Air properties calculator

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Why visitors arrive: You need dry-air density, viscosity, or conductivity at a temperature for CFD, similarity, or lab sizing.

Your question: What are air properties at my temperature and pressure, and which correlations are used?

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At 20 °C and atmospheric pressure, dry air has a density of 1.204 kg/m³, a dynamic viscosity of 1.813 × 10-5 Pa·s, a thermal conductivity of 0.02566 W/(m·K) and a Prandtl number of 0.7076. The calculator below returns eight properties for any temperature from 200 K to 2000 K, and three more when a pressure is given. Between 250 K and 400 K its viscosity, conductivity and specific heat agree with Incropera and DeWitt's published table to within 0.8 percent.

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Air properties calculator

Compute temperature-dependent thermophysical properties of dry air. Optionally include pressure to obtain density, thermal diffusivity and kinematic viscosity.

What the calculator returns

Where the calculator's outputs come from. From the temperature alone, eight: viscosity μ and conductivity k from Sutherland's law, each with its own constants; the specific heats c p from the Aly-Lee fit and c v as c p less the gas constant; γ and the Prandtl number computed from those; the ratio c p over k; and the enthalpy h, relative to 0 °C. With a pressure as well, three more: density ρ from the ideal gas law, kinematic viscosity ν = μ/ρ and thermal diffusivity α = k/(ρ c p).WHERE EACH OUTPUT COMES FROMINPUTTemperatureK, °C or °FTICK THE BOXPressureand temperatureFROM TEMPERATURE ALONE: EIGHTSUTHERLAND'S LAWμ and kseparate constantsALY-LEE FITcp and cvcv: cp less thegas constantCOMPUTEDγ and Prfrom thoseRATIOcp / kturns μ into PrENTHALPYhrelative to 0 °CWITH A PRESSURE AS WELL: THREE MOREIDEAL GAS LAWdensity ρKINEMATIC VISCOSITYν = μ / ρTHERMAL DIFFUSIVITYα = k / (ρcp)
WHERE EACH OUTPUT COMES FROMINPUTTemperatureK, °C or °FTICK THE BOXPressureand temperatureFROM TEMPERATURE ALONE: EIGHTSUTHERLAND'S LAWμ and kseparate constantsALY-LEE FITcp and cvcv: cp less thegas constantCOMPUTEDγ and Prfrom thoseRATIOcp / kturns μ into PrENTHALPYhrelative to 0 °CWITH A PRESSURE AS WELL: THREE MOREIDEAL GAS LAWdensity ρKINEMATIC VISCOSITYν = μ / ρTHERMAL DIFFUSIVITYα = k / (ρcp)
EIGHT FROM TEMPERATURE ALONEINPUTTemperature, in K, °C or °FSUTHERLAND'S LAWμ and kseparate constantsALY-LEE FITcp and cvcv: cp less thegas constantCOMPUTEDγ and Prfrom thoseRATIOcp / kturns μ into PrENTHALPYhrelative to 0 °CTHREE MORE WITH A PRESSURETICK THE BOXPressure, with the temperatureIDEAL GAS LAWdensity ρKINEMATIC VISCOSITYν = μ / ρTHERMAL DIFFUSIVITYα = k / (ρcp)
Density is the only output that needs the pressure directly; kinematic viscosity and thermal diffusivity need it through density.

Enter one air temperature, in K, °C or °F, and the tool returns eight properties. Tick the pressure box and it returns three more, because density, kinematic viscosity and thermal diffusivity all need a pressure as well as a temperature. Every result has a copy button, and the set can be downloaded as a text file.

  • Thermal conductivity of air, k: in W/(m·K). This is the quantity usually meant by the k value of air
  • Dynamic viscosity, µ: in kg/(m·s), which is the same unit as Pa·s. Divide by 0.001 for centipoise
  • Specific heat at constant pressure, cp, and at constant volume, cv: in J/(kg·K)
  • Ratio of specific heats, γ = cp/cv: dimensionless, and written k in some thermodynamics and compressible flow texts
  • Prandtl number, Pr = cpµ/k: dimensionless
  • Ratio cp/k: in (m·s)/kg, the factor that turns the viscosity into the Prandtl number
  • Specific enthalpy, h: in J/kg, taken relative to 0 °C
  • Density, ρ: in kg/m³, from the ideal gas law. Needs the pressure box
  • Kinematic viscosity, ν = µ/ρ: in m²/s. Needs the pressure box
  • Thermal diffusivity, α = k/(ρcp): in m²/s. Needs the pressure box

What is the k value of air?

In heat transfer, the k value of air is its thermal conductivity: 0.02606 W/(m·K) for dry air at 25 °C. In compressible flow, k often means the ratio of specific heats instead, cp/cv, which is 1.40206 at the same temperature and appears here as γ. The tool returns both from the one temperature.

Properties of air at atmospheric pressure

Dry air at 1 atm, which is 101325 Pa, from -50 °C to 100 °C. Every value is what the calculator above returns for that temperature, rounded to at most four significant figures, so the table and the tool cannot disagree.

T, °CT, KDensity ρ, kg/m³Dynamic viscosity µ, 10-5 Pa·sKinematic viscosity ν, 10-5 m²/sThermal conductivity k, W/(m·K)cp, J/(kg·K)Prandtl number Pr
-50223.151.5811.4570.9220.0198510000.7343
-25248.151.4221.5891.1180.0219810000.7234
0273.151.2921.7161.3290.0240510010.7140
15288.151.2241.7891.4610.0252610010.7091
20293.151.2041.8131.5070.0256610010.7076
25298.151.1831.8371.5520.0260610010.7061
30303.151.1641.8611.5990.0264510020.7046
40313.151.1271.9071.6930.0272310020.7019
50323.151.0921.9531.7890.0280010030.6993
100373.150.94552.1732.2990.0317310060.6891

Read the 20 °C row as a viscosity of 1.813 × 10-5 Pa·s and a thermal conductivity of 0.02566 W/(m·K). The calculator treats viscosity, conductivity, cp and the Prandtl number as functions of temperature alone, which holds for air near atmospheric pressure. Density does depend on pressure: double the pressure and the density doubles while the kinematic viscosity halves. For any other temperature or pressure, use the calculator.

Checked against published values

Compared with the dry air table in Incropera and DeWitt, Fundamentals of Heat and Mass Transfer, Table A.4, from 250 K to 400 K the calculator's viscosity agrees to within 0.7 percent, its thermal conductivity to within 0.8 percent and its specific heat to within 0.6 percent. Viscosity is in units of 10-5 Pa·s.

T, Kµ, this calculatorµ, publishedDifferencek, this calculatork, publishedDifference
2501.5991.596+0.19%0.022140.0223-0.72%
3001.8461.846+0.00%0.026200.0263-0.38%
3502.0742.082-0.38%0.030030.0300+0.10%
4002.2852.301-0.70%0.033640.0338-0.47%

The agreement is best near room temperature and loosens away from it. At 200 K the conductivity reads 1.6 percent low, because viscosity and conductivity each follow Sutherland's law, a two constant fit that is most accurate close to its reference temperature.

Correlations & valid range

Four properties of dry air at 1 atm against temperature from -50 to 100 degrees Celsius: Density from 1.581 to 0.946 kg/m³; Dynamic viscosity from 1.457 to 2.173 10⁻⁵ Pa·s; Thermal conductivity from 0.0198 to 0.0317 W/(m·K); Prandtl number from 0.7343 to 0.6891 .DRY AIR AT 1 ATM, FROM THE TABLE BELOWDensitykg/m³-50 °C: 1.581 kg/m³-25 °C: 1.422 kg/m³0 °C: 1.292 kg/m³15 °C: 1.224 kg/m³20 °C: 1.204 kg/m³25 °C: 1.183 kg/m³30 °C: 1.164 kg/m³40 °C: 1.127 kg/m³50 °C: 1.092 kg/m³100 °C: 0.946 kg/m³-50 °C: 1.581100 °C: 0.946Dynamic viscosity10⁻⁵ Pa·s-50 °C: 1.457 10⁻⁵ Pa·s-25 °C: 1.589 10⁻⁵ Pa·s0 °C: 1.716 10⁻⁵ Pa·s15 °C: 1.789 10⁻⁵ Pa·s20 °C: 1.813 10⁻⁵ Pa·s25 °C: 1.837 10⁻⁵ Pa·s30 °C: 1.861 10⁻⁵ Pa·s40 °C: 1.907 10⁻⁵ Pa·s50 °C: 1.953 10⁻⁵ Pa·s100 °C: 2.173 10⁻⁵ Pa·s-50 °C: 1.457100 °C: 2.173Thermal conductivityW/(m·K)-50 °C: 0.0198 W/(m·K)-25 °C: 0.0220 W/(m·K)0 °C: 0.0240 W/(m·K)15 °C: 0.0253 W/(m·K)20 °C: 0.0257 W/(m·K)25 °C: 0.0261 W/(m·K)30 °C: 0.0265 W/(m·K)40 °C: 0.0272 W/(m·K)50 °C: 0.0280 W/(m·K)100 °C: 0.0317 W/(m·K)-50 °C: 0.0198100 °C: 0.0317Prandtl number-50 °C: 0.7343 -25 °C: 0.7234 0 °C: 0.7140 15 °C: 0.7091 20 °C: 0.7076 25 °C: 0.7061 30 °C: 0.7046 40 °C: 0.7019 50 °C: 0.6993 100 °C: 0.6891 -50 °C: 0.7343100 °C: 0.6891
DRY AIR AT 1 ATM, FROM THE TABLE BELOWDensitykg/m³-50 °C: 1.581 kg/m³-25 °C: 1.422 kg/m³0 °C: 1.292 kg/m³15 °C: 1.224 kg/m³20 °C: 1.204 kg/m³25 °C: 1.183 kg/m³30 °C: 1.164 kg/m³40 °C: 1.127 kg/m³50 °C: 1.092 kg/m³100 °C: 0.946 kg/m³-50 °C: 1.581100 °C: 0.946Dynamic viscosity10⁻⁵ Pa·s-50 °C: 1.457 10⁻⁵ Pa·s-25 °C: 1.589 10⁻⁵ Pa·s0 °C: 1.716 10⁻⁵ Pa·s15 °C: 1.789 10⁻⁵ Pa·s20 °C: 1.813 10⁻⁵ Pa·s25 °C: 1.837 10⁻⁵ Pa·s30 °C: 1.861 10⁻⁵ Pa·s40 °C: 1.907 10⁻⁵ Pa·s50 °C: 1.953 10⁻⁵ Pa·s100 °C: 2.173 10⁻⁵ Pa·s-50 °C: 1.457100 °C: 2.173Thermal conductivityW/(m·K)-50 °C: 0.0198 W/(m·K)-25 °C: 0.0220 W/(m·K)0 °C: 0.0240 W/(m·K)15 °C: 0.0253 W/(m·K)20 °C: 0.0257 W/(m·K)25 °C: 0.0261 W/(m·K)30 °C: 0.0265 W/(m·K)40 °C: 0.0272 W/(m·K)50 °C: 0.0280 W/(m·K)100 °C: 0.0317 W/(m·K)-50 °C: 0.0198100 °C: 0.0317Prandtl number-50 °C: 0.7343 -25 °C: 0.7234 0 °C: 0.7140 15 °C: 0.7091 20 °C: 0.7076 25 °C: 0.7061 30 °C: 0.7046 40 °C: 0.7019 50 °C: 0.6993 100 °C: 0.6891 -50 °C: 0.7343100 °C: 0.6891
DRY AIR AT 1 ATMDensitykg/m³-50 °C: 1.581 kg/m³-25 °C: 1.422 kg/m³0 °C: 1.292 kg/m³15 °C: 1.224 kg/m³20 °C: 1.204 kg/m³25 °C: 1.183 kg/m³30 °C: 1.164 kg/m³40 °C: 1.127 kg/m³50 °C: 1.092 kg/m³100 °C: 0.946 kg/m³-50 °C: 1.581100 °C: 0.946Dynamic viscosity10⁻⁵ Pa·s-50 °C: 1.457 10⁻⁵ Pa·s-25 °C: 1.589 10⁻⁵ Pa·s0 °C: 1.716 10⁻⁵ Pa·s15 °C: 1.789 10⁻⁵ Pa·s20 °C: 1.813 10⁻⁵ Pa·s25 °C: 1.837 10⁻⁵ Pa·s30 °C: 1.861 10⁻⁵ Pa·s40 °C: 1.907 10⁻⁵ Pa·s50 °C: 1.953 10⁻⁵ Pa·s100 °C: 2.173 10⁻⁵ Pa·s-50 °C: 1.457100 °C: 2.173Thermal conductivityW/(m·K)-50 °C: 0.0198 W/(m·K)-25 °C: 0.0220 W/(m·K)0 °C: 0.0240 W/(m·K)15 °C: 0.0253 W/(m·K)20 °C: 0.0257 W/(m·K)25 °C: 0.0261 W/(m·K)30 °C: 0.0265 W/(m·K)40 °C: 0.0272 W/(m·K)50 °C: 0.0280 W/(m·K)100 °C: 0.0317 W/(m·K)-50 °C: 0.0198100 °C: 0.0317Prandtl number-50 °C: 0.7343 -25 °C: 0.7234 0 °C: 0.7140 15 °C: 0.7091 20 °C: 0.7076 25 °C: 0.7061 30 °C: 0.7046 40 °C: 0.7019 50 °C: 0.6993 100 °C: 0.6891 -50 °C: 0.7343100 °C: 0.6891
Each panel has its own vertical scale, so compare shapes, not heights. Density falls with temperature while viscosity and conductivity rise; hover a point for its value.

Viscosity and thermal conductivity each follow Sutherland's law, with a separate reference value and Sutherland constant for each property. The specific heat at constant pressure comes from the Aly-Lee hyperbolic fit for air as an ideal gas, cv is cp less the gas constant, and density follows the ideal gas law. The ratio of specific heats, the Prandtl number, the kinematic viscosity and the thermal diffusivity are then computed from those.

  • Temperature range: 200 K to 2000 K, for dry air near atmospheric pressure. Outside it the calculator declines to return a number rather than extrapolate
  • Assumption: Dry air, moisture effects are not modelled; humid flows require separate property treatment
  • Outputs: every property listed under what the calculator returns, in SI units
  • Traceability: Use documented inputs when coupling to CFD-experiment validation or experimental measurement workflows, similarity arguments fail when property tables are inconsistent between solver and experiment

Typical uses on this site

Reynolds & Prandtl sizing

Estimate Re and Pr for channel or probe similarity checks before running the viscous flow problem sets or scaling a wind-tunnel model.

Boundary conditions

Supply consistent thermophysical inputs when documenting CFD boundary conditions, see the CFD-experiment validation article for how properties enter validation statements.

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