Described is a unit for filling containing elements (2) of single-use capsules (3) with a dose (33) of product for extraction or infusion beverages, comprising: a line (4) for transporting containing elements (2); a station (SR) for filling the containing elements (2) with a dose (33) of product which comprises: at least a first containing seat (S1) designed to receive a dose (33) of product; a substation (ST1) for forming the dose (33) provided with a device (6) for releasing a predetermined quantity of product defining the dose (33) inside the first containing seat (S1), the release device (6) comprising a hopper (38) and at least one rotary element (40a) having a helical profile which extends between a first end and a second end, configured to rotate about a respective axis (X4) of rotation; a drive and control unit (15) configured to drive the rotary element (40a) according to a speed of rotation variable as a function of an angular position of the first end of the rotary unit (40a).