Deck armour covers a much broader area than a belt, while a belt protects only part of the hull. The Revenge, Nelson, and G3 layouts connect armour choices with magazines, boilers, turbines, heat isolation, shaft arrangements, and the weight available for changes.
Chapter · 3:18:38-3:25:01
Deck armour, belt armour, and the protected citadel
Armour thickness alone says little without understanding the area protected and the machinery and magazine layout beneath it.