Abstract: A water flow rate sensing device for water ducts, comprising a thermal sensor (20) capable of measuring the flow by measuring the removal of heat by a heater from the water; an elongated support (9)/in particular, tubular, having the sensor (20) mounted at a free end thereof so that the support is inserted laterally with respect to the duct through a hole and cantilevered in order to keep the sensor in a central zone of the duct. The free end of the support comprises a lamina-shaped extension member. Furthermore, the device comprises means (5,7) for transmitting a measuring signal to a remote control unit. The invention comprises also a method for measuring the water flow.