The present disclosure relates to a method for determining an actual pump rate and/or for calibrating a blood pump of a blood treatment apparatus connected to an extracorporeal blood circuit. The method includes providing a blood pump of a blood treatment apparatus or establishing a signal communication to a blood pump. The blood pump comprises, or is connected to at least a first source for a fluid and at least a first line of an extracorporeal blood circuit which is connected to the first source downstream. Furthermore, the blood pump comprises, or is connected to, a reception unit for receiving fluids of the first source wherein the reception unit is in fluid communication with the first line. The reception unit is arranged on or at a first weighing device such that the weight of the reception unit or of its content is determined using the weighing device.