A device for calibrating the product dispensed by a distributor (5) of granular materials comprises a calibration aperture (9) formed in a through-conduit (8) for the granular material between the distributor (5) of the seed and a device (4) for transporting the material, and a gate (10) associated with the calibration aperture (9) and movable between a calibration position, in which the aperture (9) is open and the material from the distributor (5) is conducted via the aperture (9), and a distribution position, in which the aperture (9) is closed and the material is conducted therebeyond along the through-conduit (8), wherein a motorized actuator (11) is connected to the gate (10) so as to make the gate (10) movable between those positions.