Standard Atmosphere Calculator & Table
Compute ISA temperature, pressure, density, density ratio, and speed of sound at any altitude using the International Standard Atmosphere.
Standard Atmosphere Table
| Alt (ft) | Temp (°C) | Press (hPa) | Press (inHg) | σ (dens ratio) | a (kt) |
|---|---|---|---|---|---|
| 0 | 15 | 1013.25 | 29.921 | 1 | 661.5 |
| 2,000 | 11.04 | 942.14 | 27.821 | 0.9428 | 656.9 |
| 4,000 | 7.08 | 875.13 | 25.843 | 0.8881 | 652.3 |
| 6,000 | 3.12 | 812.05 | 23.98 | 0.8359 | 647.7 |
| 8,000 | -0.84 | 752.71 | 22.228 | 0.7861 | 643 |
| 10,000 | -4.8 | 696.95 | 20.581 | 0.7386 | 638.3 |
| 12,000 | -8.76 | 644.58 | 19.035 | 0.6933 | 633.6 |
| 14,000 | -12.72 | 595.46 | 17.584 | 0.6502 | 628.9 |
| 16,000 | -16.67 | 549.43 | 16.225 | 0.6092 | 624.1 |
| 18,000 | -20.63 | 506.32 | 14.952 | 0.5702 | 619.2 |
| 20,000 | -24.59 | 466.01 | 13.761 | 0.5332 | 614.4 |
| 22,000 | -28.54 | 428.34 | 12.649 | 0.498 | 609.5 |
| 24,000 | -32.49 | 393.18 | 11.611 | 0.4646 | 604.5 |
| 26,000 | -36.45 | 360.4 | 10.643 | 0.433 | 599.5 |
| 28,000 | -40.4 | 329.88 | 9.741 | 0.4031 | 594.5 |
| 30,000 | -44.35 | 301.49 | 8.903 | 0.3747 | 589.4 |
| 32,000 | -48.3 | 275.11 | 8.124 | 0.348 | 584.3 |
| 34,000 | -52.25 | 250.64 | 7.401 | 0.3227 | 579.2 |
| 36,000 | -56.2 | 227.97 | 6.732 | 0.2988 | 574 |
| 38,000 | -56.5 | 207.15 | 6.117 | 0.2719 | 573.6 |
| 40,000 | -56.5 | 188.23 | 5.558 | 0.2471 | 573.6 |
| 42,000 | -56.5 | 171.04 | 5.051 | 0.2245 | 573.6 |
| 44,000 | -56.5 | 155.43 | 4.59 | 0.204 | 573.6 |
| 46,000 | -56.5 | 141.24 | 4.171 | 0.1854 | 573.6 |
| 48,000 | -56.5 | 128.35 | 3.79 | 0.1685 | 573.6 |
| 50,000 | -56.5 | 116.64 | 3.444 | 0.1531 | 573.6 |
| 52,000 | -56.5 | 106 | 3.13 | 0.1391 | 573.6 |
| 54,000 | -56.5 | 96.33 | 2.845 | 0.1264 | 573.6 |
| 56,000 | -56.5 | 87.55 | 2.585 | 0.1149 | 573.6 |
| 58,000 | -56.5 | 79.57 | 2.35 | 0.1044 | 573.6 |
| 60,000 | -56.5 | 72.31 | 2.135 | 0.0949 | 573.6 |
The standard atmosphere is an idealized reference, not actual weather. Use official weather products for real temperature, pressure, and winds. For performance planning, combine ISA values with the actual conditions and your POH/AFM. Read our full Safety Disclaimer.
Frequently Asked Questions
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OpenThe Standard Atmosphere in Depth
The International Standard Atmosphere (ISA) is a mathematical model that defines how temperature, pressure, and density change with altitude in an idealized atmosphere. It is the common reference that makes aircraft performance data, altimetry, and airspeed calibration meaningful across different days and locations. Without a shared standard, no two performance charts could be compared.
The model begins at sea level with a temperature of 15 degrees Celsius, a pressure of 29.92 inHg (1013.25 hPa), and a defined air density, then applies a fixed lapse rate of about 2 degrees Celsius per 1,000 feet up to the tropopause near 36,000 feet, above which the temperature is treated as constant. Real atmospheres deviate from this model constantly, and those deviations are exactly what density altitude and true-altitude corrections quantify.
Step-by-Step: How the Calculation Works
This calculator computes ISA temperature, pressure, density, density ratio, and the speed of sound at any altitude you enter, using the standard lapse rate below the tropopause and constant-temperature relationships above it. Pressure and density decrease exponentially with height, while temperature decreases linearly until the tropopause.
The density ratio — the local density divided by sea-level standard density — is especially useful, because true airspeed, performance, and many aerodynamic quantities scale with it. The searchable table lets you compare standard values at different altitudes and see how quickly the air thins as you climb.
More Worked Examples
Example 1 — Conditions at 10,000 ft
Standard temperature = 15 − 2 × 10 = −5°C. Pressure has fallen to roughly 20.58 inHg and density to about 74 percent of the sea-level value, which is why performance and true-airspeed corrections are already significant at this common cruising altitude.
Example 2 — At the tropopause
Near 36,000 ft the standard temperature reaches about −56.5°C and then holds constant with further climb. The speed of sound bottoms out around 574 knots, which is why jet Mach limits are most constraining in this band.
Pilot Decision-Making Context
The standard atmosphere is less a decision tool than a decision foundation. Every time you compute density altitude, correct true altitude for temperature, or read a performance chart, you are measuring the real atmosphere against this standard. Recognizing when conditions are far from standard — an unusually hot day, a strong temperature inversion, a very cold winter morning — tells you when to expect the largest performance and altimetry deviations.
Instructors use the standard atmosphere to explain why aircraft behave differently on different days, and pilots use ISA deviation (how far today differs from standard) as a quick shorthand for expected performance. A day reported as ISA+20 warns you immediately that density altitude will be high.
More Questions Answered
What is ISA deviation?
It is the difference between the actual temperature and the standard temperature at your altitude. A day that is 20 degrees warmer than standard is described as ISA+20 and will produce a notably higher density altitude.
Why does the model stop the lapse rate at the tropopause?
Above roughly 36,000 feet in the standard model the temperature stops decreasing and holds constant through the lower stratosphere. The ISA reflects this by using constant-temperature relationships above the tropopause.
Is the real atmosphere ever exactly standard?
Almost never. The standard atmosphere is an average reference. Actual temperature and pressure vary with weather, season, and location, which is precisely why pilots compute corrections rather than assuming standard conditions.
Related Reading & Tools
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