Question 1

What model does this calculator use?

Accepted Answer

It implements the International Standard Atmosphere (ISA) as defined by ICAO, identical to the U.S. Standard Atmosphere 1976 below about 32 km (roughly 105,000 ft). Sea-level reference values are 15 degC, 1013.25 hPa (29.9213 inHg), and a density of 1.225 kg/m3, with a tropospheric lapse rate of 6.5 degC per km up to the 11 km tropopause.

Question 2

What is the difference between geometric and geopotential altitude?

Accepted Answer

Geometric altitude is true geometric height above sea level. Geopotential altitude is an adjusted altitude that accounts for the slight decrease of gravity with height, and the standard atmosphere equations are defined on it. The difference is small in the troposphere but grows with altitude; this tool lets you enter either and reports both.

Question 3

What is density ratio and why does it matter?

Accepted Answer

Density ratio (sigma) is the air density at altitude divided by the sea-level standard density (1.225 kg/m3). It drives aircraft and engine performance: true airspeed, required runway, and climb rate all depend on it. Density ratio is also the link between indicated/calibrated airspeed and true airspeed.

Question 4

How is the speed of sound calculated?

Accepted Answer

Speed of sound a = sqrt(gamma x R x T), where gamma = 1.4 (ratio of specific heats for dry air), R = 287.05287 J/(kg·K), and T is absolute temperature in kelvin. Because it depends only on temperature in ISA, the speed of sound decreases with altitude until the tropopause, then stays constant through the isothermal layer.

Question 5

Is the standard atmosphere the actual weather?

Accepted Answer

No. ISA is an idealized, fixed reference used for instrument calibration, performance charts, and comparison. Real conditions deviate from ISA; the difference (ISA deviation) is exactly why density altitude and true-altitude corrections exist. Use official weather data for actual conditions.

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

Standard Atmosphere Calculator & Table

Standard Atmosphere Table

Frequently Asked Questions

Related Calculators

Density Altitude Calculator

Pressure Altitude Calculator

True Airspeed (TAS) Calculator

Mach Number Calculator

Want AI explanations, checkride questions, and saved calculations?