How Much Concrete Do I Need?

Step-by-step method for sizing a concrete order in cubic yards, 60 lb bags, and 80 lb bags for slabs, footings, and driveways.

Cubic yards = (length ft x width ft x thickness in) / 324. A 12 x 12 patio at 4 inches thick is (12 x 12 x 4) / 324 = 1.78 cubic yards. Add 5 to 10 percent for waste and rounding, and round up to the nearest quarter yard for a ready-mix order.

Open the concrete volume calculator →

The formula

Volume in cubic yards = (length ft x width ft x thickness in) / 324. Same formula for a slab, patio, or driveway. Footings use the trench cross-section instead of a slab area.

Do not forget the depth thickeners at slab edges. A 4 inch slab with a 12 inch turn-down at the perimeter needs another 5 to 8 percent volume depending on slab size.

Worked example: 20 x 20 slab at 4 inches

(20 x 20 x 4) / 324 = 4.94 cubic yards. Add 5 percent waste = 5.19. Order 5.25 cubic yards from a ready-mix supplier, or use the concrete volume calculator which does the rounding automatically.

Footings and trenches

For a continuous footing, multiply length x trench width x trench depth in feet and divide by 27 for cubic yards. A 60 ft x 16 in wide x 12 in deep footing is 60 x 1.33 x 1 / 27 = 2.96 cubic yards.

Bags or ready-mix

Under 1/2 cubic yard: use 60 lb or 80 lb premix bags. Over 1 cubic yard: ready-mix is almost always cheaper even with the short-load fee, and produces a single continuous pour without cold joints.

Ready to run the numbers?

Use the concrete volume calculator to get a specific quantity for your project.