SI unit of viscosity is Pascal-second (Pa·s) or Newton-second per meter square (N·s/m^-2).

The SI unit of viscosity is the pascal-second (Pa·s), often denoted as Pa⋅s.

Viscosity is defined as the force per unit area required to maintain a unit velocity gradient between two parallel layers of fluid.

Another common unit for viscosity is the Newton-second per square meter square (N·s/m^-2), which is equivalent to the Pascal-second.

In some contexts, viscosity is also expressed in poiseuille (Pl), named after the French physicist Jean Léonard Marie Poiseuille.

Additionally, viscosity can be measured in kilogram per meter per second (kg/(m·s)), especially in older literature and certain applications.