How much space does an iphone backup take?

I was curious how much free space is needed for an iPhone backup made from iTunes? When I try to backup I get an error: "iTunes could not back up the iPhone because there is not enough free space on this computer to save the backup" but I have 95GB free on my PC. I have a 64GB iPhone 6 that has ~50% free space if that makes a difference and I have tried updating to the latest iOS and iTunes versions as well as everything listed here:

If your iTunes backup couldn't be completed or you can't restore from a backup - Apple Support