Calculation of the AC

[WinterIsComing]

I would like to check with you the calculation of the AC of a mage
Example : The mage has a dexterity of 16 ==> + 3 ac
He casts an armor ==> + 4 ac
He casts an metamorphosis into a stone giant ==> + 11 ac (natural) - 1 (size)
He has a ring + 2 AC (alteration)

Total : 10 + 3 + 4 + 10 + 2 = 29.

Is it right ?
Thank you

[Khedrac]

This looks to be the right answer, but there should be a few clarifications:

As you say, the base AC is 10 plus modifiers, and the important thing with modifiers is their type as most modifers don't stack with others of the same type ("dodge" bonuses being the main exception which do usually stack).

So, what you have here is:
10 - Base AC
- 1 - size
+11 - natural
+ 4 - armor (mage armor grants an "armor" bonus - the same as physical armor)
+ 3 - dexterity
+ 2 - deflection (the ring may be of the abjuration school, but it probably grats a "deflection" bonus)
=29 as you said.

They key point is that the -1 size penalty from the stone giant is separate to the +11 natural armor bonus.

Edit:
The most common confusion is when (non mages) start wearing magic armor and using magical shields - a "+1 leather armor" is leather armor (2 armor bonus) with a +1 "enhancement" bonus. Some people apply the enhancement bonus to their armor class so that it would stack with a mage armor cast as well to give +5 (the mage armor and the leather armor both provide "armor" bonuses so do not stack - use the higher); this is incorrect - the "enhancement bonus" applies to the item - increasing the leather armor's "armor" bonus from " to 3 and thus would not stack with mage armor.

[VictoriousLoL]

Math seems to be correct!

[tyckspoon]

I would like to check with you the calculation of the AC of a mage
Example : The mage has a dexterity of 16 ==> + 3 ac
He casts an armor ==> + 4 ac
He casts an metamorphosis into a stone giant ==> + 11 ac (natural) - 1 (size)
He has a ring + 2 AC (alteration)

Total : 10 + 3 + 4 + 10 + 2 = 29.

Is it right ?
Thank you

Almost right - shapechanging into things usually means you take on their physical stats, so you'd be using the Stone Giant's Dexterity instead of your own. The standard Stone Giant is Dex 14 for a +2 modifier.