So I've added weights for all of our products into salesforce.  For each product line item there is a Quantity, and custom field "Weight" (multiplies the # of an item by its single unit weight...to calculate total weight for the item) i.e.   (2)  Beds  x  (100)  = 200 for Weight of line item "Beds".   I would like to add a field under the Opportunities Page layout that would calculate the total weight for all items...this would save our sales staff a few steps when calculating ship quotes. 

I have tried to create a formula for this and am struggling to find one that will work.  Can anyone help?  How do I do this?