How to calculate pounds to bushels?

A bushel is a measure of volume, not weight.

True - but every volume of something has a weight. You just need to know what product you're measuring. There are some handy tables that I ran across for some common grains and fruits & veggies. Check out http://www.alpharubicon.com/primitive/bushelssitkastan.htm and see if what you're looking for is listed!