Concrete Calculator
Cubic yards of concrete for a slab, footing or post hole — plus how many 80 lb bags that works out to.
Concrete is ordered in cubic yards and billed for what leaves the truck, not what lands in the forms. A slab that comes up 0.2 yards short means a cold joint, a second delivery fee, and a visible seam in the middle of your patio forever.
The 5% default allowance here covers uneven subgrade and minor form bowing. Add more if the ground is soft, if you are pouring against soil rather than forms on both sides, or if the slab is thick enough to need a truck chute extension.
How this calculator works
Every field is editable and the result updates as you type — nothing is submitted anywhere, all the maths runs in your browser. That matters for a number you are about to spend money on: you can see exactly which input moved the answer, and by how much.
What this calculation assumes
Assumes a level, compacted subgrade with forms on all sides. Slabs poured against open soil, footings in trenches and pours over uncompacted fill all lose more mix than the default 5% allowance covers.
Treat the result as an order-of-magnitude check rather than a final quantity. Product packaging changes, regional labour rates vary by a factor of two, and no calculator has seen your actual site.
Frequently asked questions
How thick should a concrete slab be?
4 inches for a patio, walkway or shed floor. 5 to 6 inches for a driveway carrying vehicles, with rebar or mesh. Footings go deeper and follow local code — those are structural and should be sized by an engineer, not a calculator.
How many bags of concrete make a yard?
About 45 bags of 80 lb, or 60 bags of 60 lb, per cubic yard. Because bagged concrete costs several times more per yard than ready-mix, bags only make sense below about 1 cubic yard or where a truck cannot reach.
Should I add extra for spillage?
5% is enough for a slab poured into forms. Bump to 10% for pours onto open ground, for footings in trenches, or for anything where the subgrade is uncompacted fill that will absorb mix.
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