The Trane / American Standard BAYFTAH23M (TFM235) whole-house filter is labeled “21x23.5x5” but actually measures 23.2 x 20.1 x 5 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 Trane's official product data sheet, corroborated by current resellers. 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.