A device and method are provided for determining the volume ECV.sub.hydr(t) of a body compartment of a patient at a time t by conducting measurements at the time t of the patient to determine at least one anthropometric measure X(t), the extracellular water volume ECV(t), and the intracellular water volume ICV (t) of the patient. The extracellular water volume ECV.sub.basic(t) of a first compartment with weight W.sub.basic(t) of the patient at the time t is derived by using X (t), the extracellular water volume ECV.sub.sec(t) of a second compartment of the patient at the time t is derived by using ICV (t), and ECV.sub.hydr(t) as the extracellular water volume of a third compartment of the patient is derived with weight W.sub.hydr(t). The extracellular volume ECV.sub.hydr(t) is a measure for the hydration status of the patient.