Note that the limiting volume is about for all liquids and for gases at atmospheric temperature. Within the given limiting value, air at the standard condition has approximately molecules. It justifies in defining a nearly constant density in a region which is larger than the limiting volume.

In conclusion, since most of the engineering problems deal with fluids at a dimension which is larger than the limiting volume, the assumption of fluid as a continuum is valid. For example the fluid density is defined as a function of space (for Cartesian coordinate system, x, y, and z) and time (t ) by . This simplification helps to use the differential calculus for solving fluid problems.