The Aprilaire 401 whole-house filter is labeled “16x25x6” but actually measures 17 x 28 x 7.47 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.
Native filter for 2400 cabinet — 6" thick, not 4"/5". 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.