 The ships' armor scheme was only slightly modified from the Mogami-class cruisers. Their waterline armoured belt extended all the way down to the double bottom. It extended from the forward to the rear magazines below the fore and aft turrets and was angled inwards at the top 20°from the vertical to improve its resistance to horizontal shellfire. Over the machinery spaces, it was 100 millimeters (3.9 in) thick at the top and tapered to 30 millimeters (1.2 in) at the bottom. The outer ends of the fore and aft machinery compartments was protected by a 105-millimeter (4.1 in) transverse bulkhead. On the sides of the magazines, the belt was 140 millimeters (5.5 in) thick and tapered to 30 mm at the bottom. The magazines were protected by fore and aft transverse bulkheads 95 – 140 millimeters (3.7 – 5.5 in) thick. The steering gear and the rudder compartments had sides that consisted of 100-millimeter (3.9 in) plates and their ends were protected by 50 millimeters (2.0 in) of armor.
