A dialysis apparatus, comprising: a dialyzer; a dialysis fluid distribution system connected for fluid communication with the dialyzer and comprising a first pump (P1; P2) and a second pump (P2; P1), the dialysis fluid distribution system being operable to selectively establish a main flow path that extends between the first and second pumps (P1, P2) via the dialyzer, and a bypass flow path that bypasses the dialyzer and extends between the first and second pumps (P1, P2); and a control unit electrically connected to the dialysis fluid distribution system and being operable to control a respective frequency of the first and second pumps (P1, P2), wherein the first and second pumps (P1, P2) are configured to generate a respective flow rate by repeatedly discharging, at the respective frequency, a respective stroke volume of dialysis fluid; wherein the control unit is configured to, in a calibration mode, operate the dialysis fluid distribution system to establish the bypass flow path and control the first pump (P1; P2) to operate at a first calibration frequency and second pump (P2; P1) to operate at a second calibration frequency so as to balance the flow rates generated by the first and second pumps (P1, P2), and, in a treatment mode, operate the dialysis fluid distribution system to establish the main flow path, characterized in that the control unit is configured to, in the calibration mode, determine relative calibration data that represents a relation between the stroke volumes of the first and second pumps (P1, P2) based on the first and second calibration frequencies; the control unit is configured to, in the treatment mode, control the first and second pumps (P1, P2), based on said relative calibration data, to operate at a respective treatment frequency so as to generate a selected ultrafiltration rate in the dialyzer, wherein the control unit is configured to assign a predefined nominal stroke volume to the first pump (P1; P2) and set the treatment frequency of the first pump (P1; P2) so as to generate, based on the predefined nominal stroke volume, a first flow rate (Q 1) of dialysis fluid, and to set the treatment frequency of the second pump (P2; P1), given that the stroke volume of the second pump (P2; P1) fulfils said relation with respect to the predefined nominal stroke volume, so as to generate a second flow rate (Q 2) of dialysis fluid which differs from the first flow rate by the selected ultrafiltration rate. a dialyzer; a dialysis fluid distribution system connected for fluid communication with the dialyzer and comprising a first pump (P1; P2) and a second pump (P2; P1), the dialysis fluid distribution system being operable to selectively establish a main flow path that extends between the first and second pumps (P1, P2) via the dialyzer, and a bypass flow path that bypasses the dialyzer and extends between the first and second pumps (P1, P2); and a control unit electrically connected to the dialysis fluid distribution system and being operable to control a respective frequency of the first and second pumps (P1, P2), wherein the first and second pumps (P1, P2) are configured to generate a respective flow rate by repeatedly discharging, at the respective frequency, a respective stroke volume of dialysis fluid; wherein the control unit is configured to, in a calibration mode, operate the dialysis fluid distribution system to establish the bypass flow path and control the first pump (P1; P2) to operate at a first calibration frequency and second pump (P2; P1) to operate at a second calibration frequency so as to balance the flow rates generated by the first and second pumps (P1, P2), and, in a treatment mode, operate the dialysis fluid distribution system to establish the main flow path, characterized in that the control unit is configured to, in the calibration mode, determine relative calibration data that represents a relation between the stroke volumes of the first and second pumps (P1, P2) based on the first and second calibration frequencies; the control unit is configured to, in the treatment mode, control the first and second pumps (P1, P2), based on said relative calibration data, to operate at a respective treatment frequency so as to generate a selected ultrafiltration rate in the dialyzer, wherein the control unit is configured to assign a predefined nominal stroke volume to the first pump (P1; P2) and set the treatment frequency of the first pump (P1; P2) so as to generate, based on the predefined nominal stroke volume, a first flow rate (Q 1) of dialysis fluid, and to set the treatment frequency of the second pump (P2; P1), given that the stroke volume of the second pump (P2; P1) fulfils said relation with respect to the predefined nominal stroke volume, so as to generate a second flow rate (Q 2) of dialysis fluid which differs from the first flow rate by the selected ultrafiltration rate.