Viscosity describes a fluid's resistance to deformation and flow. Honey resists motion much more than water, while gases are less viscous in everyday conditions but still create measurable drag. Two viscosity quantities appear in engineering: dynamic (absolute) viscosity describes shear stress for a given velocity gradient, while kinematic viscosity additionally accounts for density. Confusing them can produce errors of several orders of magnitude in pump, bearing, lubrication, and pipe calculations.
Dynamic Viscosity: Stress and Shear
For a Newtonian fluid, shear stress τ equals dynamic viscosity μ multiplied by the velocity gradient. The SI unit is pascal-second (Pa·s), equivalent to N·s/m². Water near room temperature is about 1 mPa·s, or 1 centipoise (cP), while common oils may be tens to hundreds of cP. The Dynamic Viscosity Converter translates Pa·s, mPa·s, poise, and centipoise without treating a cP value as if it were kinematic viscosity.
Kinematic Viscosity: Viscosity per Density
Kinematic viscosity ν is dynamic viscosity divided by density: ν = μ/ρ. Its SI unit is m²/s, although stokes (St) and centistokes (cSt) remain common in lubricant specifications. Since 1 cSt = 1 mm²/s, a liquid with ν = 25 cSt has ν = 25 × 10⁻⁶ m²/s. The Kinematic Viscosity Converter handles those area-per-time units, while the Density Converter helps recover μ when a supplier lists ν and density separately.
Converting Between the Two
A direct dynamic-to-kinematic conversion requires density; no single fixed factor can convert cP to cSt for every liquid. For example, if μ = 10 mPa·s and ρ = 800 kg/m³, then ν = 0.010/800 = 1.25 × 10⁻⁵ m²/s, or 12.5 cSt. The same dynamic viscosity with water's approximate density would produce a different kinematic value. Always record the temperature because both viscosity and density can change significantly with temperature.
Temperature and Non-Newtonian Behavior
Most liquids become less viscous as temperature rises, which is why an oil grade must be evaluated at the equipment's actual operating range. Gases generally become more viscous with temperature. The Newtonian equations assume viscosity is independent of shear rate; paints, slurries, polymer solutions, blood, and many food products can be shear-thinning, shear-thickening, or yield-stress fluids. A single catalog viscosity may not describe their behavior in a changing process.
Pipe Flow and Reynolds Number
Reynolds number compares inertial and viscous effects: Re = ρvD/μ, or equivalently Re = vD/ν. A higher viscosity generally lowers Reynolds number for a fixed density, velocity, and pipe diameter, making laminar behavior more likely. Combine viscosity with the Flow Converter to put a pump or pipe specification into compatible volumetric units before calculating velocity and Reynolds number.
Reading a Datasheet Carefully
Before converting a specification, identify whether it reports dynamic or kinematic viscosity, the temperature and test method, and whether the unit is cP, cSt, Pa·s, or another scale. Do not silently substitute centipoise for centistokes: numerically they coincide only when the density is about 1 g/cm³. Keep the original value, converted value, temperature, and density in your engineering notes so a later reviewer can reproduce the result.