Viscosity Converter

Dynamic viscosity, commonly denoted by the Greek letter mu, measures a fluid's internal resistance to flow under an applied shear stress. It is defined by Newton's law of viscosity: shear stress = mu * (velocity gradient), where the velocity gradient is the rate of change of fluid velocity perpendicular to the flow direction. The SI unit of dynamic viscosity is the Pascal-second (Pa-s), equivalent to kg/(m-s). One Pa-s represents a significant amount of viscosity; for reference, water at 20 degrees C has a viscosity of about 0.001 Pa-s. This is why the centipoise (cP) is far more common in practice: 1 cP = 0.001 Pa-s, making water's viscosity conveniently close to 1 cP. The poise (P), defined as 1 g/(cm-s), is the CGS unit named after Jean Leonard Marie Poiseuille, who studied fluid flow in narrow tubes. One poise equals 0.1 Pa-s. The centipoise is one-hundredth of a poise and is the de facto standard unit in petroleum, food, cosmetics, and pharmaceutical industries. Imperial viscosity units include lb/(ft-s) and lb/(ft-h). These appear in older US engineering references and some HVAC calculations. They are less common today but still encountered in legacy piping and heat transfer correlations. Viscosity depends strongly on temperature. For liquids, viscosity decreases exponentially with increasing temperature (the Arrhenius relationship). For gases, viscosity increases with temperature because faster-moving molecules transfer more momentum. This temperature dependence is critical in engine lubrication: motor oil must flow at cold startup (low viscosity) yet maintain adequate film thickness at operating temperature. Kinematic viscosity (nu = mu / rho) divides dynamic viscosity by density and is measured in Stokes or centistokes (mm^2/s). It represents the momentum diffusivity of the fluid and is used in the Reynolds number calculation (Re = V*L/nu), which determines whether flow is laminar or turbulent.