Concrete Calculator
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Concrete is ordered by volume — cubic yards for ready-mix, cubic feet of yield for bags — but you measure a slab in feet and inches. This calculator does the conversion. It multiplies length by width by thickness to get the volume of the slab, then divides that volume by the yield of one bag to get a bag count, adding a waste allowance for spillage and uneven ground.
Enter each area's length and width in feet and its thickness in inches. Because thickness is in inches and the other two are in feet, the thickness is divided by 12 before multiplying. Dividing the result by 27 converts cubic feet to cubic yards, since a cubic yard is 3 ft × 3 ft × 3 ft.
A 10 × 10 ft patio poured 4 inches thick, 80 lb bags, 10% waste:
Sixty-two bags is roughly 2,500 lb of material to haul, mix and place before it starts setting — which is exactly why a slab past about a cubic yard is usually a job for ready-mix delivery rather than bags.
Bag yields — 0.30, 0.45 and 0.60 cu ft. These are the yields printed on QUIKRETE Concrete Mix No. 1101's data sheet: a 40 lb bag "yields approximately 0.30 ft³," a 60 lb bag "approximately 0.45 ft³," and an 80 lb bag "approximately 0.60 ft³." Other general-purpose bagged concrete mixes are close to the same because the yield is set by the density of cured concrete, about 140 lb per cubic foot.
Strength. That same mix is rated by its data sheet at about 4,000 psi at 28 days (ASTM C39), which is ample for residential flatwork. If your project is structural — a foundation, footing or load-bearing slab — the required strength and reinforcement are set by your local building code. Residential structural concrete is governed by ACI CODE-332 (Code Requirements for Residential Concrete), which the International Residential Code adopts; exterior flatwork such as patios and driveways is covered by local code rather than by that structural standard.
Thickness. The defaults here follow long-standing residential practice: 4 inches for patios and sidewalks, 5 inches for a car driveway, and 6 inches for a garage floor or where heavier loads or freeze-thaw call for more. These are starting points, not code minimums — check your local requirements, and thicken the edges of a driveway where vehicles cross.
How many 80 lb bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic foot, so it takes about 45 bags to make a cubic yard before any waste allowance. That is a lot of mixing, which is why most people switch to ready-mix delivery once a pour passes roughly one cubic yard.
How thick should a concrete patio be?
Four inches is the usual thickness for a patio or sidewalk that carries foot traffic. A driveway for cars is normally 5 inches and a garage floor 5 to 6, with more where the soil is poor or the climate has hard freeze-thaw cycles. Confirm the requirement with your local building department, since it, not a calculator, has the final say.
Why does the calculator add a waste factor?
Some concrete is always lost to spillage, over-excavation and uneven subgrade, and running short in the middle of a pour is far worse than having a little extra. Ten percent is the default; raise it to 15 percent over rough or uneven ground.
This is a materials estimate for ordering, not a structural design. It does not size rebar or mesh, set control joints, or tell you whether a slab needs a thicker edge, a gravel sub-base or air-entrained concrete for your climate. For anything load-bearing, follow your local code and, where required, an engineer's design.
Bag yields (0.30 / 0.45 / 0.60 cu ft for 40 / 60 / 80 lb) and the ~4,000 psi 28-day strength are from the QUIKRETE Concrete Mix No. 1101 product data sheet. Residential structural concrete requirements are set by ACI CODE-332, Code Requirements for Residential Concrete, as adopted through the International Residential Code; thickness figures here are conventional residential practice and are subject to your local code. More background on slabs, ready-mix and reinforcement is in our landscaping guide.