How many bags of concrete for a slab?

An 80 lb bag of concrete mix yields about 0.6 cubic feet set; a 60 lb bag about 0.45. A 10×10 ft slab at 4 inches is ~33 cubic feet, so roughly 56 80-lb bags. Above about 1 cubic yard, ready-mix is usually cheaper than bags.

What thickness should a concrete slab be?

4 inches is standard for patios, walkways and shed floors; 5–6 inches for driveways and anything carrying vehicle loads. Thicker slabs use proportionally more concrete, so confirm the thickness before ordering.

Concrete Slab Calculator — Cubic Yards & Bags

Concrete Slab calculator

Calculate concrete for a slab, patio, floor or pad. Cubic yards, bags, ready-mix loads and weight — with a waste margin built in, nothing leaves your browser.

Dimensions

FIG. 01

Quantity

Waste margin

Volume ÷ 27 = cubic yards. Pros order to the nearest ½ yard and add a waste margin — running short mid-pour risks a cold joint.

You need

—cu yd

Order to nearest ½ yd → — · —

Cubic feet

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Cubic meters

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80 lb bags

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60 lb bags

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Est. weight

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Best buy

see above

How this is calculated

Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. For a 4-inch slab the thickness is 0.333 ft. This calculator does it for you and adds your chosen waste margin.

cubic yards = volume in cubic feet ÷ 27 × (1 + waste%)

For the full method and sources, see the methodology page. Comparing buying options? Use the bags-vs-ready-mix calculator.

Worked example

Say you're pouring a 20 × 20 ft slab at 4 inches thick. First convert the thickness to feet (4 ÷ 12 = 0.333 ft), multiply out the volume (20 × 20 × 0.333 = 133.3 ft³), add a 10% waste margin (≈ 146.7 ft³), then divide by 27:

146.7 ft³ ÷ 27 = 5.43 cubic yards → order 5.5 yd of ready-mix, or about 245 × 80 lb bags. At roughly 4,050 lb per yard that slab weighs about 22,000 lb of concrete.

Concrete by slab size (4 in thick, 10% waste)

Cubic yards and 80 lb bags for common slab sizes
Slab size	Cubic yards	Order (ready-mix)	80 lb bags
8 × 8 ft	0.87	1.0 yd	40
10 × 10 ft	1.36	1.5 yd	62
10 × 12 ft	1.63	2.0 yd	74
12 × 12 ft	1.96	2.0 yd	89
12 × 16 ft	2.61	3.0 yd	118
16 × 20 ft	4.35	4.5 yd	196
20 × 20 ft	5.43	5.5 yd	245
24 × 24 ft	7.82	8.0 yd	353
30 × 40 ft	16.30	16.5 yd	734

Thickness changes the volume proportionally. The same 20 × 20 slab needs 5.43 yd at 4 in, 6.79 yd at 5 in, or 8.15 yd at 6 in — so going from a 4-inch patio to a 6-inch slab is 50% more concrete. Use 4 inches for patios and walkways, 5–6 inches for driveways and anything carrying vehicles.

Concrete Slab calculator

Dimensions

How this is calculated

Worked example

Concrete by slab size (4 in thick, 10% waste)

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