How many cubic yards of concrete do I need for a 20x12 foot slab?

A 20x12 foot slab at 4 inches thick requires approximately 2.96 cubic yards of raw concrete. With a standard 5% waste factor, you should order 3.25 cubic yards (rounded to the nearest quarter yard, which is how batch plants sell).

What is a short load fee for concrete?

A short load fee is an extra charge that ready-mix plants add when you order less than a full load (typically under 5 cubic yards). The fee covers the fixed delivery cost that the plant cannot spread across a full truckload. Short load fees commonly range from $50 to $100+ depending on the plant and region.

How many 80lb bags of concrete make a cubic yard?

An 80lb bag of concrete yields approximately 0.6 cubic feet of mixed concrete. Since there are 27 cubic feet in a cubic yard, you need 45 bags (80lb) per cubic yard. For 60lb bags at 0.45 cubic feet each, you need 60 bags per cubic yard.

Why does this calculator round to the nearest 0.25 cubic yard?

Ready-mix batch plants sell concrete in quarter-yard (0.25 CY) increments. Rounding up to the nearest 0.25 CY ensures the volume you order matches what the plant can actually batch. Rounding down risks running short during the pour, which is far more expensive than ordering a quarter yard extra.

Concrete Volume Calculator

Combine slabs, footings, columns, walls, stairs, and post holes into one project estimate. Cubic yards, truck loads, and bag counts with adjustable waste factors.

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How to Estimate Concrete for Your Project

Why Waste Factors Matter

Concrete waste comes from over-excavation, uneven subgrade, form deflection, and spillage. Industry standard: 5% for flat slabs on prepared grade, 10% for footings and columns with form work, 15% for stairs and irregular pours. Under-ordering means a second truck at full delivery cost — always round up.

Ready-Mix vs. Bags

Ready-mix trucks typically carry 9 CY and charge per yard with a short load fee for orders under 5 CY. For small jobs (under 1 CY), bagged concrete (60lb or 80lb bags) from the building supply is often more economical. An 80lb bag yields about 0.6 cubic feet of mixed concrete — that is 45 bags per cubic yard.

Quarter-Yard Rounding

Batch plants sell concrete in 0.25 CY increments. This calculator rounds your adjusted total up to the nearest quarter yard so the number you give the dispatcher is the number they can actually batch. Rounding down risks a shortage; the short load fee for a follow-up truck is far more expensive than an extra quarter yard.

Concrete Strength (PSI)

Common residential mixes: 3,000 PSI for sidewalks and patios, 3,500 PSI for driveways, 4,000 PSI for garage slabs and footings, 4,500-5,000 PSI for structural and commercial. PSI does not change the volume — this calculator covers the takeoff. Specify strength when ordering from your batch plant.

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