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
The transition between different symmetries in convective