A variable-volume dosage device, particularly for a coffee machine and the like, comprising: - a dosage chamber having an elongated shape and extending between a front opening, through which coffee powder is introduced in said dosage chamber parallel to the longitudinal extension of said dosage chamber, and a rear opening which is opposite said front opening; said dosage chamber having a further lateral opening which is arranged proximate to said front opening at a lateral surface thereof so as to be able to empty said dosage chamber of said coffee powder through said lateral opening; - a flap which is accommodated slidingly in said dosage chamber and can move in contrast with and by virtue of the action of elastic return means between an inactive position, in which said flap is arranged substantially so as to close said dosage chamber at said front opening, and an active position, in which said flap is arranged in said dosage chamber between an intermediate position comprised between said front opening and said rear opening; - means for sensing the position of said flap within said dosage chamber with respect to said inactive position; - control and management means and actuation means which are mutually associated and functionally connected respectively to said sensor means, in order to determine the position of said flap, and to said release element, for its movement from said loading position to said release position upon reaching a predetermined quantity of coffee powder accommodated in said dosage chamber correspondingly with a predetermined position of said flap in said dosage chamber; the dosage device being characterized by: - a release element which is associated externally with respect to said dosage chamber and can move in contrast with and by virtue of the action of said elastic return means between a loading position, in which said release element is arranged so as to occlude said lateral opening, and a release position, in which said lateral opening is free from said release element. - a dosage chamber having an elongated shape and extending between a front opening, through which coffee powder is introduced in said dosage chamber parallel to the longitudinal extension of said dosage chamber, and a rear opening which is opposite said front opening; said dosage chamber having a further lateral opening which is arranged proximate to said front opening at a lateral surface thereof so as to be able to empty said dosage chamber of said coffee powder through said lateral opening; - a flap which is accommodated slidingly in said dosage chamber and can move in contrast with and by virtue of the action of elastic return means between an inactive position, in which said flap is arranged substantially so as to close said dosage chamber at said front opening, and an active position, in which said flap is arranged in said dosage chamber between an intermediate position comprised between said front opening and said rear opening; - means for sensing the position of said flap within said dosage chamber with respect to said inactive position; - control and management means and actuation means which are mutually associated and functionally connected respectively to said sensor means, in order to determine the position of said flap, and to said release element, for its movement from said loading position to said release position upon reaching a predetermined quantity of coffee powder accommodated in said dosage chamber correspondingly with a predetermined position of said flap in said dosage chamber; the dosage device being characterized by: - a release element which is associated externally with respect to said dosage chamber and can move in contrast with and by virtue of the action of said elastic return means between a loading position, in which said release element is arranged so as to occlude said lateral opening, and a release position, in which said lateral opening is free from said release element. - a release element which is associated externally with respect to said dosage chamber and can move in contrast with and by virtue of the action of said elastic return means between a loading position, in which said release element is arranged so as to occlude said lateral opening, and a release position, in which said lateral opening is free from said release element.