What is void volume in size exclusion chromatography (SEC)?

The void volume in SEC is the volume of mobile phase required to elute a molecule that has zero retention in the stationary phase. In an ideal case, it is equal to the volume of mobile phase in a column, which is the volume of the pores and spaces between the stationary phase. Void volume is often expressed as a percentage of the total column volume. For example, if the particles of stationary phase occupy 70% of the total column, the void volume would be 30% of the total column volume.