The Sagulator can be pretty handy when trying to make sure you do not have too much sag on a span.   It will take into account the material and thickness, span, width, whether it has an edge strip and its material and how the load is distributed and how much.  It will also take into account whether the ends are attached or floating as in adjustable shelves.    

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.