Viscosity of water
Viscosity is a fluid’s resistance to flow. Water’s viscosity falls sharply as it warms: at 176 °F (80 °C) it is nearly three times more fluid than at 68 °F (20 °C). This matters for pipe friction losses, pump sizing, filtration, lubrication and laboratory work.
- Dynamic viscosity (μ), in millipascal-seconds (mPa·s), equal to centipoise (cP)
- Kinematic viscosity (ν = μ ÷ density), in square millimeters per second (mm²/s), equal to centistokes (cSt)
The Vogel equation
The calculator uses the Vogel equation for liquid water, accurate to within a few percent from freezing to boiling:
μ (Pa·s) = 2.414 × 10⁻⁵ × 10^(247.8 ÷ (T − 140)), with T in kelvin
and an empirical formula for the density of water. Kinematic viscosity is then the dynamic viscosity divided by the density.
Reference values
| Temperature | Dynamic viscosity (mPa·s) | Kinematic viscosity (mm²/s) |
|---|---|---|
| 32 °F (0 °C) | 1.79 | 1.79 |
| 50 °F (10 °C) | 1.31 | 1.31 |
| 68 °F (20 °C) | 1.00 | 1.00 |
| 104 °F (40 °C) | 0.65 | 0.66 |
| 140 °F (60 °C) | 0.47 | 0.48 |
| 176 °F (80 °C) | 0.36 | 0.37 |
| 212 °F (100 °C) | 0.28 | 0.29 |
Example
At 68 °F, water has a dynamic viscosity of about 1.00 mPa·s, a kinematic viscosity of about 1.00 mm²/s and a density of 998.2 kg/m³. In US customary units, the dynamic viscosity is about 2.09 × 10⁻⁵ lb·s/ft².
Using the result
- Reynolds number: Re = velocity × diameter ÷ kinematic viscosity tells you whether flow in a pipe is laminar (under about 2,300) or turbulent (over about 4,000).
- Friction losses: warm water has slightly lower friction losses, but the effect is small in fully turbulent flow.
- Settling and filtration: particles settle faster in warm water, which is why water treatment plants see slower settling in winter.
Limits
The formulas cover pure liquid water at about atmospheric pressure. Dissolved salts raise viscosity slightly: seawater is about 8% more viscous than fresh water at the same temperature.