Free Concrete Calculator
Work out how much concrete to order for a slab, footing, column or post holes. Get cubic yards for a ready-mix order or the number of 40, 60 or 80 lb bags.
Concrete needed
How to use the concrete calculator
- Choose a slab or footing (rectangular) or a round column or post hole.
- Enter the dimensions. Length and width are in feet; thickness, diameter and depth are in inches, as they are usually measured.
- Enter how many identical pours you need (for example, 8 fence post holes).
- Keep a 5–10% waste allowance and read the result in cubic yards for ready-mix, or in bags for small jobs.
Ordering the right amount of concrete
Concrete is sold by volume. Ready-mix trucks deliver by the cubic yard (27 cubic feet), and most suppliers sell in quarter-yard increments, often with a short-load fee below a few yards. Bagged mix is easier for small jobs: an 80 lb bag makes about 0.6 cubic feet, so a single cubic yard takes 45 of them. Somewhere around one to two yards, ordering a truck usually becomes easier and cheaper than mixing bags by hand.
Always add a margin. Subgrades are never perfectly flat, forms bow a little, and some concrete is lost in the wheelbarrow and on tools. 5–10% is standard; running short in the middle of a pour creates a cold joint, which is weaker and looks worse than a little leftover. Typical thicknesses are 4 inches for patios and walkways, 5–6 inches for driveways, and deeper footings as specified by local code.
For post holes, a common rule is a hole three times the post's width and about a third to a half of the post's above-ground height in depth (below the frost line in cold climates). If the post itself sits in the hole, the concrete volume is a little less than the hole volume, so the calculator's answer includes a natural cushion.
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Frequently asked questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb mix per cubic yard.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks and shed floors. Driveways are usually 5–6 inches, and anything carrying heavy loads should follow an engineer or local code.
How do I convert cubic feet to cubic yards?
Divide by 27. A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards.
Should I round up?
Yes. Order to the next quarter yard (or next whole bag) after adding your waste allowance.