How to convert armour class from Pathfinder 1 to D&D 5?


My sandbox game uses some material from Pathfinder (and D&D 3 and 3.5). I run games there using D&D 5 (among other rules systems). How do I convert the armour class of a creature (especially one with strong natural armour), given that…

  1. I am not interested in maintaining challenge rating or game balance. (It is a sandbox setting anyways; there is no reason for me to care about how difficult something is.) A formula that makes no use of challenge ratings, or other non-diegetic information, is preferable.
  2. I do want to maintain the nature of the creature; if one is well-armoured according to one rules system, it should remain so in the other. Likewise for weak armour or excellent armour.
  3. It is fine if a converted monster has different statistics than the monster as originally designed for D&D 5. However, if the monsters are conceptually similar, than their armour class should be similar, as per point two.
  4. Pathfinder allows significant stacking of different types of armour. D&D 5 does not. This already solves the problem for some creatures that combine natural armour with a manufactured one; using the better is often a workable solution.
  5. A solution should cover the common cases of creatures endowed with natural armour. There are always edge cases like angels with charisma as a deflection bonus, but they can be handled on a case-by-case basis after getting a handle of the general principles.

Here is what is easy to do:

  • If a creature is mostly protected by armour, use the typical D&D 5 rules for that armour.
  • If a creature does not have armour but relies on speed, use its dexterity to determine armour class.
  • If a creature’s level of protection corresponds with that of an existing creature in D&D 5, one can simple use the same armour class. This requires the correspondence and a good working knowledge of the published monsters in D&D 5, and furthermore assumes consistency from those monsters. A method that also works for more exotic creature and does not require encyclopediac knowledge of D&D 5 bestiaries would be preferable.

In Pathfinder many monsters have quite significant natural armour bonuses. Using these as is is not reasonable; natural armour bonuses of +10 are common in Pathfinder and so are even higher bonuses, whereas armour class of 20 is quite good in D&D 5 and higher numbers are quite rare.

The main issue seems to be some kind of conversion formula that relates high natural armour in Pathfinder to armour class in D&D 5.