Dynamic viscosity (also called absolute viscosity or static viscosity) measures a fluid's resistance to shear — the internal force per unit area required to make one layer of fluid slide past another. It is independent of density. The SI unit is the pascal-second (Pa·s); the common engineering unit is the millipascal-second (mPa·s), numerically equal to centipoise (cP).

τ = μ × (du / dy)

where τ is shear stress [Pa] and du/dy is the velocity gradient [s⁻¹].

Kinematic viscosity is the ratio of dynamic viscosity to fluid density. It describes how easily a fluid flows under the influence of gravity or pressure, without needing to separately account for the applied force. The SI unit is m²/s; the common unit is mm²/s, numerically equal to centistokes (cSt).

ν = μ / ρ

Kinematic viscosity is derived from dynamic viscosity and density:

ν [cSt] = μ [mPa·s] × 1000 / ρ [kg/m³]

Because denser fluids have higher internal inertia, a greater dynamic viscosity is required to produce the same kinematic viscosity. Glycol solutions are significantly denser than water, so their kinematic viscosity rises more slowly with concentration than their dynamic viscosity does.

Use dynamic (absolute) viscosity for: pump power calculations, pipe pressure drop (Darcy–Weisbach), Reynolds number (Re = ρvD/μ), and any situation where force or stress is the driving input.

Use kinematic viscosity for: gravity-driven flow, lubrication ratings, most viscometer measurements, and flow calculations where density is already embedded in the formula (e.g., Stokes' law, ASTM oil viscosity grades).

Note: "Absolute viscosity" and "static viscosity" are synonyms for dynamic viscosity — they all refer to the same physical property. "Static" distinguishes it from apparent viscosity measured at elevated shear rates in non-Newtonian fluids.