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

Question

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.

Answer 1

For creatures wearing mundane armor, I would convert their equipment armor bonus using the 5E Equipment Guide or as a quick estimate Multiply the Pathfinder value by 0.8 rounded down. (AC +10 platemail becomes AC +8 platemail, AC +2 Leather becomes AC +1 Leather )

Dexterity Bonuses should probably be about the same in both game systems. I quickly skimmed through some monsters and didn't seen any crazy high Dexterity scores.

For natural armor I would probably do something like divide by 3 and round down. So 17 Natural armor becomes +5 in 5E. I might also put a minimum of +1 for those with natural armor less than 3.

For other AC bonuses, such as dodge or deflection, I would probably ignore those entirely unless they are from magic items. In that case I would take the total AC bonus and divide it in half rounded up (so a +5 ring of protection becomes +3).

I think you should transfer size bonuses 1:1, but perhaps adjust if it seems too high/low.

I did some sample conversions, like a planetar angel from Pathfinder and Planetar in 5E and it seems to work nicely. For an Adult Red Dragon PF vs 5E Dragon it is too low, perhaps a Natural armor ratio of 50% would be better. I hope that helps!