How to calculate the amount of space that iCloud Photo Library would take up? I recently got the OS X 10.10.3 update, which got me Photos for OS X and the opportunity to use iCloud Photo Library. However, I'm a little worried about maxing out my iCloud storage if I upload every photo on every device I own, since I currently only have the free 5GB and I'm already cutting it kind of close.

I was wondering, is there a way to figure out how much iCloud storage my combined photo libraries (MBP, iPhone, and iPad) would eat up before turning on iCloud Photo Library and learning the hard way?
Yes. You can start Photos app and turn off iCloud backed storage. 



You can see that the Photos Library is not my System Photo Library

Then you can import all the photos you want locally and observe how much space it takes. I'm using DaisyDisk, but Finder can show you folder size as well. I have a small system library that I'm using to test out the cloud side of things (15k photos) while I do the heavy lifting on non-system Photo libraries.

Once I'm happy with how albums and such work, I'll probably unify everything so that it fits within 200 GB of space that I'm budgeting for within iCloud.