Recently, a reader of this blog (Kenny) who was looking to try out my rules asked the following question:

Hey there, I’ve been taking some time to read through the Player’s Handbook and get familiar with the system, it seems like the game I’ve been looking for for a while now. One question is have is on AC. Building a sample Fighter, I equipped them with chain mail. This has an AC bonus of +5. Their DEX modifier is +1. I’m finding this gives them an AC of 6. Is this correct, or is there something I’m missing that would add more to their AC? Thank you

I was embarrassed to discover that NOWHERE, in around 1000 pages of rules, did I manage to ever provide a description of how Armor Class is calculated… which is a big omission!

So, now, pages 4, 65, and 230 all provide details on how Armor Class is determined. I was so used the 3.X and d20-based D&D Armor Class calculations (10+armor+Dex+etc.) that I never even noticed my error.

Thank you, Kenny, for pointing that out! You’ve made my rules that much clearer!

From page 4:

Armor Class
Your Armor Class is equal to 10 plus any Armor Class Bonus you get from armor worn (see page 43) plus your Dexterity modifier (see page 5). Armor Class may also be modified by class abilities, spells, and magical items, as noted in their descriptions.

Depending on the armor your character wears, you might add some or all of your Dexterity modifier to your Armor Class, as described in the Equipment section on page 43. Encumbrance (see page 57) may also affect your Dexterity modifier to Armor Class.

From pages 65 and230:

Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on a combatant. It’s the attack roll result that an opponent needs to equal or exceed in order to hit that combatant.

Armor Class is equal to 10 plus any Armor Class Bonus a combatant gets from armor worn (see page 43) plus its Dexterity modifier (see pages 4 and 5). Armor Class may also be modified by class abilities, spells, and magical items, as noted in their descriptions.

The higher the AC, the less vulnerable the combatant is to melee and ranged attacks.