In the current work, free volume concepts, primarily applied to glass formers in the literature, were transferred to ionic liquids (ILs). A series of 1-butyl-3-methylimidazolium (C4MIM) based ILs was investigated by Positron Annihilation Lifetime Spectroscopy (PALS). The phase transition and dynamic properties of the ILs [C4MIM][X] with X = Cl, BF4, PF6, OTf, NTf2 and B(hfip)(4) were reported recently (Yu et al., Phys. Chem. Chem. Phys., 2012, 14, 6856-6868). In this subsequent work, attention was paid to the connection of the free volume from PALS (here the mean hole volume, < v(h)>) with the molecular structure, represented by volumes derived from X-ray diffraction (XRD) data. These were the scaled molecular volume V-m,V-scaled and the van der Waals volume V-vdw. Linear correlations of < v(h)> at the "knee'' temperature (< v(h)>(T-k)) with V-m,V-scaled and V-vdw gave good results for the C4MIM series. Further relationships between volumes from XRD data with the occupied volume V-occ determined from PALS/PVT (Pressure Volume Temperature) measurements and from Sanchez-Lacombe Equation of State (SL-EOS) fits were elaborated (V-occ(SL-EOS) approximate to 1.63 V-vdw, R-2 = 0.981 and V-occ(SL-EOS) approximate to 1.12 V-m,V-scaled, R-2 = 0.980). Finally, the usability of V-m,V-scaled was justified in terms of the Cohen-Turnbull (CT) free volume theory. Empirical CT type plots of viscosity and electrical conductivity showed a systematic increase in the critical free volume with molecular size. Such correlations allow descriptions of IL properties with the easily accessible quantity V-m,V-scaled within the context of the free volume.