Aside from the methods mentioned here, you can also use the application Grand Perspective to track down exactly what is stored where on your computer, using a graphical interface. Has helped me quite a bit in 'missing hard drive space' situations.

http://grandperspectiv.sourceforge.net/

Your calculator needs new batteries:

3.5G /Applications
4.6G /Library
5.0G /private
2.3G /System
86G /Users
468M /usr
128G /Volumes

= 229 Gb, plus the small stuff plus base 10 / base 2 issues equals about 231 Gb

The final number is not drive capacity, it's the sum of whatever du finds. It also includes files in the searched folder but single files are not listed separately.

Try:

df -h

I think what you are missing is that the "-x" option to 'du' makes it exclude external drives, so that 128 GB in /Volumes is (almost certainly) detritus from some previous incident when a mounted drive was not properly unmounted.

First unmount and physically disconnect all external drives and network drives.
(To be really safe, disconnect all network cables and turn off Airport.)
Then use the menu item "Go to Folder..." from Finder's "Go" menu, and then type in "/Volumes" (without the quotes).

Look at the contents of the /Volumes folder.
If you see anything other than the one entry for your internal drive, that is likely the detritus I spoke of above.

Use Finder to trash the left over folders from /Volumes. (If you are sure that no external disks or network drives are connected then this can't affect what is on the external disks.) Finally, empty the Trash.

I saw that too, but until we know the details (such as whch OS version, and correct sizes of all volumes) I'm assuming that it's adding up an external drive.