Every medium is supposed to possess two permittivities :
Absolute Permittivity (Îo) and
Related Permittivity (Îr).
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 Îo .
The relative permittivity of vacuum with reference to itself is unity. Hence, for free space or vacuum.
Absolute Permittivity, Îo = F/m.
Related Permittivity, Îr = 1
Now, take any other medium. If it's relative permittivity, as compared to vacuum, is Îr , then it's absolute permittivity is
Î = ÎoÎr F/m