How to calculate it yourself
Bags needed = total cubic feet ÷ bag yield. For a 4-inch slab: bags = (length × width) ÷ (yield × 3). Equivalent in bags-per-yard: 45 × 80 lb bags per yd³, or 60 × 60 lb bags per yd³.
Common scenarios
4×4 ft × 4 in pad
5.3 ft³ of concrete needed. ÷ 0.6 ft³/bag = 9 bags of 80 lb. Add 10% waste = 10 bags.
Sonotube post hole (12 in × 4 ft deep)
3.1 ft³ ÷ 0.6 = ~5 bags of 80 lb per pier. For a 6-pier deck: 30 bags.
Driveway repair section (6×6 ft × 4 in)
12 ft³ ÷ 0.6 = 20 bags of 80 lb. Order 22 bags for waste; one pallet half is plenty.