How much fabric should I buy!

I am starting to make a couple of new outfits and just cant seem to decide the amount of fabric I should buy.  I don't want to buy too little and then not have enough to fill orders.  but I don't want to have an abundance of left overs.  I suppose I could just sell what I don't use???  Oh what to do???  Just one of the little dilemmas that go in circles in my head:)  I guess I will just get too much to be safe!