From FFXI Wiki

Defense is compared to Attack to determine the Attack/Defense Ratio used in the pDIF equations. It is associated with the Vitality stat. Because of level correction, increasing defense is generaly not the best way to reduce damage taken against monster targets that are a higher level than the player. Decreasing defense will increase damage taken, but if the player's defense falls below 50% of the attacking monster's attack value, further reductions in defense will have no impact on damage taken by the player.

Base Defense (before Armor and buffs) follows different formulas depending what level you are:

  • Level 1-50: floor( VIT × 1.5 ) + Level + 8
  • Level 50-60: floor( VIT × 1.5 ) + ( 2 × Level ) - 42
  • Level 61-90: floor( VIT × 1.5 ) + Level + 18
  • Level 90-99: floor( VIT × 1.5 ) + Level + 18 + floor( ( Level - 89 ) ÷ 2 )

When fighting high level enemies that have been released after Odyssey Sheol C, it has become much more relevant to consider Defense as a priority for tanks once the DT caps have been reached due to new enemies having a higher Attack than in the past. Outside of that Defense holds a special place in the heart of Blue Mages, both for their easy access to massive defense buffs (like Cocoon or Harden Shell) and for those reminiscent of when Cannonball was a relevant endgame spell.

Pet: DEF from Pet: VIT

Pet's defense derived from their VIT is the same conversion rate as a player's:

Defense = floor( VIT × 1.5 )

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