Eighteen-inch protection means a much heavier belt

How much armor might a treaty-era capital ship need to resist 18-inch rather than 16-inch gunfire?

More than fifteen inches in the main protected areas, compared with roughly thirteen to fourteen inches against 15-inch fire. Existing armor would require extensive work.

Protection against 15-inch gunfire is estimated at roughly thirteen to fourteen inches, while an 18-inch threat pushes the requirement beyond fifteen inches. That extra thickness must be carried through the protected scheme rather than added as an isolated plate. The resulting weight could force larger hulls or tighter compromises. Existing capital ships would need extensive work to meet the heavier standard.