A control unit (30) is arranged to control a dialysis fluid distribution system (12) comprising two volumetric pumps(PI, P2) and a dialyzer (13). The control unit (30) is operable in a calibration mode, to establish a bypass flow path that bypasses the dialyzer (13) and extends between the pumps (PI, P2) and to operate the pumps (PI, P2) at first and second calibration speeds so as to balance the flow rates generated by the pumps (PI, P2), e.g. based on a measured pressure or fluid level in dialysis fluid distribution system (12). The control unit (30) determines, based on the first and second calibration speeds, a relation between the stroke volumes of the pumps (PI, P2). The control unit (30) is further operable in a treatment mode, to establish a main flow path that extends between the first and second pumps (PI, P2) via the dialyzer (13) and to control the first and second pumps (PI, P2), based on the relation between their stroke volumes, to operate at a respective treatment frequency so as to generate a selected ultrafiltration rate in the dialyzer (13).
