We show that covalently attached functional groups can alter the densities of individual SWNT in a predictable and highly controllable manner. A volume additivity model based upon molecular group contributions is able to estimate the density difference between 4-hydroxy phenyl functionalized and non-functionalized HiPco SWNT as approximately 98.3 kg/m3, compared with 97.9 kg/m3 measured from density gradient centrifugation. Conversely, the density difference between the (6,5) (0.75 nm diameter) and (9,8) (1.17 nm diameter) SWNT is smaller at 23.4 kg/m3. We conclude that covalent functionalization can provide an effective handle to separate out particular SWNT from typical diameter distribution. We show that SWNT mixtures where metallic SWNT have been selectively reacted produce two distinct density fractions corresponding to functionalized metallic and pure semiconducting SWNT. The results are confirmed by Raman spectroscopy where the higher density fractions contain an increased disorder mode with a corresponding decrease in intensity for the low density fraction. This method also allows for the first independent measure of (n,m) SWNT having different chemical conversion with functional groups and this will allow rigorous analysis of SWNT chemistry than is possible with un-calibrated spectroscopies such as Raman or photoluminescence.