Concrete Calculator
Calculate cubic yards of concrete for a slab, plus how many 80 lb or 60 lb bags — with a waste allowance.
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How Much Concrete Do I Need?
For a rectangular slab: volume = length (ft) × width (ft) × thickness (in ÷ 12) in cubic feet, then ÷ 27 for cubic yards — the unit ready-mix trucks use. In bags, an 80 lb bag yields about 0.60 cu ft and a 60 lb bag about 0.45 cu ft. Pros order 5-10% extra for spillage and uneven subgrade; the calculator includes a waste toggle.
Example: a 10×12 ft patio at 4 inches thick = 40 cu ft = 1.48 cu yd — about 67 bags of 80 lb, which is why anything over ~1 cubic yard is usually cheaper and far easier as a ready-mix delivery.
Frequently Asked Questions
How many bags of concrete for a 10x10 slab, 4 inches thick?
10 × 10 × 0.333 = 33.3 cu ft (1.23 cu yd). That is roughly 56 bags of 80 lb or 74 bags of 60 lb — at that volume, ready-mix delivery usually wins on cost and effort.
How thick should a concrete slab be?
Common practice: 4 inches for patios, walkways and shed slabs; 5-6 inches for driveways that carry vehicles; footings per local code. Thickness drives volume linearly, so an extra inch adds 25% more concrete on a 4-inch plan.
When should I order ready-mix instead of bags?
A good rule: over about 1 cubic yard (≈45+ bags of 80 lb), ready-mix is typically cheaper per yard and saves hours of mixing. Under that, bags win on flexibility and no minimum-load fees.
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