A device for calibrating membrane tubes which are extrudable by an extrusion die includes a ring along an interior wall with which the extruded membrane tube can be guided, and a cooling liquid supply source which can be used to provide a cooling liquid which can be conveyed to between the interior wall and the membrane tube. The ring includes at least one open chamber in the region where the membrane tube exits the ring, with at least one opening of the open chamber facing the membrane tube and being sealed with at least one permeable webbed piece. A vacuum can be applied to the chamber.