For determining the quantity of an electrically conductive product contained in a reservoir made of electrically insulating material, having at least one storage cavity: a resonant circuit is formed, including a capacitive arrangement comprising at least part of this reservoir; a processing procedure is defined, consisting of identifying, from a plurality of signals corresponding to different frequencies, the value of a characteristic of this resonant circuit, deriving from this value of the characteristic a measurement of the resistance constituted by the product contained in the reservoir in the said capacitive arrangement and deriving therefrom the instantaneous value of an item of information representing the quantity of product contained in the reservoir from a pre-established correlation law; and a least one determination cycle is effected, consisting of triggering the measurement procedure and the processing procedure, and capturing the instantaneous value of the said item of information.