A dialysis apparatus, comprising: a dialyzer; a dialysis fluid distribution system connected for fluid communication with the dialyzer and comprising a first pump, a second pump, a calibration chamber, and a fluid level detector for sensing a level of dialysis fluid in the calibration chamber, the dialysis fluid distribution system being operable to selectively establish a main flow path that extends from the first pump to the second pump via the dialyzer so that the first and second pumps (P1, P2) are connected upstream and downstream of the dialyzer to pump dialysis fluid through the dialyzer, and a bypass flow path that bypasses the dialyzer and extends from the first pump to the second pump in fluid communication with the calibration chamber; 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; characterized in that the control unit is configured to, in a calibration mode, perform a first and second measurement procedure while operating the dialysis fluid distribution system to establish the bypass flow path; wherein the first measurement procedure comprises: controlling the first and second pumps (P1, P2) to operate at a first combination of frequencies that results in a first flow rate difference between the flow rates generated by the first and second pumps (P1, P2), and measuring, by the fluid level detector, a first time period corresponding to a first change in dialysis fluid level in the calibration chamber as a result of the first flow rate difference, the first change in dialysis fluid level corresponding to a first known volume in the calibration chamber; wherein the second measurement procedure comprises: controlling the first and second pumps (P1, P2) to operate at a second combination of frequencies that differs from the first combination of frequencies and results in a second flow rate difference between the flow rates generated by the first and second pumps (P1, P2), and measuring, by the fluid level detector, a second time period corresponding to a second change in dialysis fluid level in the calibration chamber as a result of the second flow rate difference, the second change in dialysis fluid level corresponding to a second known volume in the calibration chamber; and that the control unit is further configured to compute at least one of the stroke volumes of the first and second pumps (P1, P2) as a function of the first and second combinations of frequencies, the first and second time periods, and the first and second known volumes. a dialyzer; a dialysis fluid distribution system connected for fluid communication with the dialyzer and comprising a first pump, a second pump, a calibration chamber, and a fluid level detector for sensing a level of dialysis fluid in the calibration chamber, the dialysis fluid distribution system being operable to selectively establish a main flow path that extends from the first pump to the second pump via the dialyzer so that the first and second pumps (P1, P2) are connected upstream and downstream of the dialyzer to pump dialysis fluid through the dialyzer, and a bypass flow path that bypasses the dialyzer and extends from the first pump to the second pump in fluid communication with the calibration chamber; 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; characterized in that the control unit is configured to, in a calibration mode, perform a first and second measurement procedure while operating the dialysis fluid distribution system to establish the bypass flow path; wherein the first measurement procedure comprises: controlling the first and second pumps (P1, P2) to operate at a first combination of frequencies that results in a first flow rate difference between the flow rates generated by the first and second pumps (P1, P2), and measuring, by the fluid level detector, a first time period corresponding to a first change in dialysis fluid level in the calibration chamber as a result of the first flow rate difference, the first change in dialysis fluid level corresponding to a first known volume in the calibration chamber; wherein the second measurement procedure comprises: controlling the first and second pumps (P1, P2) to operate at a second combination of frequencies that differs from the first combination of frequencies and results in a second flow rate difference between the flow rates generated by the first and second pumps (P1, P2), and measuring, by the fluid level detector, a second time period corresponding to a second change in dialysis fluid level in the calibration chamber as a result of the second flow rate difference, the second change in dialysis fluid level corresponding to a second known volume in the calibration chamber; and that the control unit is further configured to compute at least one of the stroke volumes of the first and second pumps (P1, P2) as a function of the first and second combinations of frequencies, the first and second time periods, and the first and second known volumes.