The invention relates to a device for calculating blood flow and to an apparatus for extracorporeal blood treatment provided with the device. The device includes a memory for receiving and storing either a first datum relating to a blood flow removed from the patient Qb, or a second datum relating to a flow of liquid Qt crossing a tract of the extracorporeal circuit, and a third datum relating to a flow of infusion liquid Qinf flowing through the infusion line. The device also comprises a control unit able to calculate the first datum or the second datum as a function of the third datum relating to the flow of infusion liquid Qinf.
