I found out the hard way when I took it upon myself to start a new deck in the back yard, another "honey-do" project.  Anyhow, after I got the footings formed up and ready for concrete I measured the holes and did some math, eventually, I came to the realization that the amount of concrete I needed would be impossible to mix from the the bags.  So, I did a Google search for "ready mix concrete", I live in West Linn, and the nearest supplier was Lakeshore Concrete.  One phone call, some consultation, and the very next day I had concrete delivered to my address already mixed and configured for the specifications on the engineer's drawing for my deck.  The process from start to finish was very efficient, very affordable, and I strongly suggest others to call the crew at Lakeshore.  They are literally a life saver in the construction industry.  Thanks a million -  Juan