The Aprilaire 210 / 213 whole-house filter is labeled “20x25x4” but actually measures 20.6 x 26.63 x 4 inches. Unlike 1″ filters, 4″/5″/6″ whole-house filters don't share one universal nominal-to-actual conversion — always match by brand and model, not just the printed size.
From Aprilaire's own spec page. Fits 1210, 1620, 2210, 2216, 3210, 4200 cabinets. This is from the manufacturer's own spec sheet.
Match by brand + model, not nominal size. Confirm against your old filter or equipment manual before ordering.
Looking for a 1″ furnace filter instead? Use the full FilterKey tool.