Concrete Calculator
Volume, bag count and live cost for slabs & footings.
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80 lb · 117 bagsThese are example numbers. Tap any field below and enter your own measurements — every result updates instantly as you type.
Extra ordered for spillage and uneven subgrade — folded into every total below.
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Join the waitlistBag yields use manufacturer averages (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³). Order a full bag extra for safety.
How the concrete calculator works
The concrete calculator finds how much concrete a slab, footing, pad or round column needs. For slabs it multiplies length, width and depth to get volume in cubic feet; for round footings and sonotubes it uses pi × radius squared × height. Either way it converts to cubic yards — the unit ready-mix is ordered in — by dividing by 27.
You can mix units freely: enter a slab as 20 feet long and 4 inches deep without converting anything by hand. Add a waste factor (5 to 10 percent is typical) to cover spillage and uneven subgrade, and it is folded into both the yardage and the bag count. Enter a price per cubic yard or per bag for a live cost estimate.
Guides
- How many bags of concrete are in a yard? — the conversion every job comes back to, for 40, 60 and 80 lb bags.
- How many bags of concrete for an 8×10 slab? — 80 sq ft slab, 4-inch standard with a 6-inch comparison.
- How many bags of concrete for a 10×10 slab? — 4-inch standard, with the math and a 3/4/6-inch comparison.
- How many bags of concrete for a 12×12 slab? — 144 sq ft slab in 60 and 80 lb bag counts.
- How many bags of concrete for a 12×16 slab? — 192 sq ft slab in bags and yards.
- How many bags of concrete for a 16×20 slab? — 320 sq ft slab — where bags stop making sense vs. a truck.
- How many bags of concrete for a 20×20 slab? — 400 sq ft slab and the bag-vs-ready-mix tipping point.
- How many bags of concrete for a driveway? — yards and bags for typical residential driveway sizes.
- How many bags of concrete for a hot tub pad? — pad size and depth for the soak load, with the bag math.
- How many bags of concrete for a sonotube? — round column volume for common 8, 10 and 12 inch tubes.
- How many bags of concrete for a 4×4 post? — post-hole math for a mailbox, fence or lamp post anchor.
- How many bags of concrete for a fence post? — cylinder math for round post holes, by hole size.
Concrete calculator FAQ
How many 80 lb bags of concrete are in a cubic yard?+
An 80-pound bag of concrete mix yields about 0.6 cubic feet. A cubic yard is 27 cubic feet, so that works out to roughly 45 bags of 80 lb mix per cubic yard. A 60-pound bag yields about 0.45 cubic feet — about 60 bags per cubic yard.
How do I calculate concrete for a slab?+
Multiply the slab length by its width by its thickness to get the volume, then divide cubic feet by 27 to get cubic yards. BuildCalc lets you mix units, so a 10 by 10 foot slab at 4 inches thick is entered directly — about 1.23 cubic yards before waste.
How many cubic feet are in a cubic yard?+
There are 27 cubic feet in one cubic yard (3 feet by 3 feet by 3 feet). Ready-mix concrete is ordered by the cubic yard.
What waste factor should I use for concrete?+
A waste factor of 5 to 10 percent is standard. It covers spillage, over-excavation and uneven subgrade. For rough or hand-dug footings, lean toward 10 percent. BuildCalc folds the waste factor into both the yardage and the bag count.
Should I use 60 lb or 80 lb concrete bags?+
80-pound bags mean fewer bags to mix and carry, so they suit larger pours. 60-pound bags are lighter and easier for small jobs and repairs. For anything over about half a cubic yard, ready-mix delivery is usually cheaper than bags.
How do I estimate concrete for multiple footings?+
Enter one footing’s dimensions, then set the pour count to the number of identical footings. The calculator multiplies the volume, bag count and cost across all of them at once.
How do I calculate concrete for a sonotube or round column?+
Switch the shape to Round Column and enter the diameter and column height. BuildCalc uses the cylinder volume — pi × radius squared × height — and converts to cubic yards and bag count just like a slab. Common sonotube diameters are 8, 10 and 12 inches; a four-foot-deep 12-inch footing takes about 0.12 cubic yards of concrete, or roughly 6 bags of 80 lb mix.