This is an equation for calculating the void ratio of a sample medium.  The void represents the relative amount of space occupied by the "voids" in a volume of material, where the voids can be a combination of liquids or gases, and the relative amount of space occupied by the solid material.

This void ratio is a measure of how compact the material is.  A volume of large rocks will obviously have a great deal of space between them and thus have a large void ratio compared to fine particulate of a volume of a clay soil, which would have very limited amounts of voids relative to the solid matter.

The inputs to this equation are the total sample volume, `V_"Total"`, and the volume observed to be occupied by voids `V_V`.

Notes

In engineering application the void ratio is used to indicate a tendency of a material to expand or contract.  A high void ratio is likely to be seen in loose soils which then will contract under pressure or added loads.  A low void ratio represents a material that is dense.  This void ratio reflects the affect of how fluids will flow through a medium or how particulates can move through the medium.