Enter an altitude and get the ISA standard temperature instantly — add your OAT for the deviation (ISA+10) and its density altitude effect.
Educational only — always fly your POH & current weatherEducational estimates only — not for flight planning or dispatch. Fly your POH numbers.
The International Standard Atmosphere assumes 15 °C at sea level, cooling 1.98 °C per 1,000 ft up to the tropopause. Every performance chart is built on it — and today's air never matches it. Enter an altitude to get the ISA temperature, add your OAT to get the deviation ("ISA+10"), and see what it does to your density altitude.
Use pressure altitude for chart work (set 29.92 or use the pressure altitude calculator); indicated altitude is close enough for a quick estimate.
ISA temp = 15 °C − 1.98 °C per 1,000 ft of pressure altitude (−56.5 °C above 36,089 ft).
ISA deviation = OAT − ISA temp. Example: 25 °C at 5,000 ft → ISA 5 °C → ISA+20.
ISA deviation is the heart of density altitude: roughly 120 ft of density altitude for every °C above standard. Our guide walks through how to calculate density altitude step by step.
ISA temperature, pressure and air density from sea level to 45,000 ft (U.S. Standard Atmosphere, 1976). Temperatures use the same 1.98 °C per 1,000 ft lapse as the calculator above; density is the percentage of sea-level air density.
| Altitude | ISA temp | Pressure | Air density |
|---|---|---|---|
| Sea level 0 m Sea-level reference | +15.0 °C 59 °F | 29.92" 1,013 hPa | 100.0% |
| 1,000 ft 305 m | +13.0 °C 55 °F | 28.86" 977 hPa | 97.1% |
| 2,000 ft 610 m | +11.0 °C 52 °F | 27.82" 942 hPa | 94.3% |
| 3,000 ft 914 m | +9.1 °C 48 °F | 26.82" 908 hPa | 91.5% |
| 4,000 ft 1,219 m | +7.1 °C 45 °F | 25.84" 875 hPa | 88.8% |
| 5,000 ft 1,524 m | +5.1 °C 41 °F | 24.90" 843 hPa | 86.2% |
| 6,000 ft 1,829 m | +3.1 °C 38 °F | 23.98" 812 hPa | 83.6% |
| 7,000 ft 2,134 m | +1.1 °C 34 °F | 23.09" 782 hPa | 81.1% |
| 8,000 ft 2,438 m | −0.8 °C 30 °F | 22.22" 753 hPa | 78.6% |
| 9,000 ft 2,743 m | −2.8 °C 27 °F | 21.39" 724 hPa | 76.2% |
| 10,000 ft 3,048 m | −4.8 °C 23 °F | 20.58" 697 hPa | 73.8% |
| 12,000 ft 3,658 m | −8.8 °C 16 °F | 19.03" 644 hPa | 69.3% |
| 14,000 ft 4,267 m | −12.7 °C 9 °F | 17.58" 595 hPa | 65.0% |
| 16,000 ft 4,877 m | −16.7 °C 2 °F | 16.22" 549 hPa | 60.9% |
| 18,000 ft 5,486 m ≈ half the atmosphere below you | −20.6 °C −5 °F | 14.94" 506 hPa | 57.0% |
| 20,000 ft 6,096 m | −24.6 °C −12 °F | 13.75" 466 hPa | 53.3% |
| 25,000 ft 7,620 m | −34.5 °C −30 °F | 11.10" 376 hPa | 44.8% |
| 30,000 ft 9,144 m | −44.4 °C −48 °F | 8.89" 301 hPa | 37.4% |
| 35,000 ft 10,668 m | −54.3 °C −66 °F | 7.04" 238 hPa | 31.0% |
| 36,089 ft 11,000 m Tropopause — temperature stops falling | −56.5 °C −70 °F | 6.68" 226 hPa | 29.7% |
| 40,000 ft 12,192 m | −56.5 °C −70 °F | 5.54" 188 hPa | 24.6% |
| 45,000 ft 13,716 m | −56.5 °C −70 °F | 4.35" 147 hPa | 19.4% |
Exam shortcut: ISA temp = 15 − (2 × altitude in thousands). At 8,000 ft: 15 − 16 = −1 °C. The precise lapse is 1.98 °C/1,000 ft, which is why this calculator shows −0.8 °C — close enough that exams accept both.
In metric: ISA cools 6.5 °C per 1,000 m. At 2,000 m (6,562 ft) ISA is +2 °C, so an outside air temperature of 12 °C is ISA+10 — worth roughly 1,200 ft of extra density altitude.
Why deviation matters: every degree warmer than ISA adds ~120 ft of density altitude. ISA+15 at an 8,000 ft pressure altitude means your aircraft performs like it's at ~9,800 ft. Turbine flight planning uses ISA deviation directly — cruise tables are published for ISA, ISA+10, ISA+20.
Start at 15 °C and subtract about 2 °C per 1,000 ft (precisely 1.98 °C) up to the tropopause near 36,000 ft, where it holds at −56.5 °C. At 10,000 ft: 15 − 20 = about −5 °C.
The outside air is 10 °C warmer than the standard atmosphere predicts for that altitude. ISA+10 at 6,000 ft means an OAT of about +13 °C instead of the standard +3 °C — and roughly 1,200 ft of extra density altitude.
Yes — it's the temperature half of density altitude. Warmer than ISA means thinner air: longer takeoff rolls, weaker climb, reduced engine power, and higher true airspeed for the same indicated. Colder than ISA improves all of them.
Strictly, pressure altitude — ISA is defined against the standard 29.92 datum, and performance charts expect it. Below the transition altitude the difference is usually a few hundred feet, so indicated altitude gives a fine estimate when you don't have the altimeter setting handy.
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