How many bags of concrete do I need for a slab?

Find the slab volume in cubic feet (length x width x thickness in feet), then divide by the yield per bag: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb. Round up and add about 10% for waste.

When should I use ready-mix instead of bags?

Bagged concrete is practical up to about 1 cubic yard. Beyond that, ordering ready-mix delivered by the cubic yard is cheaper and far less labour.

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Concrete Slab Cost Calculator

Enter your slab size to get the concrete volume in cubic yards, how many 40/60/80 lb bags you'll need, and an estimated delivered ready-mix cost — including a waste allowance.

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Slab lengthLonger side.

Slab widthShorter side.

Thickness4″ typical for patios; 5–6″ for driveways.

Waste allowanceSpillage, uneven subgrade. 10% standard.

Ready-mix price (per cubic yard)US 2026: ready-mix runs about $135–$185/yd³ delivered. Use your local quote.

USD

Bag size (for the bag estimate)

Estimate (includes 10% waste)

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Cubic yards

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80 lb bags

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Est. ready-mix cost

Volume with waste	–
In cubic feet	–
In cubic metres	–
Cost range (±15%)	–

Pouring more than a slab?

Size the footings and the reinforcing mesh too.

Footing calculator →

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Bags vs. ready-mix

Under ~½ cubic yard: bags are fine.
½ to 1 cubic yard: either works — weigh bags + labour vs. a small delivery.
Over 1 cubic yard: order ready-mix; hand-mixing 50+ bags risks cold joints.

FAQ

How many 80 lb bags make a cubic yard?

About 45 — a cubic yard is 27 cubic feet, and each 80 lb bag yields ~0.60 cubic feet.

Estimates for planning only. Confirm with your ready-mix supplier before ordering.

Price guidance is a US 2026 market range gathered from public cost guides, shown in US dollars regardless of the currency you select above. Real prices vary widely by region, grade and supplier — always use your own quote.

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