In the present research we investigate pressure driven flow in the transition and free-molecular flow regimes with the objective of developing unified flow models for microchannels. These models are based on a velocity scaling law, which is valid for a wide range of Knudsen number. Simple slip-based descriptions of flowrate in microchannels are corrected for effects in the transition and free-molecular flow regimes with the introduction of a rarefaction factor. The resulting models can predict the velocity distribution, mass flowrate, pressure and shear stress distribution in rectangular microchannels in the entire Knudsen flow regime.