The transition thresholds between hexagons and rolls in convective patterns are obtained in the framework of the amplitude equations. We show that the discrepancies between the theoretical thresholds, calculated for unbounded systems, and the experimental ones, made in finite containers, can be partially corrected by a phenomenological argument. The finite-size effects are responsible for the decreasing in the efficiency of the heat transport across the cell. Using this fact we are able to approach the calculated thresholds to those observed in real experiments. The transition between different symmetries in convective