[Berlin_Freeper]

iOS 8 is a HUGE update though (and not just in terms of features), demanding anywhere between 5GB to 7GB of space aboard your iPhone or iPad, meaning those with 8GB or 16GB devices need to think very carefully before downloading, as you’ll need to free up A LOT of space.

[DeFault User]

The article has a suggested remedy:

Since it's the install package itself that gobbles up so much space, the easiest way to get around the issues is to plug your phone or tablet into your computer and update straight from iTunes. That way, your phone won't get stuck with having to bear the update's weight and you should have more than enough room for the measly 1.1 GB of iOS 8 itself. Simple!

[ReignOfError]

It takes a lot of space to shuffle files around if you opt for the over-the-air update, but that’s not the permanent footprint. I did the update by connecting to iTunes on the desktop; on my 16GB iPhone, I had 1.5GB free before the update, 1.2GB after.