Estimate how much concrete you need for a slab, footing, wall or column. Enter the dimensions and get cubic yards and bag count instantly.
One cubic yard = 27 cubic feet, or about 45 bags of 80 lb concrete.
Concrete is always ordered by the cubic yard, but different projects measure differently. A slab uses length times width times thickness. A footing uses the trench's length, width and depth. A wall uses height times length times thickness. The formula is the same every time: multiply the three dimensions in feet, then divide by 27.
For a standard 4-inch slab, the thickness in feet is 4 ÷ 12 = 0.333. A 10 by 4 foot slab is 10 × 4 × 0.333 ÷ 27 = 0.49 cubic yards. Add about 10% to every estimate to cover uneven ground, form bowing and spillage.
If your project is not a simple rectangle, break it into rectangles, run each through the calculator, and add the results. Steps, curbs and irregular footings all break down into a few basic boxes.
Multiply length by width by thickness in feet, then divide by 27 to get cubic yards. Always add about 10% for waste and uneven ground.
A 10 by 10 foot slab at 4 inches thick is 10 × 10 × 0.333 ÷ 27 = 1.23 cubic yards, or about 56 bags of 80 lb concrete.
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches, and 54 square feet at 6 inches.
A cubic yard of ready-mix concrete typically costs $125 to $150 delivered, depending on the mix and your location. Bagged concrete runs more per yard.
For small jobs under about a yard, bags are easier. For a full slab or footing, ready-mix is cheaper per yard and pours faster. As a rule of thumb, switch to ready-mix past roughly 50 bags.