In this paper is presented a criterion for identification of heat transfer regime through convection ( natural, forced or mixed) by making use of the Mathematica system symbolic computation capabilities. The criterion is based on a comparison of buoyancy and viscous forces. The analysis is realized at the interior of two vertical plates submitted to uniform heat flux density, in steady and laminar state in a fully developed flow. Thus, it was proposed the dimensionless numbers product RiRe to be the only representative parameter in order to identify the convective thermal regime, instead of the idea based only on the dimensionless number Richardson or Grashof. (C) 2015 The Authors. Published by Elsevier Ltd.