Calculating Spell DC

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10+Spell Level+Relevant Ability Modifier before relevant feats and class abilities.

Also can be found here:

http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#savingThrow

or

Spell: If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you’re casting.

If the spell interferes with you or distracts you in some other way, the DC is the spell’s saving throw DC + the level of the spell you’re casting. For a spell with no saving throw, it’s the DC that the spell’s saving throw would have if a save were allowed (10 + spell level + caster’s ability score).

https://naggaroth.daerma.com/prd/coreRulebook/magic.html

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Metamagic feats that increase the DC of the spell include Cooperative Spell, Fey Power, Heighten Spell, and Improved Heighten Spell. Other ways to increase the DC of the spell include Spell Focus, Greater Spell Focus, school specialization, and leveling up your character.

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Spell Enhancer
(Magic of Faerun)

Transmutation
Level: Sorcerer 4, Wizard 4,
Components: V,
Casting Time: See text
Range: Personal
Effect: One of your spells
Duration: 1 round

This spell enhances the next spell you cast, making it more difficult for targets to resist.
Casting spell enhancer is a free action, allowing you to cast the spell to be enhanced in the same round as this spell.
The saving throw DC of the enhanced spell is increased by +2.

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Wand Mastery
( Eberron Campaign Setting, p. 62)

[Item Creation]

Wands are far more potent in your hands.

Prerequisite
Craft Wand (PH) , caster level 9th,

Benefit
When you use a wand, the DC of saving throws against the wand’s effect is increased by 2 and the wand’s effective caster level is increased by 2.

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Heighten Spell [Metamagic]

Benefit
A heightened spell has a higher spell level than normal (up to a maximum of 9th level). Unlike other metamagic feats, Heighten Spell actually increases the effective level of the spell that it modifies. All effects dependent on spell level (such as saving throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level. The heightened spell is as difficult to prepare and cast as a spell of its effective level.

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Spellcasting Prodigy
( Forgotten Realms Campaign Setting, p. 38)

[General]

You have an exceptional gift for magic.

Prerequisite
Benefit
For the purpose of determining bonus spells and the saving throw DCs of spells you cast, treat your primary spellcasting ability score (Charisma for bards and sorcerers, Wisdom for divine spellcasters, Intelligence for wizards) as 2 points higher than its actual value. If you have more than one spellcasting class, the bonus applies to only one of those classes.

Special
You may only take this feat as a 1st-level character. If you take this feat more than once (for example, if you are a human , or another type of creature that gets more than one feat at 1st level), it applies to a different spellcasting class each time. You can take this feat even if you don’t have any spellcasting classes yet.

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Greater Spell Focus
( Player’s Handbook v.3.5, p. 94)

[General]

Choose a school of magic to which you already have applied the Spell Focus feat. Your spells of that school are now even more potent than before.

Prerequisite
Spell Focus (PH) ,

Benefit
Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select. This bonus stacks with the bonus from Spell Focus.

Special
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic to which you already have applied the Spell Focus feat.

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Owl’s Insight
(Spell Compendium, p. 152)

Transmutation
Level: Druid 5,
Components: V, S,
Casting Time: 1 standard action
Range: Touch
Target: Creature touched
Duration: 1 hour
Saving Throw: Fortitude negates (harmless)
Spell Resistance: Yes (harmless)

As you cast this spell, your face is overlaid with the image of a snow owl. The image fades as you bestow your ally with comprehension.

The subject gains an insight bonus to Wisdom equal to 1/2 your caster level.