A method for operating a pneumatic system with a compressed air supply unit and an air spring unit, wherein the pneumatic system has: - a pressure accumulator with a pressure accumulator volume, in particular an air dryer with an air dryer volume, - a gallery and at least one air spring (92, 92.1, 92.2, 92.3, 92.4) selectively gas-conducting connectable to the gallery via a valve (93, 93.1, 93.2, 93.3, 93.4) of a valve block, with a number of bellows volumes (91V, 91.1V, 91.2V, 91.3V, 91.4V), which are assigned to the air springs (92, 92.1, 92.2, 92.3, 92.4), and the method has the following steps: - determining at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4), - determining of the at least one bellows volume (91V, 91.1V, 91.2V, 91.3V, 91.4V) of a spring bellows (91, 91.1, 91.2, 91.3, 91.4) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4) on the basis of the at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A), characterized by - specification of a pneumatic replacement model for the volumes (91V, 91.1V, 91.2V, 91.3V, 91.4V,VD,VL), on the basis of a mass flow balance by a balance volume (VBIL), - calculation of at least one pressure value (P, PL, PV, PS, PE, PF1, PF2, PF3, PF4) of a volume (VBIL, 91V, 91.1V, 91.2V, 91.3V, 91.4V, VD, VL) on the basis of the pneumatic replacement model. - a pressure accumulator with a pressure accumulator volume, in particular an air dryer with an air dryer volume, - a gallery and at least one air spring (92, 92.1, 92.2, 92.3, 92.4) selectively gas-conducting connectable to the gallery via a valve (93, 93.1, 93.2, 93.3, 93.4) of a valve block, with a number of bellows volumes (91V, 91.1V, 91.2V, 91.3V, 91.4V), which are assigned to the air springs (92, 92.1, 92.2, 92.3, 92.4), and the method has the following steps: - determining at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4), - determining of the at least one bellows volume (91V, 91.1V, 91.2V, 91.3V, 91.4V) of a spring bellows (91, 91.1, 91.2, 91.3, 91.4) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4) on the basis of the at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A), characterized by - specification of a pneumatic replacement model for the volumes (91V, 91.1V, 91.2V, 91.3V, 91.4V,VD,VL), on the basis of a mass flow balance by a balance volume (VBIL), - calculation of at least one pressure value (P, PL, PV, PS, PE, PF1, PF2, PF3, PF4) of a volume (VBIL, 91V, 91.1V, 91.2V, 91.3V, 91.4V, VD, VL) on the basis of the pneumatic replacement model. - determining at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4), - determining of the at least one bellows volume (91V, 91.1V, 91.2V, 91.3V, 91.4V) of a spring bellows (91, 91.1, 91.2, 91.3, 91.4) of the at least one air spring (92, 92.1, 92.2, 92.3, 92.4) on the basis of the at least one deflection (92A, 92.1A, 92.2A, 92.3A, 92.4A), characterized by - specification of a pneumatic replacement model for the volumes (91V, 91.1V, 91.2V, 91.3V, 91.4V,VD,VL), on the basis of a mass flow balance by a balance volume (VBIL), - calculation of at least one pressure value (P, PL, PV, PS, PE, PF1, PF2, PF3, PF4) of a volume (VBIL, 91V, 91.1V, 91.2V, 91.3V, 91.4V, VD, VL) on the basis of the pneumatic replacement model.