In D&D 3.5, I know that base attack bonus scales with level. How does AC increase over levels to meet it? I know saves do, so there should be some kind of AC thing? Is it done more with HP, or is there something I'm missing?
Other answers have explained that AC doesn’t automatically scale, which is true. There is no level component to determining AC.
Instead, AC is a function primarily of five factors:
- Armor, including magic enhancement bonuses
- Shields, including magic enhancement bonuses
- Rings of protection
- Amulets of natural armor
There are certainly other sources of AC—size bonuses, natural armor bonuses, and so on—but these are the common factors available to most characters.
So, what are we talking about with each of these?
Dexterity is obvious; everyone has a Dexterity score. Due to armor’s maximum Dexterity, however, the amount that Dexterity’s bonus to AC can actually scale is limited, and many characters will never improve upon whatever Dexterity they start with. It is certainly possible to attain extremely high Dexterity, and there are armors with very-high maximum Dexterity ratings (and at the extreme, gnomish twist cloth has no limit at all), but that isn’t most characters. So for most characters, Dexterity is not a factor in the scaling of AC.
Armor basically means a chain shirt or full-plate, the best light and heavy armors, respectively. The medium armors are poor; if you have medium armor proficiency and want better armor than a chain shirt, you should get mithral full-plate (wear a chain shirt until you can manage that). So your mundane armor, in most cases, offers either +4 or +8. The only real exceptions here would be those high-Dexterity characters, who have too much Dexterity to wear even a mithral chain shirt.
On top of the mundane +4 or +8, magic armor gains enhancement bonuses; all magic armors are required to start with a +1 enhancement, so really we are discussing +5 or +9. The enhancement bonus can scale up to +5, but gets very expensive.
There are roughly two varieties of shield worthy of consideration—buckler or and the hide shield from Sandstorm. Everything a light or heavy shield can do, a hide shield can do better, unless you actually wish to perform shield bashes. Tower shields are always a poor choice for literally every character, and in any event the best thing about them, the total cover trick, is something the hide shield can also do and without a special feat. The other “exotic” shields found in Races of Stone are really only considerations for a handful of classes who get free proficiency with them—none of them are worth a feat—so I won’t be considering those. Anyway, like armor, shields can gain enhancement bonuses, +1 to +5, and +1 is compulsory before any other magic can be added to the shield. Thus we are talking about +2 for a +1 buckler or +4 for a +1 hide shield as the “starting point” for shield usage.
Note that many melee characters should not use a shield at all at first—two-handed weapons have far too many advantages to use a shield. However, the animated property is not too terribly expensive, can allow such characters to have their cake and eat it too—but the cost definitely is there.
Also expensive are rings of protection and amulets of natural armor, both of which also scale from +1 to +5. Notably, a ring of protection provides a deflection bonus to AC—that is, it counts against touch attacks, which armor, shields, and natural armor do not.
For most characters, then, their starting Dexterity and their initial +1 armor and +1 shield (or +1 animated shield) provide a big chunk of AC early on. If we’re talking full-plate and hide shield, it’s an easy 25 (assuming at least 14 Dex) even at fairly low levels. After one has acquired those items, though, improving AC more quickly becomes very expensive, even compared to, say, mithral full-plate.
The only real options for further AC improvements are improving the enhancement bonuses on armor and shield, and purchasing a ring of protection and/or amulet of natural armor. Each of these four options vary from +1 to +5, and their costs each scale quadratically with that number. Due to the quadratic growth, it is best to spread the bonuses around—have a +1 armor, +1 shield, ring of protection +1, and amulet of natural armor +1 before you get +2 on anything, and then again +2 on everything before getting +3 on anything, and so on.
We could therefore work out some kind of progression, improving each of these items in a cycle as we get towards maxing out all four for +20 AC. It would have to be a bit fudged, since we would have to guess about other things a character invests their wealth in, but we could come up with something. However, I’m not going to do that. It would be a waste of time for one simple reason: you shouldn’t do that.
That is, buying a ring of protection, buying an amulet of natural armor, upgrading your armor and/or shield’s enhancement bonus, those are all very poor choices, because they simply cost far, far too much. There are vastly more important things to spend gold on, that provide far greater protection and cost much less.
How does AC scale in D&D 3.5?
After the earliest levels, it doesn’t
By default, AC does not automatically scale with level at all. The only way to increase it is by improving your Dexterity, upgrading your equipment, or using AC-boosting spells, class features, or other such effects.
Some of those effects may themselves scale (for instance, barkskin's AC bonus increases based on its caster level, and a monk's AC bonus increases based on their class level), but they will explicitly state if and how they do.
In standard D&D edition 3.5 or 3 it does not unless the character in question is a Monk that is unencumbered and unarmored. However there are examples presented in both the D20 Modern books and the D20 Star Wars books that you could efficiently adapt for a campaign setting that you want to have this effect. A setting like a stone age world where crafting armor is not a common skill could use this kind of system. However be careful with game balance. Some one with training in not getting hit is still at a disadvantage to someone who is prepared to be hit and is mitigating the damage of that hit with some type of armor. Remember AC isn't a calculation of the defenders ability to dodge its a calculation of their ability to not take damage while being attacked. To that end if you are going to have a level bonus to defense and armor, have armor do more than just deflect attacks; have it mitigate damage and deflect attacks. To do so divide the armor's AC by 2 make the higher resulting number the armor's AC and the lower number a mundane damage reduction (i.e.: Studded leather has an AC bonus of 3 as standard, with this variant it would have an AC bonus of 2 [which you then add the character's level defense bonus to] and a damage reduction of 1 that is by passed by anything but mundane attacks). You can find more detailed information for this variant rule set in the Unearthed Arcana book for 3.5 edition under the Variant Rules section.