The present invention relates to a device and to a method for determining the dialysis fluid flow rate for an extracorporeal blood treatment device. The dialysis fluid flow rate Qd is determined in accordance with the blood flow rate Qb or the blood flow rate Qb is determined in accordance with the dialysis fluid flow rate Qd, wherein by increasing about a specific value, the increase of a variable characterising the efficiency of the blood treatment, in particular clearance K, does not exceed the specific value. The present invention also relates to a blood treatment device comprising a device for determining the dialysis fluid flow rate or blood flow rate and to a method for operating an extracorporeal blood treatment device. As a result, an optimal dialysis fluid flow rate and/or blood flow rate calculates the requirement in accordance with a high efficiency of the dialysis treatment and in accordance with a reduced consumption of the dialysis liquid.
