Device for controlling an extracorporeal blood treatment device, the blood treatment device comprising: an exchange unit, which is subdivided into a first chamber and a second chamber by a semipermeable membrane, the first chamber being part of an extracorporeal blood circuit in which a blood pump for conveying blood at a particular blood flow rate Qb is arranged, and/or means for conveying dialysate through the second chamber of the exchange unit at a particular dialysate rate Qd and/or means for supplying substitution fluid to the extracorporeal blood circuit at a particular substitution fluid rate Qs, and/or means for withdrawing ultrafiltrate from the first chamber through the semipermeable membrane into the second chamber of the exchange unit at a fluid rate which corresponds to the total of the substitution fluid rate Qs and a particular ultrafiltration rate Qf, the device for controlling the blood treatment device comprising: a control unit for generating a control signal for setting the feed rate of the blood pump to a particular blood flow rate Qb and for generating a control signal for setting at least one particular flow rate from the set of flow types of dialysate rate Qd ultrafiltration rate Qf and substitution fluid rate Qs, a memory unit for storing a preset clearance K or dialysance D, characterised in that a central processing unit is provided, which is formed in such a way that at at least one preset flow rate from the set of flow rates of blood flow rate Qb, dialysate rate Qd ultrafiltrate rate Of and substitution fluid rate Qs, at least one of the respective other flow rates from the set of flow rates of blood flow rate Qb, dialysate rate Qd ultrafiltrate rate Of and substitution fluid rate Qs is calculated purely on the basis of a preset dependence of the clearance K or dialysance D on the flow rates, for which the preset clearance K or dialysance D is maintained. an exchange unit, which is subdivided into a first chamber and a second chamber by a semipermeable membrane, the first chamber being part of an extracorporeal blood circuit in which a blood pump for conveying blood at a particular blood flow rate Qb is arranged, and/or means for conveying dialysate through the second chamber of the exchange unit at a particular dialysate rate Qd and/or means for supplying substitution fluid to the extracorporeal blood circuit at a particular substitution fluid rate Qs, and/or means for withdrawing ultrafiltrate from the first chamber through the semipermeable membrane into the second chamber of the exchange unit at a fluid rate which corresponds to the total of the substitution fluid rate Qs and a particular ultrafiltration rate Qf, a control unit for generating a control signal for setting the feed rate of the blood pump to a particular blood flow rate Qb and for generating a control signal for setting at least one particular flow rate from the set of flow types of dialysate rate Qd ultrafiltration rate Qf and substitution fluid rate Qs, a memory unit for storing a preset clearance K or dialysance D, characterised in that a central processing unit is provided, which is formed in such a way that at at least one preset flow rate from the set of flow rates of blood flow rate Qb, dialysate rate Qd ultrafiltrate rate Of and substitution fluid rate Qs, at least one of the respective other flow rates from the set of flow rates of blood flow rate Qb, dialysate rate Qd ultrafiltrate rate Of and substitution fluid rate Qs is calculated purely on the basis of a preset dependence of the clearance K or dialysance D on the flow rates, for which the preset clearance K or dialysance D is maintained.