Water Viscosity Calculator
Enter the temperature of liquid water and choose your preferred unit system. The calculator returns dynamic viscosity in mPa·s (centipoise), kinematic viscosity in mm²/s (centistokes), and water density in kg/L across the full liquid range from 0 to 370 °C (32 to 698 °F). Values are interpolated from IAPWS 2008 reference data. A step-by-step breakdown and temperature-viscosity chart are included.
What is water viscosity?
Viscosity measures a fluid's resistance to flowing. Dynamic viscosity (symbol mu, unit Pa·s or mPa·s) describes the internal friction between adjacent layers of a moving fluid. Kinematic viscosity (symbol nu, unit m²/s or mm²/s) is dynamic viscosity divided by density, and it describes how quickly momentum diffuses through a fluid under its own weight. Water is approximately 55 times more viscous than air at room temperature, yet far less viscous than most oils. Its viscosity drops sharply as temperature rises, roughly halving between 0 °C and 60 °C, which is why warm water flows more easily through pipes.
How viscosity is calculated from temperature
For liquid water in the range 0-100 °C at atmospheric pressure, viscosity is accurately described by the Vogel-Fulcher-Tammann equation: ln(mu) = A + B / (T - C), where T is in Kelvin and the constants A = -3.7188, B = 578.919, and C = -137.546 K were fitted by Korson, Drost-Hansen and Millero (1969). This calculator uses linear interpolation between a published IAPWS (International Association for the Properties of Water and Steam) reference dataset to cover the full liquid range up to 374 °C. Kinematic viscosity is then computed as nu = mu / rho, where rho is the density at the same temperature.
Dynamic versus kinematic viscosity
Dynamic viscosity (absolute viscosity) measures the force needed to move one layer of a fluid over another. It is the value in the Navier-Stokes equations, Darcy-Weisbach pipe-flow formula, and Reynolds number. Kinematic viscosity is dynamic viscosity divided by fluid density, and it naturally appears in equations where gravitational or buoyancy forces are involved, such as the Stokes settling velocity or the definition of the Grashof number. In SI units, 1 mPa·s equals 1 centipoise (cP), and 1 mm²/s equals 1 centistoke (cSt). At 20 °C, water has a dynamic viscosity of about 1.00 mPa·s, which is why water is commonly used as the reference fluid for the cP unit.
Practical uses and engineering context
Viscosity values are required in almost every fluid-flow calculation. The Reynolds number Re = rho * v * L / mu determines whether flow is laminar or turbulent in a pipe or around a body. The Hagen-Poiseuille equation relates volumetric flow rate to dynamic viscosity, pipe radius, and pressure drop. In heat exchangers, the Prandtl number Pr = mu * Cp / k links viscosity to thermal conductivity and specific heat capacity. For cooling water systems, knowing that viscosity at 80 °C is only about half its 20 °C value matters for pump sizing and flow-balance calculations. In laboratory settings, knowing the exact kinematic viscosity at a controlled temperature is essential for calibrating viscometers and computing diffusion coefficients.
Water viscosity and density at standard temperatures
| Temp (°C) | Temp (°F) | Dynamic (mPa·s) | Kinematic (mm²/s) | Density (kg/L) |
|---|---|---|---|---|
| 0 | 32 | 1.7916 | 1.7920 | 0.9998 |
| 10 | 50 | 1.3077 | 1.3081 | 0.9997 |
| 20 | 68 | 1.0016 | 1.0034 | 0.9982 |
| 25 | 77 | 0.8909 | 0.8936 | 0.9970 |
| 30 | 86 | 0.7975 | 0.8009 | 0.9956 |
| 40 | 104 | 0.6531 | 0.6581 | 0.9922 |
| 50 | 122 | 0.5468 | 0.5534 | 0.9880 |
| 60 | 140 | 0.4665 | 0.4747 | 0.9832 |
| 70 | 158 | 0.4042 | 0.4137 | 0.9778 |
| 80 | 176 | 0.3547 | 0.3653 | 0.9718 |
| 90 | 194 | 0.3147 | 0.3264 | 0.9653 |
| 100 | 212 | 0.2818 | 0.2940 | 0.9584 |
Reference data from IAPWS 2008. Dynamic viscosity is in mPa·s (= cP), kinematic viscosity in mm²/s (= cSt), density in kg/L. Values assume liquid water at sufficient pressure.
Frequently asked questions
Why does water viscosity decrease as temperature increases?
In liquids, viscosity arises from intermolecular forces and hydrogen bonding between neighboring molecules. As temperature rises, the molecules have more thermal energy, which weakens the hydrogen bonds holding them in place and allows layers to slide past each other more easily. This is the opposite behavior to gases, where viscosity increases with temperature because faster molecules collide more often and transfer momentum more efficiently.
What is the viscosity of water at room temperature?
At 20 °C (68 °F), the dynamic viscosity of water is approximately 1.0016 mPa·s (centipoise) and the kinematic viscosity is approximately 1.0034 mm²/s (centistoke). At 25 °C (77 °F), the values are about 0.8909 mPa·s and 0.8936 mm²/s respectively. These are the most-cited reference values in engineering and laboratory work.
What is the difference between mPa·s, cP, mm²/s, and cSt?
These are all common viscosity units. For dynamic viscosity: 1 mPa·s = 1 centipoise (cP) = 0.001 Pa·s. For kinematic viscosity: 1 mm²/s = 1 centistoke (cSt) = 0.000001 m²/s. The centipoise and centistoke are historical CGS units still widely used in industry; the SI equivalents are Pa·s and m²/s. Water at 20 °C is the practical definition of 1 cP, which is why cP remains popular in lab and process settings.
Does pressure affect water viscosity?
At the pressures encountered in most engineering and laboratory work (up to a few hundred bar), the effect of pressure on water viscosity is small compared to the effect of temperature. This calculator assumes saturation pressure for temperatures up to 100 °C (atmospheric) and increasing pressure above that to keep water liquid. For extreme pressures (thousands of bar), consult specialized IAPWS property tables.
How do I use viscosity in a Reynolds number calculation?
The Reynolds number is Re = (rho × v × L) / mu, or equivalently Re = (v × L) / nu, where rho is fluid density, v is flow velocity, L is a characteristic length (e.g. pipe diameter), mu is dynamic viscosity, and nu is kinematic viscosity. Use the dynamic viscosity in Pa·s with density in kg/m³ and velocity in m/s for SI-consistent results. Flow is laminar below Re approximately 2300 in a pipe and turbulent above approximately 4000.