Viscosity Conversion Calculator

Viscosity comes in two flavors that are constantly confused. Dynamic or absolute viscosity, symbol mu, is the constant of proportionality between shear stress and velocity gradient, with the SI unit of pascal-second. Kinematic viscosity, symbol nu, is mu divided by density and has units of area per time, which is why it governs momentum diffusion and appears directly in the Reynolds number. The CGS units survive in daily use: one poise is 0.1 Pa.s, so one centipoise is exactly one millipascal-second, and one stokes is 1 x 10^-4 m2/s, so one centistokes is exactly one square millimeter per second. Water at 20 degrees C has a dynamic viscosity of 1.002 mPa.s and a kinematic viscosity of 1.0034 mm2/s, and the near-equality of the two numbers is a coincidence of water having a density close to 1 g/cm3. For any other fluid the density must be supplied to move between the two families. The empirical timed-efflux scales are a separate lineage. Saybolt Universal Seconds, Redwood seconds, and Engler degrees all report how long a fixed sample takes to drain through a standard orifice, so they are instrument definitions rather than physical quantities. Conversions to centistokes are curve fits: above about 75 cSt, SUS is close to 4.632 times cSt, with corrections needed lower down and no relation valid below roughly 2 cSt. All three are tied to specific test temperatures, commonly 100 or 210 degrees F for Saybolt, and all assume Newtonian behavior, so they are meaningless for greases, polymer melts, or shear-thinning fluids. Industrial lubricant grades under ISO 3448 are defined instead by kinematic viscosity at 40 degrees C, where ISO VG 46 spans 41.4 to 50.6 cSt.

Isaac Newton framed the concept in Book II of the Principia in 1687, proposing that the resistance between sliding layers of a fluid is proportional to the rate at which they are separated, which is why fluids obeying that linear law are still called Newtonian. The mathematics matured in the nineteenth century through Claude-Louis Navier in 1822 and George Gabriel Stokes in 1845, whose equations of viscous motion earned Stokes the naming of the kinematic unit. Jean Leonard Marie Poiseuille, a French physician investigating blood flow in capillaries, published precise measurements of flow through fine tubes in the 1840s that established the practical measurement method and gave his name to the poise. The empirical seconds scales arose independently in the petroleum and lubricants trade, where a stopwatch and a calibrated cup were the affordable instruments. Carl Oswald Viktor Engler introduced his degree scale in Germany in the 1880s, Redwood seconds were named for Boverton Redwood and adopted in Britain in the same era, and George M. Saybolt developed the Saybolt viscometer for the American oil industry in the late nineteenth century. The resulting incompatibility of national scales became a real trade problem, and ASTM eventually resolved it by standardizing the glass capillary determination of kinematic viscosity in ASTM D445 and publishing authoritative conversion tables from centistokes to Saybolt units in ASTM D2161. The pascal-second entered general use after the coherent SI system was adopted by the General Conference on Weights and Measures in 1960, though the centipoise and centistokes remain dominant in industrial practice.