Method of operating a system for supporting a patient, adapted to calculate the supporting pressure of an inflatable mattress on the base of parameters such as, for instance, weight, position, said system comprising: - a first series of cells and a second series of cells included in the inflatable mattress; - detecting means to detect the air pressure in the inside of the first series of cells and the air pressure in the inside of the second series of cells; - a compressor to inflate said inflatable mattress; - a valve having a discharge connection for the discharge in the outer ambient, said valve being connected through a first conduit to the first series of cells of the inflatable mattress, through a second conduit to the second series of cells, and through a feed conduit to the compressor; - a controller connected to the valve, to the pressure detecting means and to the compressor; said method comprising the steps of: - inflating the first series of cells or the second series of cells at a supporting pressure; - deflating the first series of cells at a pressure lower than the supporting pressure if the second series of cells has been inflated at the supporting pressure, or deflating the second series of cells at a lower pressure than the supporting pressure if the first series of cells has been inflated at the supporting pressure, said method being characterized by the fact that the system comprises further: - at least one control chamber (30, 38, 48, 66, 94, 96, 120) placed under the patient and connected to a pressure sensor, to the compressor in order to be inflated, and to a discharge in order to be deflated; - said pressure sensor being connected to the controller; and by the fact of comprising the steps of: - choosing initially a first pressure and a second pressure so as to have a first predetermined pressure and a second predetermined pressure; - inflating or deflating the at least one control chamber (30, 38, 48, 66, 94, 96, 120); - measuring an indicative time to be spent by the at least one control chamber (30, 38, 48, 66, 94, 96, 120) to pass from a first pressure to a second pressure, both pressures being detected by the pressure sensor; - calculating the supporting pressure on the base of the indicative time as measured in the previous phase. - a first series of cells and a second series of cells included in the inflatable mattress; - detecting means to detect the air pressure in the inside of the first series of cells and the air pressure in the inside of the second series of cells; - a compressor to inflate said inflatable mattress; - a valve having a discharge connection for the discharge in the outer ambient, said valve being connected through a first conduit to the first series of cells of the inflatable mattress, through a second conduit to the second series of cells, and through a feed conduit to the compressor; - a controller connected to the valve, to the pressure detecting means and to the compressor; - inflating the first series of cells or the second series of cells at a supporting pressure; - deflating the first series of cells at a pressure lower than the supporting pressure if the second series of cells has been inflated at the supporting pressure, or deflating the second series of cells at a lower pressure than the supporting pressure if the first series of cells has been inflated at the supporting pressure, - at least one control chamber (30, 38, 48, 66, 94, 96, 120) placed under the patient and connected to a pressure sensor, to the compressor in order to be inflated, and to a discharge in order to be deflated; - said pressure sensor being connected to the controller; - choosing initially a first pressure and a second pressure so as to have a first predetermined pressure and a second predetermined pressure; - inflating or deflating the at least one control chamber (30, 38, 48, 66, 94, 96, 120); - measuring an indicative time to be spent by the at least one control chamber (30, 38, 48, 66, 94, 96, 120) to pass from a first pressure to a second pressure, both pressures being detected by the pressure sensor; - calculating the supporting pressure on the base of the indicative time as measured in the previous phase.