Grille size air filters are specially made to fit Metal Aire brand return air grilles. Disposable panel type for furnace and central air systems. Metal grid on both sides gives extra strength. Filters are Class II, UL approved. No. 10355.011212: Nominal size: 12 x 12 x 1, actual size: 11-1/8 x 11-3/8 x 3/4 No. 10355.011818: Nominal size: 18 x 18 x 1, actual size: 17-1/8 x 17-3/8 x 3/4 No. 10355.012020: Nominal size: 20 x 20 x 1, actual size: 19-1/8 x 19-3/8 x 3/4 No. 10355.012222: Nominal size: 22 x 22 x 1, actual size: 21-1/8 x 21-3/8 x 3/4 No. 10355.012412: Nominal size: 24 x 12 x 1, actual size: 23-1/8 x 11-3/8 x 3/4 No. 10355.012418: Nominal size: 24 x 18 x 1, actual size: 23-1/8 x 17-3/8 x 3/4 No. 10355.012420: Nominal size: 24 x 20 x 1, actual size: 23-1/8 x 19-3/8 x 3/4 No. 10355.012424: Nominal size: 24 x 24 x 1, actual size: 23-1/8 x 23-3/8 x 3/4 No. 10355.013018: Nominal size: 30 x 18 x 1, actual size: 29-1/8 x 17-3/8 x 3/4 No. 10355.013020: Nominal size: 30 x 20 x 1, actual size: 29-1/8 x 19-3/8 x 3/4 No. 10355.013024: Nominal size: 30 x 24 x 1, actual size: 29-1/8 x 23-3/8 x 3/4