Calculating Spell DC
10+Spell Level+Relevant Ability Modifier before relevant feats and class abilities.
Also can be found here:
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).
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.