Free Concrete Calculator
Enter your slab, footing, or column dimensions to get cubic yards and exactly how many bags to buy, with a waste buffer built in.
This free concrete calculator converts slab, footing, and round column dimensions into cubic yards, then into bag counts for 80 lb (0.6 cu ft), 60 lb (0.45 cu ft), and 40 lb (0.3 cu ft) bags, with an adjustable waste percentage for spillage and uneven subgrade.
Covers spillage, uneven subgrade, and form spread. 10% is standard.
Planning estimates. Order 5 to 10 percent extra for spillage, and confirm structural mixes with your supplier or engineer.
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Concrete volume math, without the guesswork
Concrete is sold by the cubic yard: 27 cubic feet. The core calculation is simple, length times width times thickness in feet, divided by 27. A 10 by 10 foot slab at 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Everything else is unit conversion and a waste buffer.
Bag math depends on bag size because each size yields a different volume: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. Divide your total cubic feet by the per-bag yield and round up. That 33.3 cubic foot slab needs 56 eighty-pound bags.
Thickness is where DIY estimates go wrong. Four inches is the minimum for a patio or walkway, and it must be a true 4 inches everywhere; a subgrade that dips to 5 inches in spots adds concrete you did not plan for. Measure in several places and use the average, or just bump the waste factor.
Round columns use pi times radius squared times height. A 12 inch diameter sonotube 4 feet deep is 3.14 x 0.5 x 0.5 x 4 = 3.14 cubic feet, about a tenth of a yard, or 6 eighty-pound bags. Footings are just rectangular boxes: length times width times depth.
The breakpoint for calling a ready-mix truck is usually around 1 to 2 cubic yards. Below that, bags are cheaper; above it, the truck wins on both cost and your back. Many suppliers have a 1-yard minimum and short-load fees, so price both options near the boundary.
Concrete calculator questions
How many bags of concrete do I need for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. That is about 56 eighty-pound bags, 74 sixty-pound bags, or 111 forty-pound bags, before adding a waste buffer.
How many 80 lb bags of concrete are in a yard?
About 45. A cubic yard is 27 cubic feet, and an 80 lb bag yields roughly 0.6 cubic feet, so 27 / 0.6 = 45 bags per yard.
Should I get concrete delivered or mix bags?
Bagged concrete wins for small jobs under about 1 cubic yard. Above 2 yards a ready-mix truck is cheaper per yard and saves enormous labor. Between 1 and 2 yards, price both, watching for short-load fees.
How thick should a concrete slab be?
Use 4 inches minimum for patios, walkways, and sheds. Driveways and slabs carrying vehicles want 5 to 6 inches, usually with rebar or wire mesh.