Try one of these:

http://www.awc.org/calculators/span/...eversecalc.asp

http://www.awc.org/calculators/span/...rcalcstyle.asp

http://www.woodbin.com/calcs/sagulator.htm

If I read your post correctly you have a 2x12 being used as a shelf.
If this is correct, probably none of thes will give you what you want
to know but may point you in the right direction.