Properties of Air: Density, Viscosity, Cp, k at 1 atm

Dry Air: Properties at a Glance

This page gives the properties of dry air from −100 to 1000 °C (−148 to 1832 °F) and from 1 atm (101.325 kPa) to 100 bar. Density, specific heats, speed of sound and compressibility factor come from the Lemmon et al. (2000) equation of state. Viscosity and thermal conductivity come from the Lemmon & Jacobsen (2004) equations. Both were evaluated with CoolProp 7.2.0 (fluid “Air”). The calculator below returns all properties for one temperature and pressure.

Air in this model is a mixture of nitrogen, oxygen and argon only (no CO₂ or water). For humid air see the wet bulb calculator. For density at altitude see the density altitude calculator.

Air Property Calculators

Explore the full set of air property calculators on KasperCalc:

Density Specific Heat Capacity Thermal Conductivity Dynamic and Kinematic Viscosity Prandtl Number Thermal Diffusivity Thermal Expansion Coefficient Speed of Sound

Air All-Properties Calculator

Degrees (°F):

Degrees (°C):

Pressure:

Density

Density (kg/m³):

Density (lb/ft³):

Compressibility factor Z:

Viscosity

Dynamic viscosity (µPa·s):

Dynamic viscosity (lbm/(ft·s) ×10⁻⁵):

Kinematic viscosity (mm²/s):

Thermal properties

cp (J/(kg·K)):

cp (Btu/(lb·°F)):

cv (J/(kg·K)):

γ = cp/cv:

Thermal conductivity (mW/(m·K)):

Thermal conductivity (Btu/(h·ft·°F)):

Thermal diffusivity (mm²/s):

Prandtl number:

Expansion and sound

β (×10⁻₃ 1/K):

Speed of sound (m/s):

Speed of sound (ft/s):

Dry air, absolute pressure. Range: −100 to 1000 °C (−148 to 1832 °F) and 1 atm to 100 bar (14.696 to 1450 psia). Outside it the calculator shows “Out of Range”. Values are interpolated linearly in temperature (10 °C grid) and in ln P between 1 atm, 2, 5, 10, 20, 50 and 100 bar. Interpolation adds about 0.1 % at most (checked against CoolProp at six off-grid points).

Combines the Density, Viscosity, Specific Heat, Thermal Conductivity, Thermal Diffusivity, Prandtl Number, Thermal Expansion and Speed of Sound calculators. Kinematic viscosity, diffusivity and Prandtl number are formed from the interpolated μ, ρ, cp and k.

Composition and Constants of Dry Air

Quantity	Value	Source
Composition used for the equation of state (mole fraction)	N₂ 0.7812, O₂ 0.2096, Ar 0.0092	Lemmon et al. 2000, Table 3
U.S. Standard Atmosphere 1976 sea-level dry air (mole fraction)	N₂ 0.78084, O₂ 0.209476, Ar 0.00934, CO₂ 0.000314, plus Ne, He, Kr, Xe, CH₄, H₂ (total about 0.00002)	US Std Atm 1976, Table 3
Molar mass, U.S. Standard Atmosphere	28.9644 g/mol (28.9645 from Table 3 by hand)	US Std Atm 1976
Molar mass, Lemmon et al. 2000	28.9586 g/mol (±0.0002)	Lemmon et al. 2000, Table 2
Molar mass, CoolProp (used for the numbers here)	28.96546 g/mol	CoolProp 7.2.0 Air entry
Universal gas constant R*	8.31432 J/(mol·K)	US Std Atm 1976
Specific gas constant R = R*/M₀	287.05 J/(kg·K) (53.35 ft·lbf/(lbm·°R))	Calculated from the two lines above
Critical point	132.5306 K (−140.62 °C), 3.7860 MPa (37.86 bar), 342.68 kg/m³	Lemmon et al. 2000, Table 2
Maxcondentherm (highest-temperature point on the dew/bubble curve, reducing point)	132.6312 K (−140.52 °C), 3.78502 MPa, 10.4477 mol/dm³	Lemmon et al. 2000, Table 2
Solidification point (lower limit of the equation)	59.75 K, 0.005265 MPa	Lemmon et al. 2000, Table 2
Normal boiling point (bubble point at 1 atm)	78.903 K (−194.25 °C)	Lemmon et al. 2000, Table 2
Range of the equation of state	60 to 2000 K, up to 2000 MPa	Lemmon et al. 2000

Every temperature on this site’s air pages (≥ 173 K) and every pressure (≤ 100 bar) is above the critical temperature (132.5 K). All 777 grid points were checked with CoolProp and are gas or supercritical, never liquid or two-phase. Air is a mixture, so the saturation line has a dew point and a bubble point, which differ by about 2.8 K at 1 atm (78.9 and 81.7 K).

Dry Air at Standard Reference Temperatures, 1 atm

Property	0 °C (32 °F)	15 °C (59 °F), ISA	20 °C (68 °F), NTP	25 °C (77 °F)
Density (kg/m³)	1.2931	1.2255	1.2046	1.1843
Density (lb/ft³)	0.080724	0.076508	0.075199	0.073935
cp (J/(kg·K))	1005.7	1006.0	1006.1	1006.3
cv (J/(kg·K))	716.95	717.46	717.67	717.89
γ	1.4027	1.4022	1.4020	1.4018
Dynamic viscosity (µPa·s)	17.218	17.962	18.206	18.448
Kinematic viscosity (mm²/s)	13.316	14.656	15.114	15.577
Thermal conductivity (mW/(m·K))	24.361	25.499	25.874	26.247
Thermal diffusivity (mm²/s)	18.733	20.682	21.349	22.023
Prandtl number	0.71083	0.70864	0.70796	0.70730
β (×10⁻₃ 1/K)	3.6740	3.4809	3.4210	3.3631
Speed of sound (m/s)	331.441	340.410	343.344	346.251

Calculated with CoolProp 7.2.0 at 101.325 kPa. Conventions: ISA sea level is 15 °C and 101.325 kPa. NTP (NIST) is 20 °C and 101.325 kPa. The older “STP” is 0 °C and 101.325 kPa. IUPAC STP since 1982 is 0 °C and 100 kPa, which gives a density 1.3 % lower. Always state which one is meant.

Check Against the U.S. Standard Atmosphere 1976 (15 °C, 1 atm)

Property	CoolProp (this site)	US Std Atm 1976	Difference
Density (kg/m³)	1.2255	1.2250	+0.044 %
Dynamic viscosity (Pa·s)	1.7962×10⁻⁵	1.7894×10⁻⁵	+0.377 %
Speed of sound (m/s)	340.410	340.294 (Table III: 340.29)	+0.034 %
Thermal conductivity (W/(m·K))	0.025499	0.025326 (eq. 53)	+0.682 %
Sutherland's law μ (Pa·s)	1.7893×10⁻⁵	1.7894×10⁻⁵	-0.006 %
Ideal gas density, R = 287.05 (kg/m³)	1.22501	1.2250	+0.001 %

The density difference comes mostly from the molar mass: CoolProp uses 28.96546 g/mol, the Standard Atmosphere 28.9644 g/mol (and 314 ppm CO₂, an older value than today’s). The Standard Atmosphere conductivity table prints 0.025362 W/(m·K) (after the NASA errata for its units), 0.14 % above its own eq. 53. All differences are far inside the 5 % stated for viscosity and conductivity.

Property Summary at 1 atm

Property	-100 °C (-148 °F)	0 °C (32 °F)	100 °C (212 °F)	200 °C (392 °F)	400 °C (752 °F)	600 °C (1112 °F)	800 °C (1472 °F)	1000 °C (1832 °F)
Density (kg/m³)	2.047	1.293	0.9459	0.7458	0.5242	0.4041	0.3288	0.2772
cp (J/(kg·K))	1008.8	1005.7	1011.2	1025.0	1068.5	1115.1	1154.2	1184.7
γ	1.408	1.403	1.398	1.390	1.368	1.347	1.331	1.320
Dynamic viscosity (µPa·s)	11.78	17.22	21.90	26.05	33.28	39.60	45.32	50.63
Kinematic viscosity (mm²/s)	5.756	13.32	23.15	34.92	63.50	97.98	137.8	182.7
Thermal conductivity (mW/(m·K))	16.21	24.36	31.62	38.25	50.24	61.14	71.35	81.10
Thermal diffusivity (mm²/s)	7.849	18.73	33.06	50.04	89.70	135.7	188.0	247.0
Prandtl number	0.7334	0.7108	0.7003	0.6980	0.7079	0.7222	0.7331	0.7397
β (×10⁻₃ 1/K)	5.850	3.674	2.683	2.115	1.486	1.145	0.9316	0.7853
Speed of sound (m/s)	263.48	331.44	387.00	434.61	514.26	581.19	640.53	694.69

Calculated with CoolProp 7.2.0 at 101.325 kPa. Each property has its own page with a chart, a calculator and a table every 20 °C, at 1 atm, 10 bar and 100 bar.

Accuracy

Property	Stated uncertainty	Source
Density	about 0.1 % up to 873 K (600 °C) and 70 MPa; about 0.5 % above 873 K (rising to 1 % at 2000 K and 2000 MPa)	Lemmon et al. 2000 (abstract)
Speed of sound	about 0.2 % up to 873 K and 70 MPa	Lemmon et al. 2000 (abstract)
Specific heat (cp, cv)	about 1 % up to 873 K and 70 MPa; not stated above	Lemmon et al. 2000 (abstract)
Viscosity and thermal conductivity	about 5 % for air, higher in the critical region; the temperature and pressure limits of the equations were not read (abstract only)	Lemmon & Jacobsen 2004 (abstract)
Values above about 600 °C and the 1000 °C end	Less certain than the figures above, especially μ, k and Pr	This site (see note)

The uncertainty of Prandtl number, thermal diffusivity and kinematic viscosity combines the uncertainties of the properties they come from. The full text of Lemmon & Jacobsen (2004) was not opened; do not read more into its 5 % than the abstract says.

Reference Data Sources and Standards for Air

Source	What it gives	Link
Lemmon, Jacobsen, Penoncello & Friend (2000), Thermodynamic Properties of Air and Mixtures of Nitrogen, Argon, and Oxygen from 60 to 2000 K at Pressures to 2000 MPa	Equation of state for dry air: density, cp, cv, speed of sound, compressibility factor. Basis of every number on these pages.	doi.org/10.1063/1.1285884
Lemmon & Jacobsen (2004), Viscosity and Thermal Conductivity Equations for Nitrogen, Oxygen, Argon, and Air	Correlations for the dynamic viscosity and thermal conductivity of air.	doi.org/10.1023/B:IJOT.0000022327.04529.f3
NIST REFPROP (Standard Reference Database 23)	NIST reference software that implements the Lemmon air equations.	www.nist.gov/srd/refprop
CoolProp (Bell, Wronski, Quoilin & Lemort 2014)	Open-source property library used to compute the tables on these pages.	doi.org/10.1021/ie4033999
U.S. Standard Atmosphere, 1976 (NOAA / NASA / USAF, NASA-TM-X-74335)	Dry-air composition, molar mass 28.9644 g/mol, sea-level density, viscosity, speed of sound and thermal conductivity, and the standard atmosphere profile.	ntrs.nasa.gov/citations/19770009539
ISO 2533:1975, Standard Atmosphere	International standard atmosphere (cited only; the standard is not reproduced).	www.iso.org/standard/7472.html

These are the references the data on the air pages rest on. NIST REFPROP and ISO 2533 are listed for reference. REFPROP was not run for this site and ISO 2533 is a paid standard that was not opened. Air has no manufacturer data sheets, so data sources and standards are listed here in place of products.

Sources

E. W. Lemmon, R. T. Jacobsen, S. G. Penoncello and D. G. Friend, “Thermodynamic Properties of Air and Mixtures of Nitrogen, Argon, and Oxygen From 60 to 2000 K at Pressures to 2000 MPa,” J. Phys. Chem. Ref. Data 29(3), 331–385, 2000. doi:10.1063/1.1285884. Equation of state for dry air: density, specific heats, speed of sound, compressibility factor. Opened the NIST reprint, accessed 25 Oct 2026.
E. W. Lemmon and R. T. Jacobsen, “Viscosity and Thermal Conductivity Equations for Nitrogen, Oxygen, Argon, and Air,” Int. J. Thermophys. 25(1), 21–69, 2004. doi:10.1023/B:IJOT.0000022327.04529.f3. Viscosity and thermal conductivity of air. Only the abstract was read (NIST publication page), accessed 25 Oct 2026.
I. H. Bell, J. Wronski, S. Quoilin and V. Lemort, “Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp,” Ind. Eng. Chem. Res. 53(6), 2498–2508, 2014. doi:10.1021/ie4033999. The software used (CoolProp 7.2.0, fluid “Air”) to evaluate the two correlations above.
NOAA, NASA and USAF, U.S. Standard Atmosphere, 1976, NASA-TM-X-74335. ntrs.nasa.gov/citations/19770009539. Sea-level reference values, dry-air composition and Sutherland constants, accessed 25 Oct 2026.

All numbers on this page are calculated, not measured here. See the Air General Properties page for the accuracy statements and the cross-check against the U.S. Standard Atmosphere.

General Properties of Dry Air: Composition, Constants and Calculator

Cite This Work