text: Mass flow rate can also be calculated by
  where 
  The above equation is only true for a flat, plane area. In general, including cases where the area is curved, the equation becomes a surface integral:
  The area required to calculate the mass flow rate is real or imaginary, flat or curved, either as a cross-sectional area or a surface, e.g. for substances passing through a filter or a membrane, the real surface is the (generally curved) surface area of the filter, macroscopically - ignoring the area spanned by the holes in the filter/membrane. The spaces would be cross-sectional areas. For liquids passing through a pipe, the area is the cross-section of the pipe, at the section considered. The vector area is a combination of the magnitude of the area through which the mass passes through, A, and a unit vector normal to the area, . The relation is .
  The reason for the dot product is as follows. The only mass flowing through the cross-section is the amount normal to the area, i.e. parallel to the unit normal. This amount is
  where θ is the angle between the unit normal  and the velocity of mass elements. The amount passing through the cross-section is reduced by the factor , as θ increases less mass passes through. All mass which passes in tangential directions to the area, that is perpendicular to the unit normal, doesnt actually pass through' the area, so the mass passing through the area is zero. This occurs when :
  These results are equivalent to the equation containing the dot product. Sometimes these equations are used to define the mass flow rate.
  Considering flow through porous media, a special quantity, superficial mass flow rate, can be introduced. It is related with superficial velocity, v, with the following relationship:
  The quantity can be used in particle Reynolds number or mass transfer coefficient calculation for fixed and fluidized bed systems.