How much concrete do I need?

Concrete is ordered by volume. Multiply length by width by thickness, keep the units consistent, and convert to cubic yards for a ready-mix order or to bags for a small job. The Concrete Calculator handles slabs, footings, and round post holes.

Slab

Length (ft) × width (ft) × thickness (in) ÷ 12 gives cubic feet. Divide by 27 for cubic yards. A 10 × 10 ft patio at 4 inches is 33.33 cu ft, or 1.23 cubic yards.

Bags

Pre-mixed bags list their yield on the label. Common figures are 0.60 cu ft for an 80 lb bag and 0.45 cu ft for a 60 lb bag. That same patio needs 56 bags of 80 lb mix or 75 bags of 60 lb mix. One cubic yard is 45 bags of 80 lb mix or 60 bags of 60 lb mix.

Post holes and columns

Volume is π × radius² × depth. Four holes 12 inches across and 4 feet deep hold about 12.57 cu ft, or 21 bags of 80 lb mix.

Add a margin

Many people add 5 to 10% for spillage and uneven ground. For structural work, follow local code and a qualified professional. If you are putting a gravel base under the slab, the Gravel Calculator covers that.