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

Altitude and Air Density

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

 

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 (68^oF, 30 in Hg)

Note! - other definitions of STP and NTP are commonly used.

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 and Air Pressure

Altitude		Pressure
feet	meter	inHg	torr	psia	kPa
0	0	29.92	759.97	14.7	101.3
500	152	29.39	746.51	14.43	99.4
1000	305	28.86	733.04	14.17	97.6
1500	457	28.34	719.84	13.91	95.8
2000	610	27.82	706.63	13.66	94.1
2500	762	27.32	693.93	13.41	92.4
3000	914	26.82	681.23	13.17	90.7
3500	1067	26.33	668.78	12.93	89.1
4000	1219	25.84	656.34	12.69	87.4
4500	1372	25.37	644.4	12.46	85.8
5000	1524	24.9	632.46	12.23	84.3
5500	1676	24.44	620.78	12	82.7
6000	1829	23.98	609.09	11.77	81.1
6500	1981	23.53	597.66	11.55	79.6
7000	2134	23.09	586.49	11.34	78.1
7500	2286	22.66	575.56	11.13	76.7
8000	2438	22.23	564.64	10.91	75.2
8500	2591	21.81	553.97	10.71	73.8
9000	2743	21.39	543.31	10.5	72.3
9500	2896	20.98	532.89	10.3	71.0
10000	3048	20.58	522.73	10.1	69.6
10500	3200	20.19	512.83	9.91	68.3
11000	3353	19.8	502.92	9.72	67.0
11500	3505	19.41	493.01	9.53	65.7
12000	3658	19.03	483.36	9.34	64.4
12500	3810	18.66	473.96	9.16	63.1
13000	3962	18.3	464.82	8.99	61.9
13500	4115	17.94	455.68	8.81	60.7
14000	4267	17.58	446.53	8.63	59.5
14500	4420	17.24	437.9	8.46	58.3
15000	4572	16.89	429.01	8.29	57.1
15500	4724	16.56	420.62	8.13	56.0
16000	4877	16.22	411.99	7.96	54.8
16500	5029	15.9	403.86	7.81	53.8
17000	5182	15.58	395.73	7.65	52.7
17500	5334	15.26	387.6	7.47	51.5
18000	5486	14.95	379.73	7.34	50.6
18500	5639	14.65	372.11	7.19	49.5
19000	5791	14.35	364.49	7.05	48.6
19500	5944	14.05	356.87	6.9	47.5
20000	6096	13.76	349.5	6.76	46.6
20500	6248	13.48	342.39	6.62	45.6
21000	6401	13.2	335.28	6.48	44.6
21500	6553	12.92	328.17	6.34	43.7
22000	6706	12.65	321.31	6.21	42.8
22500	6858	12.38	314.45	6.08	41.9
23000	7010	12.12	307.85	5.95	41.0
23500	7163	11.86	301.24	5.82	40.1
24000	7315	11.61	294.89	5.7	39.3
24500	7468	11.36	288.54	5.58	38.4
25000	7620	11.12	282.45	5.46	37.6
25500	7772	10.88	276.35	5.34	36.8
26000	7925	10.64	270.26	5.22	36.0
26500	8077	10.41	264.41	5.11	35.2
27000	8230	10.18	258.57	5	34.5
27500	8382	9.96	252.98	4.89	33.7
28000	8534	9.74	247.4	4.78	32.9
28500	8687	9.53	242.06	4.68	32.2
29000	8839	9.31	236.47	4.57	31.5
29500	8992	9.11	231.39	4.47	30.8
30000	9144	8.9	226.06	4.37	30.1

Temperature vs. Elevation Density Correction

The chart above can be used to estimate "temperature vs. elevation density correction factor". The density at a given temperature and elevation can then be calculated as

Example - Density at Elevation 4000 ft and Temperature 300 ^oF

From the chart above - at elevation 4000 ft and temperature 300 ^oF the correction factor is approximate 1.7. The density can be calculated as