Concrete Calculator (Slabs, Footings & Columns)
Calculate concrete for a slab, footing or round column in cubic yards and cubic metres — and how many 40, 60 or 80 lb bags you need.
How to use
- 1Choose a slab/footing or a round column.
- 2Enter the dimensions (feet and inches) and quantity; add a waste allowance.
How it's calculated
Slab volume = length × width × thickness. Column = π × r² × height. Cubic yards = cubic feet ÷ 27. Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³.
Frequently asked questions
How thick should a slab be?
4 inches is standard for patios and walkways; 5–6 inches for driveways and slabs carrying vehicles.
Bags or ready-mix?
Bags suit small jobs. Above about 1 cubic yard (~45 × 80 lb bags), ready-mix delivery is usually cheaper and easier.
How much extra should I order?
5–10% covers spillage, uneven ground and form flex.