About the Viscosity Calculator
This viscosity calculator converts between dynamic (absolute) viscosity and kinematic viscosity. The two are linked by density: kinematic viscosity is dynamic viscosity divided by density, ν = μ/ρ. Enter either value with the fluid density and the calculator returns the other in every common unit — centipoise, pascal-seconds, poise and lb/(ft·s) for dynamic viscosity, and centistokes, stokes, m²/s and ft²/s for kinematic viscosity.
Engineers need this conversion all the time: oil and lubricant data sheets quote kinematic viscosity in cSt (ISO VG grades are defined at 40 °C), while lab viscometers and many fluid tables give dynamic viscosity in cP or Pa·s. Reynolds number, pipe friction and pump calculations need one or the other depending on the form of the equation.
Viscosity depends strongly on temperature, so the density and viscosity you enter must be at the same temperature. A handy shortcut: with density in g/cm³, cP ÷ density = cSt, so for water near room temperature the two numbers are almost identical.
How to use the viscosity calculator
- 1Choose whether you are converting dynamic to kinematic viscosity or the reverse.
- 2Enter the viscosity and pick its unit.
- 3Enter the fluid density at the same temperature and pick its unit.
- 4Read the converted viscosity in every common unit.
Formula and method
Dynamic (absolute) viscosity μ measures a fluid’s internal resistance to shear: the shear stress needed for a given velocity gradient, in Pa·s or centipoise (1 cP = 1 mPa·s). Kinematic viscosity ν is dynamic viscosity divided by density, in m²/s or centistokes (1 cSt = 1 mm²/s = 10⁻⁶ m²/s). It describes how fast momentum diffuses through the fluid and is what gravity-flow viscometers measure directly.
The calculator converts every input to SI (Pa·s, kg/m³, m²/s), applies ν = μ/ρ or μ = νρ, and converts back. Exact factors are used: 1 P = 0.1 Pa·s, 1 St = 10⁻⁴ m²/s, 1 ft²/s = 0.09290304 m²/s, 1 lb/(ft·s) = 1.48816 Pa·s and 1 lb/ft³ = 16.0185 kg/m³.
- ν
- Kinematic viscosity (m²/s, cSt)
- μ
- Dynamic (absolute) viscosity (Pa·s, cP)
- ρ
- Fluid density at the same temperature (kg/m³)
Worked examples
Water at 20 °C: 1.002 cP, 998.2 kg/m³
Kinematic viscosity = 0.001002 Pa·s ÷ 998.2 kg/m³ = 1.0038 × 10⁻⁶ m²/s, or 1.004 cSt. Because water’s density is almost exactly 1 g/cm³, its viscosity in cP and cSt is nearly the same number.
ISO VG 46 hydraulic oil: 46 cSt at 870 kg/m³
Dynamic viscosity = 46 × 10⁻⁶ m²/s × 870 kg/m³ = 0.04002 Pa·s, or 40.0 cP. The quick check is 46 cSt × 0.87 g/cm³ = 40.0 cP.
Glycerin: 1.41 Pa·s at 1.26 g/cm³
1.41 Pa·s is 1,410 cP. Dividing by a density of 1.26 g/cm³ gives 1,410 ÷ 1.26 = 1,119 cSt, or 11.19 stokes.
Frequently asked questions
What is the difference between dynamic and kinematic viscosity?+
Dynamic viscosity (cP, Pa·s) is the force needed to shear the fluid. Kinematic viscosity (cSt, m²/s) is dynamic viscosity divided by density, so it describes how easily the fluid flows under its own weight. Two fluids with the same cP can have different cSt if their densities differ.
How do I convert cP to cSt?+
Divide centipoise by the density in g/cm³ (which equals specific gravity for liquids). For example, an oil of 40 cP with a density of 0.87 g/cm³ has a kinematic viscosity of 40 ÷ 0.87 = 46 cSt.
What is the kinematic viscosity of water?+
At 20 °C water has a dynamic viscosity of about 1.002 mPa·s and a density of 998.2 kg/m³, giving a kinematic viscosity of about 1.004 cSt (1.004 × 10⁻⁶ m²/s). At 40 °C it falls to roughly 0.66 cSt.
Why does temperature matter so much?+
Liquid viscosity drops sharply as temperature rises — many oils lose half their viscosity for every 15–25 °C of heating. Always use viscosity and density measured at the same temperature, and quote it, e.g. 46 cSt at 40 °C.
What units are centistokes and centipoise?+
One centistoke (cSt) equals 1 mm²/s or 10⁻⁶ m²/s. One centipoise (cP) equals 1 mPa·s or 0.001 Pa·s. The stokes and poise are the older CGS units, 100 times larger than the centi- units.