Quikcrete has a calculator on their website based on sq.ft., 4" or 6" slab, 60# or 80# bags:
http://www.quikrete.com/Calculator/Main.asp
Here is another calculator:
http://www.concretenetwork.com/concrete/howmuch/calculator.htm
Food for thought if doing it on your own. 4.5 cu yds. = 121.5...