Abstract: A volumetric dosing unit with doser cells or chambers of adjustable volume for continuously operating machines producing dual-use filter sachets for infusion products. The metering unit has a doser with a cylindrical drum rotatable about a horizontal axis and with radial bores opening on its external cylindrical surface. A feeding hopper for the product to be dosed is located above the drum. A compacting element extends from its hopper to the lower deadpoint of the drum tangentially relative to a strip of paper for filter sachets onto which are discharged fractional doses for processing by means of piston elements sliding in the bores. The piston elements are linked to cam control means having an actuator track of adjustable width.