Anyone who has ever shown up at the hardware store unsure whether to buy 40 bags or 60 knows the frustration of guessing concrete quantities. Buy too few and you're making a second trip mid-project. Buy too many and you're stuck storing — or returning — bags you didn't need. The Problem With Eyeballing Bag Counts Concrete bags come in different sizes (40 lb, 60 lb, 80 lb), and each yields a different volume once mixed. A 60 lb bag yields roughly 0.45 cubic feet, while an 80 lb bag yields about 0.60 cubic feet. Multiply that across a patio, walkway, or post-hole project, and small miscalculations add up fast — either in wasted money or wasted trips.