A breathing apparatus (1) is disclosed that is adapted to determine a transpulmonary pressure in a patient (125) when connected to said breathing apparatus. A control unit (105) is operable to set a first mode of operation for ventilating said patient with a first Positive End Expiratory Pressure (PEEP) level; set a second mode of operation for ventilating said patient with a second PEEP level starting from said first PEEP level; and determine said transpulmonary pressure (Ptp) based on a change in end-expiratory lung volume (ΔEELV) and a difference between said first PEEP level and said second PEEP level (ΔPEEP). Furthermore, a method and computer program are disclosed.