A device is directly or indirectly connectable to a patient for determining a volume related to the lungs of the patient. The device has an inspiration part through which a breathing gas passes towards the patient, and an expiration part through which the breathing gas passes away from the patient. The inspiration part has a mouthpiece for introducing a change of the gas composition in the breathing gas. The device implements a method for determining the volume with a detector being arranged in the expiration part that measures a measuring parameter which is dependent on the change in the gas composition, and with a calculation unit connected to the detector unit that determines the variation in the measuring parameter, and determines the volume from the determined variation.