Standard Atmosphere Table (ISA)
ICAO and ISO 2533 standard atmosphere: temperature, pressure, density and speed of sound from sea level to 20 km.
The International Standard Atmosphere (ISA) is a reference model of how air temperature, pressure and density change with altitude. Aircraft instruments, engine ratings, wind and fan calculations all use it as a common baseline. At sea level it is 15 C, 101.325 kPa and 1.225 kg/m³; the temperature falls by 6.5 K per kilometer up to 11 km and then stays constant at −56.5 C.
The table
| Altitude (m) | Altitude (ft) | Temperature (C) | Pressure (kPa) | Pressure (inHg) | Density (kg/m3) | Speed of sound (m/s) |
|---|---|---|---|---|---|---|
| 0 | 0 | 15 | 101.325 | 29.921 | 1.225 | 340.3 |
| 1000 | 3281 | 8.5 | 89.875 | 26.54 | 1.1116 | 336.4 |
| 2000 | 6562 | 2 | 79.495 | 23.475 | 1.0065 | 332.5 |
| 3000 | 9843 | -4.5 | 70.109 | 20.703 | 0.9091 | 328.6 |
| 4000 | 13123 | -11 | 61.64 | 18.202 | 0.8191 | 324.6 |
| 5000 | 16404 | -17.5 | 54.02 | 15.952 | 0.7361 | 320.5 |
| 6000 | 19685 | -24 | 47.181 | 13.933 | 0.6597 | 316.4 |
| 7000 | 22966 | -30.5 | 41.061 | 12.125 | 0.5895 | 312.3 |
| 8000 | 26247 | -37 | 35.6 | 10.513 | 0.5252 | 308.1 |
| 9000 | 29528 | -43.5 | 30.742 | 9.078 | 0.4663 | 303.8 |
| 10000 | 32808 | -50 | 26.436 | 7.807 | 0.4127 | 299.5 |
| 11000 | 36089 | -56.5 | 22.632 | 6.683 | 0.3639 | 295.1 |
| 12000 | 39370 | -56.5 | 19.33 | 5.708 | 0.3108 | 295.1 |
| 13000 | 42651 | -56.5 | 16.51 | 4.876 | 0.2655 | 295.1 |
| 14000 | 45932 | -56.5 | 14.102 | 4.164 | 0.2268 | 295.1 |
| 15000 | 49213 | -56.5 | 12.045 | 3.557 | 0.1937 | 295.1 |
| 16000 | 52493 | -56.5 | 10.287 | 3.038 | 0.1654 | 295.1 |
| 17000 | 55774 | -56.5 | 8.787 | 2.595 | 0.1413 | 295.1 |
| 18000 | 59055 | -56.5 | 7.505 | 2.216 | 0.1207 | 295.1 |
| 19000 | 62336 | -56.5 | 6.41 | 1.893 | 0.1031 | 295.1 |
| 20000 | 65617 | -56.5 | 5.475 | 1.617 | 0.088 | 295.1 |
Source and notes: ICAO / ISO 2533 standard atmosphere: sea level 288.15 K, 101 325 Pa, lapse rate 6.5 K per km to 11 km, isothermal 216.65 K above.
How to use this table
- Find the altitude in meters or feet.
- Read the pressure, density and speed of sound at that altitude, for the standard (not the actual) day.
- To correct for a hotter or colder day, scale the density by the ratio of absolute temperatures; the pressure at a given altitude changes much less.
Worked example: air at 3 000 m (9 843 ft)
- Temperature -4.5 C, pressure 70.109 kPa (69.2 percent of sea level), density 0.9091 kg/m³.
- Density ratio σ = 0.742, so a fan or an engine breathing this air moves 26 percent less mass per unit volume than at sea level.
- Speed of sound 328.6 m/s, so Mach 1 is 1 183 km/h at this altitude.
Notes and cautions
- The real atmosphere varies with weather and season. Standard values are a convention, not a forecast.
- Humidity lowers air density slightly. For density altitude with humidity, use the Psychrometric calculator and the My Dashboard pressure and density altitude widget.
Related tools and guides
- Psychrometric Calculator and Chart
- Moist Air and the Psychrometric Chart (guide)
- Pressure Converter
- All reference tables
FAQ
What is the standard atmosphere?
A fixed model of the atmosphere used so everyone gets the same numbers: 15 C and 101.325 kPa at sea level, a lapse rate of 6.5 K/km in the troposphere, and a constant −56.5 C above 11 km.
How does air density change with altitude?
It falls roughly exponentially: about 9 percent lower at 1 km, and about half of its sea-level value at 5 500 m.
Sources
- ISO 2533:1975, Standard Atmosphere; ICAO Doc 7488.
- ICAO / ISO 2533 standard atmosphere: sea level 288.15 K, 101 325 Pa, lapse rate 6.5 K per km to 11 km, isothermal 216.65 K above.