When it comes to combat in any game, you typically want to hit with lots of attacks without getting hit yourself. Whether you want to dance over swords while singing “Can’t Touch This”, simply have arrows bounce off your rippling muscles, or stop a blade with your gleaming plate mail, in dungeons and dragons, we have something called Armor Class -often shortened to AC – which helps you do this very thing.
Now figuring out how to calculate AC is a good skill to have, and is very easy! For any unarmored character/creature, just start with 10, and add the Dexterity Modifier, alongside any other bonuses. Simple! Armor is a little more tricky, but for light armor and the mage armor spell, they simply change the 10 in that formula, medium armor however only adds up to +2 to your AC from Dexterity (+3 if you have the Medium Armor Master feat), and Heavy armor just ignores Dexterity altogether, just be careful of Strength Requirements. If you don’t meet the Strength requirement listed for your chosen Heavy Armor, then your movement speed will be reduced by 10ft unless you’re a Dwarf.
Of course there are features such as the Barbarian’s and Monk’s Unarmored Defense abilities that can also increase AC. However, such features typically tell you how to calculate AC. There are some cases that just give you a flat AC number to use, such as the Tortle’s natural AC, or the Barkskin spell. Other than that, you really only need to keep track of magic items, magical effects, shields, and other bonuses to AC.
Tip: AC calculations don’t stack! If you have two different ways of determining your AC, for example a Tortle Barbarian, you would need to choose between the Tortle’s fixed 17 AC, or the Barbarians Unarmored Defense formula. Bonuses to AC like shields and magic items will still apply unless the AC giving feature says otherwise.
Now that you know, I see a high AC build in your future. Good Luck, and don’t die!