Kinematic Viscosity of Air Calculator

About the Kinematic Viscosity of Air Calculator

Kinematic viscosity ν = μ/ρ is one of the most important fluid properties in engineering. It appears in the Reynolds number (Re = VD/ν), boundary-layer equations, and virtually every convection heat-transfer correlation. For air, both dynamic viscosity μ and density ρ depend on temperature and pressure, so ν varies significantly across conditions.

This calculator uses Sutherland's law to compute the dynamic viscosity of air as a function of temperature, combined with the ideal gas law for density. Sutherland's formula — μ = μ₀ (T/T₀)^(3/2) (T₀+C)/(T+C) with C ≈ 120 K — is accurate to within 2% from −40°C to 1 500°C. Density is calculated from ρ = PM/(RT), with optional humidity correction.

The temperature-sweep table provides a complete reference from −40°C to 500°C, letting you look up viscosity at any condition without needing a textbook. A bar chart visualises how ν increases sharply with temperature — roughly doubling between 0°C and 200°C.

When This Page Helps

Use this calculator when Reynolds number, boundary-layer behavior, or convective heat transfer depends on realistic air properties instead of a single handbook value.

It is useful for CFD setup, duct and wind-speed calculations, altitude work, and quick engineering checks where temperature and pressure move away from standard conditions. That makes it easier to keep downstream flow calculations consistent with the actual operating state instead of a default ambient assumption.

How to Use the Inputs

1. Enter the air temperature in °C, °F, or Kelvin.
2. Enter the absolute pressure in Pa, kPa, bar, psi, or atm.
3. Use a pressure preset to quickly switch between common pressures.
4. Optionally enter the relative humidity for moist-air corrections.
5. Read the kinematic viscosity, dynamic viscosity, and air density.
6. Consult the temperature sweep table for a full range of values.

Example Calculation

Tips & Best Practices

• For non-standard gases, replace the Sutherland constants. Each gas has its own μ₀, T₀, and C values.
• To quickly estimate: ν (m²/s) ≈ 1.5×10⁻⁵ at 20°C, doubling roughly every 120°C.
• In CFD simulations, always set viscosity consistent with your turbulence model temperature.
• For altitude calculations, combine This calculator with the air-pressure-at-altitude calculator.
• At high Mach numbers, use the Eckert reference temperature for viscosity evaluation.

Practical Guidance

Air viscosity matters most when it feeds another calculation such as Reynolds number, pressure loss, or heat-transfer coefficient. Under standard indoor conditions a memorized value may be good enough, but once temperature, altitude, or process gas conditions move away from ambient, property changes become large enough to affect the answer materially.

Common Pitfalls

The most common mistake is mixing dynamic viscosity and kinematic viscosity. Another is using standard-density air at reduced pressure, which can shift Reynolds number and flow regime more than expected. If humidity is important, remember that its effect is usually secondary compared with temperature and pressure.

Frequently Asked Questions

• Why does kinematic viscosity increase with temperature for gases?

  Dynamic viscosity of gases increases with temperature (more molecular momentum transfer), while density decreases. Both effects increase ν = μ/ρ, making the rise quite steep — roughly proportional to T^1.7.

• Does pressure affect viscosity?

  Dynamic viscosity of an ideal gas is nearly independent of pressure. However, density is proportional to pressure, so kinematic viscosity ν = μ/ρ is inversely proportional to pressure.

• How accurate is Sutherland's law?

  Within about 2% for temperatures from −40°C to 1 500°C at moderate pressures. At very high pressures (> 100 atm) or near the critical point, real-gas corrections are needed.

• What are typical ν values for air at room conditions?

  At 20°C and 1 atm, ν ≈ 1.51 × 10⁻⁵ m²/s. This is about 15 times the kinematic viscosity of water, which is ~1.0 × 10⁻⁶ m²/s.

• How does humidity affect air viscosity?

  Humid air is slightly less dense than dry air (water vapor is lighter than N₂/O₂). This increases kinematic viscosity slightly but the effect is usually small (< 2% at 60% RH).

• Why do Reynolds numbers differ at altitude?

  At higher altitudes, pressure drops and ν increases. A higher ν means Re = VD/ν is lower for the same velocity and length, potentially delaying the transition to turbulence or reducing heat-transfer coefficients.