A correlation is obtained for the drag coefficient $c '_f$ of the turbulent boundary layer as a function of the effective boundary layer Reynolds number $Re$ that we previously introduced. A comparison is performed also with another correlation for the drag coefficient as a function of the traditional Reynolds number $Re_{\th}$, based on the momentum thickness of the boundary layer proposed recently by R.D.Watson, R.M.Hall and J.B.Anders (NASA Langley Research Center) on the basis of different set of experimental data. We show that the correlation obtained by us agrees with experimental data from the Illinois Institute of Technology, but is incompatible with the data obtained in the Royal Institute of Technology at Stockholm. On the other hand, both sets of data are in disagreement with the Langley correlation.