Wavelength Division Multiplexing (WDM) networks of the future are likely to use Flexgrid, providing operators with additional flexibility when assigning spectrum compared to traditional WDM networks using the 50GHz ITU grid. Flexgrid breaks the spectrum up into small (typically 12.5GHz) slots, but its key feature is that contiguous slots can be joined together to form arbitrary sized blocks of spectrum. This additional flexibility will allow faster transponders that utilise high spectral efficiency modulation techniques, but no longer fit within a 50GHz slot due to their larger spectral width requirements, to be carried by the optical network. From the use of these new spectrum efficient modulation formats and finer control over spectrum allocations, a key benefit that Flexgrid offers network operators is that their WDM networks can carry more traffic. This paper will look at the capacity improvements that can be realised on a WDM network that is using Flexgrid over those that are using a traditional fixed grid.