You determine your defense scores as follows.
Base Defense: All defenses start with 10 + one-half your level.
Armor Class: Add the armor bonus of the armor you wear and the shield bonus of the shield you carry. If you’re wearing light armor or no armor, also add your Dexterity modifier or Intelligence modifier, whichever is higher.
Fortitude: Add your Strength modifier or Constitution modifier, whichever is higher.
Reflex: Add your Dexterity modifier or Intelligence modifier, whichever is higher. If you’re using a shield, add its shield bonus.
Will: Add your Wisdom modifier or Charisma modifier, whichever is higher.
Also add any of the following that apply:
• Racial or feat bonuses
• An enhancement bonus (usually from a neck slot magic item)
• An item bonus
• A power bonus
• Untyped bonuses
Published in Player's Handbook, page(s) 275.