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SimuPipe

Properties of Air (−25 to 200 °C)

Density, dynamic and kinematic viscosity, and speed of sound for dry air at atmospheric pressure — the working range of compressed-air, HVAC and duct calculations. Viscosity comes from the same CoolProp-fitted correlation SimuPipe's solver evaluates; density is ideal-gas at 1 atm (real-air deviation under 0.1%).

Molar mass M

28.965 g/mol

Gas constant R

287.05 J/(kg·K)

γ = cₚ/cᵥ (20 °C)

1.402

cₚ = γR/(γ−1)

1.001 kJ/(kg·K)

Air properties vs temperature (at 1 atm)
T (°C)Density ρ (kg/m³)Dyn. viscosity μ (µPa·s)Kin. viscosity ν (mm²/s)Speed of sound a (m/s)
-251.422515.9411.21316.0
-201.394416.2011.62319.2
-151.367416.4612.04322.3
-101.341416.7112.46325.4
-51.316416.9712.89328.5
01.292317.2213.33331.6
51.269017.4713.77334.6
101.246617.7214.21337.6
151.225017.9614.66340.5
201.204118.2115.12343.5
251.183918.4515.58346.4
301.164418.6916.05349.3
401.127219.1717.00355.0
501.092319.6417.98360.6
601.059520.1018.97366.2
800.999521.0121.02377.0
1000.946021.8923.14387.5
1250.886622.9725.91400.3
1500.834224.0328.80412.7
1750.787625.0531.80424.7
2000.746026.0534.92436.4

Dry air at 101.325 kPa absolute. Density scales with absolute pressure (ρ ∝ P); viscosity and sonic velocity are pressure-insensitive at moderate pressures. 1 µPa·s = 10⁻⁶ Pa·s; the ν column is in mm²/s, so its values read directly as centistokes (1 mm²/s = 1 cSt). The sonic-velocity column holds γ at its 20 °C value — the error stays under ~1% across the table (real cₚ rises about 2% by 200 °C for the same reason).

Definitions & Key Relations

Air density follows the ideal-gas law, and the speed of sound depends on temperature only:

ρ=PR Ta=γ R T\rho = \frac{P}{R\,T} \qquad a = \sqrt{\gamma\,R\,T}
  • PP — absolute pressure (Pa), TT — absolute temperature (K)
  • R=287.05R = 287.05 J/(kg·K) — specific gas constant of air, γ=Cp/Cv\gamma = C_p/C_v — specific-heat ratio
  • ρ\rho — density (kg/m³), aa — sonic velocity (m/s), the choked-flow limit

To convert this table's free-air density to line conditions, scale by absolute pressure: at P bar(g), multiply ρ by (P + 1.013)/1.013. The kinematic viscosity ν = μ/ρ scales down by the same factor — which raises the Reynolds number of compressed-air flow accordingly. See the Reynolds number calculator and gas flow converter.

Data source and basis
  • Viscosity is evaluated from the CoolProp 7.2.0-fitted ln(μ) polynomial the SimuPipe solver itself uses (fit range 190–600 K, well beyond this table) — the published values cannot drift from what a simulation computes.
  • Density is ideal-gas at 101.325 kPa from M = 28.965 g/mol; real-air deviation (Z ≈ 0.9996 at 20 °C) is below 0.1% at atmospheric pressure.
  • γ and M are CoolProp values at 20 °C; cₚ and the sonic velocity are derived from them via the relations above, holding γ constant — a perfect-gas approximation good to ~1% up to 200 °C (real cₚ rises about 2% by 200 °C).
  • Spot anchors: μ = 18.21 µPa·s and a = 343.5 m/s at 20 °C, ν = 15.1 mm²/s — matching CRC Handbook / standard references within a fraction of a percent.
  • For altitude-dependent atmospheric pressure and density, see the atmospheric pressure vs altitude table; for other gases, the gas properties table.

Frequently Asked Questions

What is the density of air at room temperature?
1.204 kg/m³ at 20 °C and 1 atm (101.325 kPa), on the ideal-gas basis — real air deviates by less than 0.1% at these conditions. At 0 °C it is 1.292 kg/m³ (the Nm³ reference basis) and at 15 °C it is 1.225 kg/m³ (the Sm³ / ISA sea-level basis). Density is inversely proportional to absolute temperature and directly proportional to absolute pressure, so compressed air at 7 bar(g) is roughly eight times denser than these free-air figures.
Why does air viscosity increase with temperature?
Unlike liquids, gas viscosity comes from molecular momentum exchange between layers, and hotter molecules move faster — so μ rises with temperature, roughly as T^0.7. Air goes from 17.2 µPa·s at 0 °C to 21.9 at 100 °C and 26.0 at 200 °C. Kinematic viscosity ν = μ/ρ rises even faster, because density falls at the same time: 13.3 mm²/s at 0 °C but 23.1 mm²/s at 100 °C — a hot-air duct runs at a noticeably lower Reynolds number than a cold one at the same velocity.
How does air density change with pressure?
Linearly, for practical purposes: ρ = PM/(RT), so doubling the absolute pressure doubles the density. This table is at 1 atm; for a compressed-air line at gauge pressure P_g, multiply by (P_g + 101.325)/101.325 using absolute pressures. Real-gas deviation (Z factor) stays below about 1% up to roughly 20 bar at ambient temperature, so the ideal scaling is fine for compressed-air work.
What are the specific heat and gas constant of air?
The specific gas constant is R = 287.05 J/(kg·K) (universal gas constant divided by air's molar mass of 28.965 g/mol). The specific-heat ratio γ = cp/cv is 1.402 at 20 °C, giving cp = γR/(γ−1) ≈ 1,001 J/(kg·K) — the commonly quoted engineering value is 1,005 J/(kg·K), the small difference reflecting rounding of γ. cp rises slowly with temperature (about 2% by 200 °C).
What is the speed of sound in air?
a = √(γRT): 331.6 m/s at 0 °C, 343.5 m/s at 20 °C, and about 388 m/s at 100 °C — it depends on temperature only, not on pressure (the pressure and density effects cancel). It matters in piping because it is the choked-flow limit: gas in a pipe, valve or orifice throat cannot exceed the local sonic velocity, which is why SimuPipe's compressible solvers track the Mach number and warn as flow approaches Mach 1.
Does humidity affect these values?
Slightly. Water vapour (M = 18 g/mol) is lighter than air, so humid air is marginally less dense — saturated air at 20 °C is about 1% lighter than dry air, growing to roughly 6% at 60 °C. Viscosity changes are similarly small. This table is for dry air; for the water content itself (dew point, condensate load in compressed-air systems), see the compressed air moisture calculator.

Model compressed-air systems

SimuPipe solves air networks with temperature-correct viscosity, pressure-correct density, and sonic-choking checks — from a single dryer line to a full ring main.

Get Started FreeCompressed Air Example

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Gas PropertiesAtmospheric Pressure vs AltitudeCompressed Air MoistureReynolds Number Calculator