Air - Altitude, Density and Specific Volume

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The density and the specific volume of air varies with the elevation above sea level

The density of air will vary with the pressure (the Ideal Gas Law) and the altitude above sea level. Mean absolute pressure at sea level is approximately 760 mmHg (101.325 kPa, kN/m²) with a variation is about +/- 5%.

Altitude and Air Pressure

The air pressure varies with altitude as shown in the table below:

Altitude (m)	Barometer (mmHg)
0	760
75	753
150	746
250	739
300	733
450	719
600	706
750	693
900	681
1,000	668
1,200	656
1,350	644
1,500	632
1,800	609
2,100	586
2,400	564

Altitude and Air Density

Air density and specific volume as functions of altitude above sea level are indicated in the diagrams below:

air altitude meters density and specific volume diagram

air altitude feet density and specific volume diagram

STP - Standard Temperature and Pressure

STP - Standard Temperature and Pressure is 0^oC and 101.325 kN/m².

NTP - Normal Temperature and Pressure

NTP - Normal Temperature and Pressure, commonly used for testing and documentation of fan capacities, is 20^oC and 101.6 kN/m² (60^oF , 30 in Hg)

Altitude and Specific Volume Correction

The specific volume of standard air at a certain altitude can be calculated by multiplying with the volume correction factor below

Altitude		Volume Correction Factor
(m)	(ft)	Volume Correction Factor
0	0	1.00
480	1600	1.05
990	3300	1.11
1500	5000	1.17
1980	6600	1.24
2460	8200	1.31
2970	9900	1.39

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