See Table: Common Armor, Weapon, and Shield Hardness and Hit Points
Item | Hardness | Hit Points
Armor | special2 | armor bonus ×5
note 2. Varies by material; see Table: Substance Hardness and Hit Points.
Substance Hardness Hit Points
Leather or hide 2 5/inch of thickness
Iron or steel 10 30/inch of thickness
Mithral 15 30/inch of thickness
Adamantine 20 40/inch of thickness
SO...a mithril chain shirt is AC4, it has hardness 15 and 20 hit points.
You should be able to figure out most armor using that formula...
Shields are detailed:
Item Hardness Hitpoints
Buckler 10 5
Light wooden shield 5 7
Heavy wooden shield 5 15
Light steel shield 10 10
Heavy steel shield 10 20
Tower shield 5 20