Stokes’ Law is named after George Gabriel Stokes, an Irish-born mathematician who studied the behavior of fluids. He developed an equation that describes the force required to move a sphere through a fluid at a particular velocity. His formula can be rearranged to calculate the terminal velocity of a sphere falling through a fluid, which makes it useful for determining the time it takes for solids to settle out of suspension in a brewery tank. Brewers can also use it to understand how to manipulate the variables involved to increase the rate of settling. The law in the form used by brewers is $$V_S=\frac{2}{9}\frac{\left({\rho }_p-{\rho }_f\right)}{\mu }g{R}^2$$

where V is the particle’s velocity in m/s, ρ_p is the mass density of the particle in kg/m³, ρ_f is the mass density of the fluid in kg/m³, g is the acceleration as a result of gravity m/s², R is the radius of the particle in m, and µ is the viscosity of the fluid in kg/m/s.

A careful look at the equation shows that the greatest impact a brewer can have on the settling velocity of a particle—without affecting beer quality—is an increase in the particle’s radius. Doubling the particle’s radius, for instance, would cause a fourfold decrease in the settling time. Brewers use this principle to speed up clarification through sedimentation by adding inert agents to the brew that cause yeast and other particulates to bind together, which creates larger-size particulates and speeds up sedimentation. See finings. The equation is sometimes written with the 2/9 term replaced with 1/18. The equation can also be adapted to apply to centrifugation, where beer is spun at high velocity, which, in effect, increases particulate acceleration through gravity. Centrifugation is used mainly in the preliminary reduction of suspended particles, especially yeast, after fermentation and before cold storage and maturation.