An apparatus is described for extracorporeal blood treatment, comprising a treatment unit, an extracorporeal blood circuit and a fluid evacuation line. The apparatus comprises a control unit connected to a pressure sensor and a blood pump, the blood pump generating a variable flow with a constant component and a variable component. The control unit receives, from the pressure sensor, a plurality of values and calculates the average pressure value, acquires an estimated value of volume variation in the expansion chamber connected to the variable flow component, calculates, as a function of the pressure values, an estimated value of pressure variation in the expansion chamber that is representative of an oscillating pressure component and determines a representative magnitude of a blood level in the expansion chamber as a function of the average pressure value, the estimated value of volume variation and the estimated pressure variation in the expansion chamber.