A forming member for controlling the volume of packages of pourable food products formed from a tube of packaging material and sealed at a number of sections of the tube crosswise to an axis of the tube; the forming member having: a main wall which cooperates cyclically with a first portion of the tube extending between two consecutive sections; and at least one flap extending alongside the wall. The flap is movable, with respect to the wall, between a first position, in which it cooperates with a second portion of the tube, extending between the two consecutive sections, to control the volume, between the two consecutive sections, of the package being formed, and a second position, in which it is detached from the second portion.