere from inversions (warming with height) to 3 °C/1,000 ft in dry convection; real sea-level temperatures span −40 to +50 °C. ISA's value is precisely that it ignores all this: deviations get measured, not the reference moved.
Where does the 36,089 ft tropopause figure come from?+
It's where the ISA's linear lapse from 15 °C reaches −56.5 °C (11,000 m exactly — the feet are the metric definition converted). Above it ISA holds isothermal to 20 km. The real tropopause migrates — 55,000+ ft over the tropics, under 30,000 ft in polar winter — but the standard pins it for calculation.
How do I use ISA temperature with a forecast?+
Subtract: forecast (or measured) OAT minus ISA at that altitude equals the deviation that enters performance work. Winds-temps-aloft showing −15 °C at 10,000 ft, where ISA is −4.8, is ISA−10 — denser, better-performing air. POH cruise charts with ISA / ISA+20 columns expect exactly this arithmetic, which our deviation calculator automates.
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