How are item effects applied?

Question

If I have a ring that gives -5% spell cost, an amulet that gives -10% spell cost, and robes that give -20% spell cost am I getting a full 35% off spell cost or are they all applied separately?

As an example, if a spell costs 100 magicka getting 35% off of that would leave it costing 65 magicka.

But if the effects are applied one after another the same 100 magicka spell would cost 20% off of 100 which is 80 then 10% off of that 80 leaves you with 72 and then applying the final 5% off of 72 leaves the final cost at 68.3.

Are item buffs added and then applied or applied separately?

Answer 1

The effects are added, and then applied. So if you were to enchant four items (head, neck, chest, ring) with a -25% Fortify Destruction enchant, you could cast any destruction spell without draining your magicka at all!

25% is the highest you can reduce the spell cost with a single enchant, here are some instructions on how to accomplish this:

http://www.reddit.com/r/skyrim/comments/mjulb/0_magicka_cost_spells_fully_explained/