Air’s viscosity rises with temperature (unlike liquids), while its density falls, so kinematic viscosity rises quickly when air is heated. It is needed to compute Reynolds numbers for aircraft, cars and ventilation ducts.
How to use the Kinematic Viscosity of Air Calculator
- Temperature
- Pressure
- Specific gas constant R_s (J/(kg·K))
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the air temperature and pressure (and the specific gas constant if not standard air).
Tips
- Temperatures in humidity formulas are usually in °C; the menus convert °F.
- Relative humidity depends on temperature; dew point does not.
- Air pressure drops about 12% per kilometre near the ground.
- Weather formulas are empirical – expect small differences between sources.
Common mistakes to avoid
- Mixing station pressure and sea-level pressure.
- Using relative humidity above 100%.
- Confusing heat index (humidity) with wind chill (wind).
Inputs explained
| Field | What to enter |
|---|---|
| Temperature | Units: °C, °F, K |
| Pressure | Units: hPa, kPa, inHg, mmHg, psi, atm |
| Specific gas constant R_s (J/(kg·K)) | A number |
What each result means
- Density = [Pressure]÷([Specific gas constant R_s (J÷(kg·K))]×([Temperature] + 273.15))
- Dynamic viscosity = 1.716e-5×(([Temperature] + 273.15)÷273.15)^1.5×(273.15 + 110.4)÷([Temperature] + 273.15 + 110.4)×1e6
- Kinematic viscosity = [Dynamic viscosity]×1e-6÷[Density]
Example values
How the main result changes when Temperature changes and everything else stays at the example values above:
| Temperature (°C) | Kinematic viscosity (mm²/s (cSt)) |
|---|---|
| 10 | 14.1586 |
| 15 | 14.6064 |
| 20 | 15.0593 |
| 25 | 15.5174 |
| 30 | 15.9806 |
| 40 | 16.9221 |
Formula
ρ = p/(R_s·T), μ = μ₀·(T/T₀)^{3/2}·(T₀ + S)/(T + S) (μ₀ = 1.716×10⁻⁵ Pa·s, T₀ = 273.15 K, S = 110.4 K), ν = μ/ρWorked example
Inputs — Temperature: 20 °C; Pressure: 1,013.25 hPa; Specific gas constant R_s (J/(kg·K)): 287.05.
Results — Density = 1.20412 kg/m³; Dynamic viscosity = 18.13322 µPa·s; Kinematic viscosity = 15.05934 mm²/s (cSt).
Key terms
- Sutherland's law
- μ ∝ T^{3/2}/(T + S).
Frequently asked questions
What is the kinematic viscosity of air at 20 °C?
About 15.1 mm²/s.
Why does gas viscosity rise with temperature?
Faster molecules transfer momentum more effectively.
What is Sutherland's law?
An empirical formula for gas viscosity versus temperature.
Does pressure affect viscosity?
Dynamic viscosity hardly; kinematic viscosity falls as pressure rises.
What formula does the Kinematic Viscosity of Air Calculator use?
ρ = p/(R_s·T), μ = μ₀·(T/T₀)^{3/2}·(T₀ + S)/(T + S) (μ₀ = 1.716×10⁻⁵ Pa·s, T₀ = 273.15 K, S = 110.4 K), ν = μ/ρ.