Does the iOS 8 permanently take up 5.9gs of data?

I have a iPhone 5s with iOS 7 and i have 16g data and I was wondering if the update takes up 5.9 gs data permanently or is that just needed to update it.

It does take up 5.9 GB of space but the old OS will be deleted once iOS 8 is installed, giving you a little over 3 GB. You will end up with roughly 3GB less space that way.

No, that's just the space you need to update. You get much of it back when the update is done.

Just do it on iTunes.