From Jeffery Moore:
BCTW is a bitrate calculator that actually works. I had no luck with other ones I found on the web, so I made this one. It should work great when encoding DivX, XviD, MPEG-1, or MPEG-2 (SVCD or DVD). I mainly use it for SVCD's, so you might see some warning messages relating to the SVCD format. If you're not making an SVCD, simply ignore them.