  The primordial gravitational waves (\textbf{PGW}) have been generated by
inflationary amplification of the primordial (quantum) fluctuations. It is true
that they have not been recorded directly so far, but their spectrum can help a
lot in solving the basic puzzles of the early universe as Inflation (high)
energy scale. In the present work, we give a straightforward method to
calculate the spectral energy density (\textbf{SED}) of the relic gravitons
different from that used in e.g. \cite{ mirza04,Latham 2005, Yuki 2006}. In our
approach, the evolution equations are in terms of the scale factor (instead of
conformal time) through the Lagrange formalism (instead of the transfer
function). The presence of the Hubble parameter allows to calculate the power
spectrum in the different dynamical regimes.
