For dilute suspensions, Stokes law predicts the settling velocity of small spheres in fluid, either air or water.  This originates due to the strength of viscous forces at the surface of the particle providing the majority of the retarding force. Stokes law finds many applications in the natural sciences, and is given by:
where w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is the acceleration due to gravity, r is the radius of the particle and μ is the dynamic viscosity of the fluid.
Stokes law applies when the Reynolds number, Re, of the particle is less than 0.1. Experimentally Stokes law is found to hold within 1% for , within 3% for   and within 9% . With increasing Reynolds numbers, Stokes law begins to break down due to the increasing importance of fluid inertia, requiring the use of empirical solutions to calculate drag forces.