Instead of trying to find an online calculator, here's a file I made that you can download to convert back and forth yourself.
http://dl.dropbox.com/u/24438380/Bac...onversions.xls
If you know the wheel width and backspace, it will calculate the offset.
If you know the wheel width and offset, it will calculate the backspace.
Let me know if you have any questions
**My recommendation, always convert to backspace. It's easier to compare how much further wheels will "stick out" if you compare via width & backspace because its easier to visualize the relationship.
Edit: updated the file so you guys can see how much further out certain wheels will sit