Armour thickness is only a rough guide to shell resistance

Can armour resistance be estimated directly from the calibre of the gun it is meant to defeat?

Only roughly: side armour near the shell calibre and deck armour near half that figure offer an initial estimate, with considerable variation from steel quality and metallurgy.

Armour thickness near a gun's calibre offers a rough initial guide to side protection, with about half that thickness suggested for deck armour. Steel quality and purity can alter the result substantially. The comparative judgment here places British armour ahead of several European rivals in the interwar years, with American metallurgy catching up during the 1940s. Continued British shipbuilding is offered as an explanation for greater experimentation and investment. These are approximate comparisons with considerable room for variation.