Dynamic Viscosity of Air at 20 °C
| Value | 0.0000181 Pa·s |
| Status | Measured: ± 2.00e-07 Pa·s (0.011 relative) |
| Source | ASHRAE Handbook of Fundamentals, Ch. 1 |
| Categories | Material PropertiesEngineering & TradeFluid Properties |
| micropascal second | 18.1 μPa·s |
| millipoise | 0.181 mP |
| pound per foot hour | 0.043785498 lb/(ft·h) |
| millipascal second | 0.0181 mPa·s |
| centipoise | 0.0181 cP |
| microreyn | 0.0026251831 μreyn |
| poise | 0.000181 P |
| pascal second | 0.0000181 Pa·s |
| pound per foot second | 0.000012162638 lb/(ft·s) |
| pound-force second per square foot | 3.7802636e-07 lbf·s/ft² |
| kilopascal second | 1.8100000e-08 kPa·s |
| reyn | 2.6251831e-09 reyn |
Dry air at any temperature
-50 to 300 °CThe value above is one point on this curve. Dry air at 1 atm from −50 to 300 °C — the ideal-gas density and Sutherland viscosity behind every duct, fan and drag calculation.
Learning zone
Air's dynamic viscosity is tiny, but the number that governs flow regimes is the kinematic viscosity ν = µ/ρ, and because air is a thousand times less dense than water its kinematic viscosity is 1.51 × 10⁻⁵ m²/s — fifteen times water's. That is why, at the same velocity and duct size, airflow has a Reynolds number fifteen times lower than water, and why boundary layers in air are comparatively thick.
Gases invert the liquid temperature trend: air gets more viscous as it heats, since viscosity in a gas comes from momentum transfer by molecular collisions rather than from intermolecular attraction. Sutherland's law gives about 21.8 µPa·s at 150 °C. Pressure has almost no effect until you approach vacuum. Use this value for duct friction, fan-law corrections, particle settling in air-cleaning equipment and drag calculations — and remember that the density term in Reynolds number changes with altitude and temperature far more than the viscosity does.