Superficial velocity can be expressed as:

When the packed bed has a fluid passed over it, the pressure drop of the fluid is approximately proportional to the fluid's superficial velocity.

Using the concept of porosity, the dependence between the advection velocity of fluid and the superficial velocity can be expressed as (for one-dimensional flow):

For fluid flow through a bed of approximately spherical particles of diameter D in contact, if the "voidage" is ε and the "superficial velocity" is V, the Reynolds number can be defined as:

Superficial velocity is used in many engineering equations because it is the value which is usually readily known and unambiguous, whereas real velocity is often variable from place to place, its mean not readily available in complex flow systems, and subject to assumptions.

Superficial velocity (or superficial flow velocity), in engineering of multiphase flows and flows in porous media, is a hypothetical (artificial) fluid velocity calculated as if the given phase or fluid were the only one flowing or present in a given cross sectional area.