Apple - Determining The Complete Size Of A Folder & Its Sub-Folders in Finder?

In Finder:

1. Either:
   1. Option + click a folder.
   2. Or: Click the gear icon in the header.
   3. Or: Select the View menu, and then "Show View Options".
   4. Or: Command + J
2. Select "Show View Options"
3. In the 4th subset of options select the box "Calculate all sizes".

If a folder is empty it will only show "--" like all the folders previously did.