An easy-to-use, easy-to-read gage to monitor filter media loading so that filter media may be changed when needed. No wiring needed, only a 3/8 drill bit. The gage is installed between the blower and the filter, where a slight vacuum exists due to the air-flow resistance of the air filter. Air flowing into the gage, around the calibration screw,lifts a vane in proportion to the negative pressure in the blower compartment. As the air filter loads, the vacuum increases, raising the vane to indicate a filter change is necessary.