AC confusion

[Draig]

Alright I used to know this before but for some reason I can't remember the rules on ac and it being affected by rings of protection and natural armor and all that. So can someone please tell me what kinds of magic AC effecting items can stack and which cannot? And also if mage armor or shield can or cannot stack with it also? I apologize for the simplicity of this question just it has caused numerous debates in my group and I'd like to lay it out straight once and for all

[Drglenn]

As a rule of thumb: the same type bonuses do not stack with themselves but stack with each other.

So a guy with +1 fullplate, a +1 heavy shield, +1 ring of protection and +1 amulet of natural armour has AC 24 (+9 armour, +3 shield, +1 Deflection, +1 Natural Armour) touch 11 (+1 deflection) Flat footed 23 (+9 armour, +3 shield, +1 Natural Armour)

But if he has mage armour cast on him his AC stays the same as the 2 armour bonuses don't stack

[Keld Denar]

Bonuses with different names stack. (http://www.d20srd.org/srd/theBasics.htm#stacking)

Armor bonuses come from armor like breastplates and Mage Armor
Shield bonuses come from shields like large steel shields and the Shield spell
Natural armor bonuses from from natural armor
Deflection bonuses come from things like Rings of Protection

Note that some bonuses affect other bonuses. The +bonus from magical armor is an enhancement bonus to the ARMOR, not to your AC. Thus, +5 armor stacks with +5 shields, even though they are both enhancement bonuses, since they affect other base bonuses that stack.

There are other bonuses as well, things like Sacred bonuses, Luck bonuses, and such. Like named bonuses almost never stack.

[Telonius]

The exception is a Dodge bonus - those stack with everything.

Mage Armor provides an Armor bonus. Shield provides a Shield bonus. So yes, they stack with each other.

Do note that if the target already has an Armor bonus (from whatever armor they're wearing) or a shield bonus (from a shield they're carrying), the spells don't stack with the armor and/or shield. So if your Armor bonus to AC is less than 4, Mage Armor makes sense and will set your Armor bonus to 4. If it's 4 or above, Mage Armor has no effect. Same way with Shield.

Also remember that magic armor usually has an armor bonus plus an enhancement bonus. So if you have +4 Padded, you'd still benefit from a Mage Armor.

The normal AC would be: 10 + 1 (armor bonus from Padded) +4 (magic enhancement bonus)+dex = 15+dex.

With Mage Armor, it would be 10 + 4 (armor bonus from Mage Armor) + 4 (magic enhancement bonus)+dex = 18+dex.

[Allanimal]

To make it just a bit more complicated, from the SRD:

Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.

[Popertop]

[URL="http://www.d20srd.org/srd/theBasics.htm#stacking"]Like named bonuses almost never stack.

The only exception to this rule is dodge bonuses.

Edit: I got ninja'd. Circumstance bonuses will stack if they are from different circumstances.

So let's repeat it again class:
Like named bonuses almost never stack

[MeeposFire]

Most AC bonuses are typed. For instance armor provides an armor bonus. Shields provide a shield bonus. Rings of protection provides a deflection bonus. natural armor provides a natural armor bonus. These all stack (as do other bonuses like sacred, perfection, profane, and the like).

Some bonuses are called enhancement bonuses which means they enhance a bonus that you already have (which can be 0 to start in most cases). For instance let us say you have a chain shirt at 4 AC and you cast a level 1 magic vestment that provides an enhancement bonus of 1 on your chain shirt. You get a total of 5. Now if you also cast that same spell on your heavy shield (2AC) your shield is now giving you +3. Since your shield and armor are different types they stack giving you 4(+1 enhancement)+2(+1enhancement) which is 5+3=+8AC. Remember the enhancement bonus is applied to a base AC and it does not stack on the same base (for instance your armor +1 already has a +1 enhancement bonus. Casting magic vestment at level 1 on the armor would not change it since that also gives a +1 enhancement bonus though it would still work on the mundane shield).

Same goes with the necklace of natural armor. This also provides an enhancement bonus though this time it is to your natural armor bonus (which for most PCs is 0). So if you had a +1 necklace and you had 0 base ntural armor (since you are human for example) you would have a total of +1 natural armor (0+1=1) which would stack with the armor and shield (that both have separate enhancement bonuses which is what can be confusing) to give a total of 5 armor+3 shield+1 natural=+9AC.

Hope that helps.

[Drglenn]

also AC enhancment bonuses are generally to another type of bonus, the enhancment bonuses can stack on the differently named bonuses but not on the same bonus
e.g. in the example I gave above that amulet of natural armour gives a +1 enhancment bonus to his natural armour, if he gets a source of natural armour that's not an enhancment bonus (through having a race that grants NA for example) it'll stack with that and it still stacks with the +1 enhancment bonus to his armour.

Edit: ninja'd... 4 times:smallredface:

[Forged Fury]

Also remember that magic armor usually has an armor bonus plus an enhancement bonus. So if you have +4 Padded, you'd still benefit from a Mage Armor.

The normal AC would be: 10 + 1 (armor bonus from Padded) +4 (magic enhancement bonus)+dex = 15+dex.

With Mage Armor, it would be 10 + 4 (armor bonus from Mage Armor) + 4 (magic enhancement bonus)+dex = 18+dex.
Wait... what? Are you sure this works this way? My understanding was that the enhancement bonus of magic armor enhances the armor not the individual. Reading the description in the SRD for magic armor, I can see how it could be read that way, but it doesn't seem to be RAI.

[Yuki Akuma]

Wait... what? Are you sure this works this way? My understanding was that the enhancement bonus of magic armor enhances the armor not the individual. Reading the description in the SRD for magic armor, I can see how it could be read that way, but it doesn't seem to be RAI.

You're right, he's wrong. The enhancement bonus of a suit of armour enhances the suit of armour's armour bonus to AC.

So someone wearing +4 leather who gets mage armour cast on him doesn't recieve a change in his AC. He does have an AC four higher than before for the purposes of incorporeal 'touch' attacks, though.

[Person_Man]

FYI, if you ever write your own rules system, include the following sentence:

"All bonuses are named. For example, full plate provides a +8 Armor Bonus to your armor class, and flanking provides a +2 Flanking Bonus to your melee attack rolls. Bonuses of the same type never stack. Penalties and damage always stack, regardless of their source."

It would have made 3.5 a lot simpler.

[KillianHawkeye]

FYI, if you ever write your own rules system, include the following sentence:

"All bonuses are named. For example, full plate provides a +8 Armor Bonus to your armor class, and flanking provides a +2 Flanking Bonus to your melee attack rolls. Bonuses of the same type never stack. Penalties and damage always stack, regardless of their source."

It would have made 3.5 a lot simpler.

I don't know, I think the removal of Synergy Bonuses from 3.0 was a good move.