Defense is the number that determines how much damage can be absorbed per hit. Damage the player takes is reduced by half their defense value, rounded up.
Defense (or Defense Rating) gives an arithmetic reduction in damage: i.e. reduces damage by a fixed amount per hit (as opposed to a geometric damage reduction, which reduces damage by a percentage).
The formula is:
Damage Reduction = ROUND_UP(Defense * 0.5)
For example, with 10 (or 9) Defense, a 20 Damage attack is reduced by 5, to 15 Damage.
Because of this, high Defense is particularly effective against low damage attacks, which can be reduced almost completely, but not as effective against high damage attacks, where the reduction is just flat. This also means that added defense gets increasingly more important. A jump from 10 to 20 defense has smaller benefits on average than a jump from 30 to 40 defense.
Monsters have the same Defense mechanics as players. Monsters with high defense will take virtually no damage from low damage weapons (e.g. Fire Imps take 5 damage from the Minishark with Musket Balls). Accordingly when fighting high defense monsters, high damage weapons are preferable.
 Armor and Accessory Defense
Here is a list of armors, accessories, and their respective defense bonuses, including any applicable set bonuses. As of the 1.1 update the highest possible defense bonus obtainable is 85. Hallowed armor gives 50 defense, you can then equip an Obsidian Shield (+2), Obsidian Skull (+1), Cobalt Shield (+1), Moon Charm (+1 during a full moon) Shackle (+1) to provide a defense of 56. If all these accessories have the warding attribute, that provides an additional 20 defense, totaling 76 defense. Ironskin and bowl of soup then give an additional 9 giving 85 total defense.
|Cobalt armor with Hat||17|
|Cobalt armor with Helmet||26|
|Cobalt armor with Mask||19|
|Mythril armor with Hood||24|
|Mythril armor with Helmet||37|
|Mythril armor with Hat||27|
|Adamantite armor with Headgear||28|
|Adamantite armor with Helmet||46|
|Adamantite armor with Mask||32|
|Hallowed armor with Headgear||31|
|Hallowed armor with Helmet||35|
|Hallowed armor with Mask||50|
|Ironskin Potion||8 (for 5 minutes)|
|Bowl of Soup||1 (for 10 minutes)|
|Ale||-4 (for 2 minutes)|
|Goblin Tinkerer's / random Modifiers||4 (Max per accessory) 20 total|
- No attack can be reduced to less than 1 Damage.
- Defense applies to all forms of damage, including, but not limited to fall damage, gravity influenced blocks (sand and silt), Lava, spikes, meteorite, and hellstone.
- Modifiers on accessories can increase the defense of an accessory by up to 4 points.
|Attack Speed • Breath meter • Buffs • By hand • Consumable • Crafting station • Critical hit • Damage types • Data IDs|
Day and night cycle • Death • Debuffs • Defense • Difficulty • Environment • Fall Damage • Game controls • Ghost • Health • Inventory
Knockback • Lighting mode • Mana • Map Size • Modifier • Moon phase • Music • NPC drops • NPC Spawning • Placement
Player stats • Rarity • Shoot speed • Spawn • Storage items • Use time • Value