The ships would have been protected by a waterline belt of armor thick, sloped 15° outwards at the top to increase its ability to resist penetration at short range. The belt armor was designed to be able to defeat shells from a distance of . The main battery turrets and barbettes would have had between of armor plating, and the conning tower would have had been protected by armor thick. The decks would have been 120 mm thick. The "Kii"-class battleships would have had a torpedo bulkhead thick, which connected at the top to a splinter deck beneath the main deck.