Here's a good-looking area-to-volume calculator that I made using the spreadsheet component from the free office suite, OpenOffice
For those of you who would rather keep Mr. Gates rolling in dough by paying for MS Office, it also works in excel.
Click here to get VolumeCalc.xls
The loss adjustment is handy when you're using irregular objects which settle after a trip in the dump truck. I use 103% to 105% adjustment for 1.5" river rock, for example.