A pressure vessel 12 with a CO2 liquid level indicator 14, for use with a paintball marker 10, includes a magnetic float assembly 42 disposed within the pressure vessel 12 and adapted to float on liquid phase CO2 39 contained within the pressure vessel 12, in combination with a magnetically sensitive indicator 44 on an outer surface 40 of the pressure vessel 12 and which is adapted to detect and to display the location of the magnetic float assembly 42 within the pressure vessel 12. When connected to a conventional paintball marker 10, the pressure vessel 12 magnetically shows the indicated location of the magnetic float assembly 42, thereby showing the amount of liquid phase CO2 39 remaining within the pressure vessel 12. This remaining amount of liquid phase CO2 39 can be correlated to an approximate number of additional paintball rounds that can be shot before the CO2 is depleted.