Here are the standard filter sizes we carry. We also offer custom filter sizes if you don't see yours. Either way, we've got the right size filter for you! Remember, most experts advise that homeowners change standard 1″ air filters once a month when their A/C, heat pump or furnace is used on a daily basis. When you run your HVAC equipment intermittently, you may be able to extend that time frame to perhaps once every six weeks or every two to three months.