This is an experimental study aiming at understanding the relationship between particle diameters measured with different analytical techniques: optical analysis (microscopy aided by computer image analysis), laser light scattering and permeability tests. Permeability test can provide an equivalent mean diameter that can be used in several cases where particle specific surface is the relevant property to focus on (for example problems related to particle reactivity or involving interactions with fluids as in pneumatic transport or in fluidization). This diameter can be determined by measuring the pressure loss in a granular bed of known porosity at various gas flow rate and by using a proper mathematical model (e.g., Darcy or Karman-Kozeny models) to correlate them. Four different materials (glass beads, granulated microcrystalline cellulose, MCC, sodium chloride crystals, tetraacetylethylenediamine powders, TAED) in the size range 700-1000 µm (by sieve analysis) were analysed. A strong discrepancy was observed by calculating the Sauter diameters from image analysis (and laser light scattering) and those from permeability measurements. The introduction of a function considering the pores morphology and connectivity was able to reconcile the size measured with the three different methods.