where  and  are defined as
 and 
where:
 is the modified Reynolds number,<br>
 is the packed bed friction factor<br>
 is the pressure drop across the bed,<br>
 is the length of the bed (not the column),<br>
 is the equivalent spherical diameter of the packing,<br>
 is the density of fluid,<br>
 is the dynamic viscosity of the fluid,<br>
 is the superficial velocity (i.e. the velocity that the fluid would have through the empty tube at the same volumetric flow rate)<br>
 is the void fraction (porosity) of the bed.<br>
 is the particle Reynolds Number (based on superficial velocity)