[RPG] How to calculate save DCs for monster abilities


While reading up on the Bog Nixie (Bestiary 3, page 201), I noticed it mentioned that it had the harpy's Captivating Song DC and that the save DC was "Charisma-based."

My question, then, is how do I calculate the save DC of a monster's abilities?

The harpy's save DC, for example, is 16. With 17 Charisma, it should have a +3 Charisma bonus. My best guess, then, is that the formula is DC = 10 + 2×(Stat Modifier).

Best Answer

Glad you could reverse-engineer the save DC calculation. It is actually described in the SRD - and in several other places. Kept that one as an example.

Step 8: Special Abilities and Qualities

Most special abilities that cause damage, such as breath weapons, give a save (Fortitude, Reflex, or Will depending on the ability). The DC for almost all special abilities is equal to 10 + 1/2 the creature’s Hit Dice + a relevant ability modifier (usually Constitution or Charisma depending on the ability). Special abilities that add to melee and ranged attacks generally do not allow a save, as they rely on the attacks hitting to be useful.

So a special attack that doesn't already rely on a hit roll will have a save DC. Some special attacks that are particularly devastating might have both hit rolls and saving throw, but those are rare.

As Geoffrey points out in his answer, some spell-like abilities that emulate published spells don't use the 1/2 HD to calculate the save DC, instead using the spell level of the emulated spell for the DC calculation. Some unique (Sp) abilities still use 1/2 HD though.

Finally, Supernatural (SU) abilities ignore spell resistance, but spell-like (SP) abilities usually need a caster level (almost aways equal to HD, unless otherwise specified) to beat SR.