Concrete Calculator
Choose a slab, a round footing or post hole, or a square footing. You get cubic yards for a ready-mix order and bag counts for mixing it yourself.
How this calculator works
- Slab volume = length × width × thickness (inches ÷ 12). Round = π × radius² × depth. Square = side² × depth.
- Cubic yards = cubic feet ÷ 27. Bag yields are the standard figures printed by makers: 0.30, 0.45 and 0.60 cu ft for 40, 60 and 80 lb bags.
Concrete questions
How many 80 lb bags make a yard of concrete?
About 45. One cubic yard is 27 cu ft and an 80 lb bag yields about 0.60 cu ft.
How thick should a patio slab be?
4 inches is standard for patios and walks; 5 to 6 inches for driveways or where vehicles park, with reinforcement per local practice.
How much concrete for a post hole?
A 12 inch diameter hole 42 inches deep holds about 2.75 cu ft, roughly five 80 lb bags before waste, less the volume of the post.