How many bags of concrete do I need?
Divide your volume by the yield per bag. Standard yields: a 40 lb bag makes 0.30 cu ft, a 60 lb bag 0.45 cu ft, and an 80 lb bag 0.60 cu ft. One cubic yard (27 cu ft) needs 90 x 40 lb bags, 60 x 60 lb bags, or 45 x 80 lb bags.
For anything over about 1 cubic yard, ready-mix delivery is usually cheaper than bags and far less work. Under that, bags are the practical choice for posts, small footings and repairs — just add 10% for spillage and uneven holes.
Working out a slab or footing from scratch? Use the concrete calculator.