Every medium is supposed to possess two permittivities :

                           Absolute Permittivity (o)  and 

                           Related Permittivity (r).

    For measuring relative permittivity, vacuum is to be chosen as the reference medium. It is alloted an absolute permittivity of 8.854 x 10-12 farad/metre. It's symbol is o

The relative permittivity of vacuum with reference to itself is unity. Hence, for free space or vacuum.

                      Absolute Permittivity,          o  =  F/m. 

                      Related Permittivity,            r  = 1

Now, take any other medium. If it's relative permittivity, as compared to vacuum, is r , then it's absolute permittivity is

                                               = oF/m