I would order 4.75 yards. Concrete ready mix has a usual "minimum" one load price. most trucks carry 8 or 10 yds. 4.75 is WAYYY more than you need. However, given same price for 4 yds or 5 yards or 6 yards. I always order way more, being short will cost you double. If you don't need it, it doesn't cost not to use it, and the concrete company will take it away free. I can usually find a way to use the left over anway. Good Luck!

