Q: Why Doesn't my storage space add up? And is it correct?

Do not use this display as it is unreliable unless you have just re-indexed the disk for Spotlight. Spotlight- How to re-index folders or volumes - Apple Support. If you need to know disk space then select the disk's Desktop icon, then Press COMMAND-I to open the Get Info window. It's displayed in the top panel: Capacity, Used, and Available. Also, see:

Freeing Up Space on The Hard Drive

  1. See Lion/Mountain Lion/Mavericks' Storage Display.

  2. You can remove data from your Home folder except for the /Home/Library/ folder.

  3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.

  4. Also see Freeing space on your Mac OS X startup disk.

  5. See Where did my Disk Space go?.

  6. See The Storage Display.

You must Empty the Trash in order to recover the space they occupied on the hard drive.

You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.

Try using OmniDiskSweeper 1.8 or GrandPerspective to search your drive for large files and where they are located.

If that's what you see via the Get Info dialog.