Standard atmosphere table
Computed from the ICAO standard atmosphere (U.S. Standard Atmosphere 1976). Pressure altitude in feet; density as a ratio to 1.225 kg/m³.
| Altitude (ft) | Temp (°C) | Temp (°F) | Pressure (inHg) | Pressure (hPa) | Density ratio σ | Speed of sound (kt) |
|---|---|---|---|---|---|---|
| 0 | 15.0 | 59.0 | 29.92 | 1,013.3 | 1.0000 | 661 |
| 1,000 | 13.0 | 55.4 | 28.86 | 977.2 | 0.9711 | 659 |
| 2,000 | 11.0 | 51.9 | 27.82 | 942.1 | 0.9428 | 657 |
| 3,000 | 9.1 | 48.3 | 26.82 | 908.1 | 0.9151 | 655 |
| 4,000 | 7.1 | 44.7 | 25.84 | 875.1 | 0.8881 | 652 |
| 5,000 | 5.1 | 41.2 | 24.90 | 843.1 | 0.8617 | 650 |
| 6,000 | 3.1 | 37.6 | 23.98 | 812.0 | 0.8359 | 648 |
| 7,000 | 1.1 | 34.0 | 23.09 | 781.9 | 0.8106 | 645 |
| 8,000 | −0.8 | 30.5 | 22.22 | 752.6 | 0.7860 | 643 |
| 9,000 | −2.8 | 26.9 | 21.39 | 724.3 | 0.7620 | 641 |
| 10,000 | −4.8 | 23.3 | 20.58 | 696.8 | 0.7385 | 638 |
| 11,000 | −6.8 | 19.8 | 19.79 | 670.2 | 0.7156 | 636 |
| 12,000 | −8.8 | 16.2 | 19.03 | 644.4 | 0.6932 | 634 |
| 13,000 | −10.8 | 12.6 | 18.29 | 619.4 | 0.6713 | 631 |
| 14,000 | −12.7 | 9.1 | 17.58 | 595.2 | 0.6500 | 629 |
| 15,000 | −14.7 | 5.5 | 16.89 | 571.8 | 0.6292 | 626 |
| 16,000 | −16.7 | 1.9 | 16.22 | 549.2 | 0.6090 | 624 |
| 17,000 | −18.7 | −1.6 | 15.57 | 527.2 | 0.5892 | 622 |
| 18,000 | −20.7 | −5.2 | 14.94 | 506.0 | 0.5699 | 619 |
| 19,000 | −22.6 | −8.8 | 14.34 | 485.5 | 0.5511 | 617 |
| 20,000 | −24.6 | −12.3 | 13.75 | 465.6 | 0.5328 | 614 |
| 25,000 | −34.5 | −30.2 | 11.10 | 376.0 | 0.4481 | 602 |
| 30,000 | −44.4 | −48.0 | 8.89 | 300.9 | 0.3741 | 589 |
| 35,000 | −54.3 | −65.8 | 7.04 | 238.4 | 0.3099 | 576 |
| 40,000 | −56.5 | −69.7 | 5.54 | 187.5 | 0.2462 | 574 |
| 45,000 | −56.5 | −69.7 | 4.35 | 147.5 | 0.1936 | 574 |
The standard atmosphere
Air changes with weather, so aviation needs a fixed reference. The International Standard Atmosphere (ISA) defines one:
- sea level: 15 °C, 29.92 inHg (1013.25 hPa) and a density of 1.225 kg/m³;
- temperature falls 6.5 °C per kilometer (1.98 °C per 1,000 feet) up to the tropopause at 11,000 meters (36,089 feet);
- above that it stays at −56.5 °C to 20,000 meters.
Pressure follows from the temperature profile through the hydrostatic equation. The calculator computes all of it from these defining constants, and the table above is generated the same way.
Checked against the FAA's table
The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C, Figure 11-2) prints the standard atmosphere from sea level to 20,000 feet. Every row matches this calculator to the handbook's printed precision: for example 22.22 inHg and −0.9 °C at 8,000 feet, and 20.57 inHg and −4.8 °C at 10,000 feet. The defining values at the layer boundaries match exactly: 226.32 hPa at the tropopause and 54.75 hPa at 20 kilometers.
Using ISA in the cockpit
Performance charts and the E6B's altitude window assume these standard values and then correct for how the real air differs:
- ISA deviation: outside air temperature minus standard temperature at your pressure altitude. Warm days (positive deviation) mean higher density altitude and less performance.
- Density altitude: the ISA altitude with the same density as the real air. See the density altitude calculator.
- Pressure altitude: the ISA altitude with the same pressure. See the pressure altitude calculator.
- True airspeed: grows as density falls. See the true airspeed calculator.
Enter an altitude above the table's rows, up to 65,000 feet, and the calculator switches to the isothermal stratosphere automatically.
A worked example: ISA deviation at cruise
At a pressure altitude of 8,500 feet the standard temperature is −1.8 °C. If the outside air temperature there is +10 °C, the air is ISA +11.8 °C, and the density altitude is about 9,870 feet: the airplane performs as if it were almost 1,400 feet higher. Performance charts often show cruise speeds and fuel flows at ISA, ISA +20 and ISA −20 for exactly this reason. Enter an actual temperature in the calculator to get the deviation and density altitude together.
Speed of sound and Mach
The speed of sound depends only on temperature: 661 knots on a standard sea-level day, falling to 574 knots at −56.5 °C above the tropopause. That is why the same true airspeed is a higher Mach number at altitude. The calculator lists it for every altitude, and the true airspeed calculator turns it into a Mach number.
Questions pilots ask
What is the International Standard Atmosphere?
It is a reference model of the atmosphere: 15 °C and 29.92 inHg (1013.25 hPa) at sea level, temperature falling 1.98 °C per 1,000 feet up to 36,089 feet, then a constant −56.5 °C. Altimeters, airspeed indicators and performance charts are calibrated to it.
How do I find ISA deviation?
Subtract the standard temperature at your pressure altitude from the outside air temperature. At 10,000 feet the standard temperature is −4.8 °C, so an outside air temperature of 5 °C is ISA +10 °C. Enter the actual temperature to see it.
What is the standard lapse rate?
About 2 °C (3.5 °F) per 1,000 feet in the troposphere; precisely 6.5 °C per kilometer, or 1.98 °C per 1,000 feet. Pressure falls about 1 inch of mercury per 1,000 feet in the lower atmosphere.
Which standard does this use?
The ICAO standard atmosphere, identical to the U.S. Standard Atmosphere 1976 up to 20 kilometers (65,617 feet). Altitudes are geopotential, as aviation uses them.