The present invention relates to a method for calculating or approximating a value representing the relative blood volume (RBV) at a certain point of time, or a value representing the refilling volume of a patient that may be observed or found during or due to a blood treatment of the patient, the method involving considering one or more calculated or measured value(s) reflecting an overhydration level of the patient or an approximation thereof. It relates further to an apparatus and a device for carrying out the present invention, a blood treatment device, digital storage means, a computer program product, and a computer program.