macOS – Finder Not Showing Correct Free Space?

Check the Storage section of About This Mac. You will probably see the missing 15GB listed there as "Backups". These are Time Machine local snapshots, and Finder ignores them when calculating free disk space, because they get deleted automatically when your free disk space drops below 20%. In other words, they give up their space automatically as needed, so the number in Finder is the one you should pay attention to.

For others with similar problems, you don't need a special utility to find out where all your GB have gone on a Mac.

First, turn on invisible files. On newer Macs, you can do that with Command Shift Period. On older Macs, you do need a quick defaults terminal command.

Once you've done that, open your hard drive, and put it in to List View

Then go to the View menu and choose Show View Options Check Calculate all Sizes

Now wait. It can take a while because there are lots of sizes to add up, but eventually it will show you the sizes of every folder. You can use the disclosure triangles to open more and more folders and drill down to where all of your data is being used. I also usually sorts by the Size column so the big stuff is right at the top.

As a professional Mac consultant, this is the method I use to find people's data, I don't bother with any 3rd party utilities for this.