Metering unit for fine-grained powders including - a hopper for loading the powder, - a support provided with a vertical hole in correspondence with a bottom outlet of said hopper which is mounted on said support, - a volumetric drawer provided with a volumetric metering chamber, said volumetric drawer being mobile back and forth through the support between a filling position, in which said volumetric metering chamber is filled by the powder that descends by gravity from the hopper through said vertical hole in the support, and an emptying position, in which the volumetric metering chamber is emptied of the powder that descends by gravity through a hole formed in a plate on which the support is mounted, and - actuator means that provide the volumetric drawer with said back and forth motion, wherein it further includes - a final metering chamber, placed under said plate and communicating with the volumetric metering chamber through a second hole formed in the plate in correspondence with said filling position, - a metering tube that extends through said final metering chamber so as to protrude in front with its end in correspondence with said emptying position, said metering tube being provided with a slot for the entry of the powder formed in its portion contained in the final metering chamber, - a variable-speed motor that turns the metering tube, - a weighing tray placed in correspondence with the emptying position so as to receive the powder from the volumetric metering chamber and from the metering tube, - a scale that carries the weighing tray and constantly records the exact weight of the powder contained therein; - second actuator means which empty the weighing tray into a discharge pipe, and - a control unit operatively connected to said scale, to both the actuator means (5, 11) and to said variable-speed motor so as to control the operation of the actuator means (5, 11) and of the motor on the basis of the weight of the powder detected by the scale. - a hopper for loading the powder, - a support provided with a vertical hole in correspondence with a bottom outlet of said hopper which is mounted on said support, - a volumetric drawer provided with a volumetric metering chamber, said volumetric drawer being mobile back and forth through the support between a filling position, in which said volumetric metering chamber is filled by the powder that descends by gravity from the hopper through said vertical hole in the support, and an emptying position, in which the volumetric metering chamber is emptied of the powder that descends by gravity through a hole formed in a plate on which the support is mounted, and - actuator means that provide the volumetric drawer with said back and forth motion, wherein it further includes - a final metering chamber, placed under said plate and communicating with the volumetric metering chamber through a second hole formed in the plate in correspondence with said filling position, - a metering tube that extends through said final metering chamber so as to protrude in front with its end in correspondence with said emptying position, said metering tube being provided with a slot for the entry of the powder formed in its portion contained in the final metering chamber, - a variable-speed motor that turns the metering tube, - a weighing tray placed in correspondence with the emptying position so as to receive the powder from the volumetric metering chamber and from the metering tube, - a scale that carries the weighing tray and constantly records the exact weight of the powder contained therein; - second actuator means which empty the weighing tray into a discharge pipe, and - a control unit operatively connected to said scale, to both the actuator means (5, 11) and to said variable-speed motor so as to control the operation of the actuator means (5, 11) and of the motor on the basis of the weight of the powder detected by the scale.