A method for allocating spectral capacity in a WDM optical network comprising: —providing (4) a group of connection demands, —defining a group of candidate lightpaths that are adapted to carry an optical signal transparently, —defining a highest admissible spectral efficiency for each candidate lightpath, —computing (6) a respective spatial path for each connection demands, —for each candidate lightpath, determining the connection demands that match the candidate lightpath and computing its (1 2) spectral resource saving as a function of the difference between the spectral resources that would be used with its highest admissible spectral efficiency and the spectral resources that would be used with its reference spectral efficiency for the total capacity of the matching connection demands, —selecting (14) and allocating spectral resources to the candidate lightpath having the highest admissible spectral resource saving, —removing (14) the selected candidate lightpath and iterating to the step of determining the matching connection demands.