A unified similarity transformation is proposed to extract all possible similarity solutions for free, forced and mixed convection within Darcy and non-Darcy porous media. The slip velocity at the wall resulting from both the externally forced flow and the buoyancy force is chosen as a velocity scale to form a modified Peclet number which naturally transforms into the conventional Peclet number and Rayleigh number under certain physical limiting conditions. This unified treatment reveals three limiting flow regimes, namely, the forced convection regime, the Darcy free convection regime and the Forchheimer free convection regime, as well as three other intermediate flow regimes, namely, the Darcy mixed convection regime, the Darcy-Forchheimer free convection regime and the Forchheimer mixed convection regime. Relevant flow parameters for distinguishing these flow regimes are found to be the ‘micro-scale’ Reynolds and Grashof numbers based on the square root of the permeability. A flow regime map has been constructed to show these six different flow regimes, taking the two micro-scale dimensionless numbers as the abscissa and ordinate variables. Asymptotic expressions derived for these flow regimes appear quite useful for practical estimation of convective heat transfer within Darcy and non-Darcy porous media.