How often should you change your furnace filters?

How often you should change your furnace filters typically depends on the thickness of the filter. If your filter is 1-2 inches thick, you should change it every three months. If your filter is 4 inches thick, you should change it every six months, and if your filter is 5 inches thick, you should change it every 12 months (once a year). You should also change your filter as-needed if the filter is visibly very dirty or you can't see the material of the filter itself.