The force of viscosity on a small sphere moving through a viscous fluid is given by:
where (in SI units):
*  is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle (newtons, kg m s);
*  (some authors use the symbol ) is the dynamic viscosity (Pascal-seconds, kg m s);
*  is the radius of the spherical object (meters);
*  is the flow velocity relative to the object (meters per second).
Stokes' law makes the following assumptions for the behavior of a particle in a fluid:
*Laminar flow
*No inertial effects (zero Reynolds number)
*Spherical particles
*Homogeneous (uniform in composition) material
*Smooth surfaces
*Particles do not interfere with each other.
Depending on desired accuracy, the failure to meet these assumptions may or may not require the use of a more complicated model. To 10% error, for instance, velocities need be limited to those giving Re < 1.
For molecules Stokes' law is used to define their Stokes radius and diameter.
The CGS unit of kinematic viscosity was named "stokes" after his work.