A slab 12 × 12 ft poured 4" thick holds 48.00 cubic feet of concrete, which is 1.78 cubic yards. In premix that is 80 bags of 80 lb or 107 bags of 60 lb with nothing extra. Add 10% for waste, the same as the concrete calculator does by default, and buy 88 bags of 80 lb or 118 bags of 60 lb (1.96 cubic yards).
Pour it 6" thick for a driveway apron, a hot tub pad or heavier loads and the same slab takes 72.00 cubic feet (2.67 cubic yards): 120 bags of 80 lb or 160 bags of 60 lb, or 132 and 176 with 10% waste.
How the numbers are worked out
Concrete is bought by volume. Length × width gives the area: 12 × 12 = 144 square feet. Turn the thickness into feet (4 ÷ 12 = 0.333) and multiply: 144 × 0.333 = 48.00 cubic feet. A cubic yard is 27 cubic feet, so 48.00 ÷ 27 = 1.78 cubic yards.
For bags, divide by what one bag makes: about 0.60 cubic feet for an 80 lb bag and 0.45 for a 60 lb bag. 48.00 ÷ 0.60 = 80.0, so 80 bags of 80 lb; 48.00 ÷ 0.45 = 106.7, so 107 bags of 60 lb. Bags are always rounded up. For 10% waste, multiply the volume by 1.1 first: 52.80 cubic feet.
Bags or ready-mix?
At 1.96 cubic yards with waste, this slab is past the point where many crews order ready-mix: one cubic yard is 45 bags of 80 lb, and this pour is 88 bags weighing 7,040 lb. A truck brings it as one batch, so the whole slab sets up evenly. Many suppliers charge extra for small loads, so ask about their minimum. If you go with bags, line up enough hands and mixers to keep the pour moving.
12x12 slab at other thicknesses
12x12 slab at other thicknesses| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 80 lb bags + 10% |
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| 3 1/2" | 1.56 | 70 | 94 | 77 |
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| 4" | 1.78 | 80 | 107 | 88 |
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| 5" | 2.22 | 100 | 134 | 110 |
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| 6" | 2.67 | 120 | 160 | 132 |
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Nearby slab sizes, 4" thick, no waste
Nearby slab sizes, 4" thick, no waste| Slab (ft) | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
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| 10 × 10 | 1.23 | 56 | 75 | 112 |
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| 10 × 12 | 1.48 | 67 | 89 | 134 |
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| 12 × 12 | 1.78 | 80 | 107 | 160 |
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| 12 × 16 | 2.37 | 107 | 143 | 214 |
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| 12 × 20 | 2.96 | 134 | 178 | 267 |
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Before you pour
Measure the thickness in several spots once the base is graded. A slab that averages 4 1/2" instead of 4" takes 12.5% more concrete, which is why 5% to 10% extra is normal. IRC R506.1 sets 3 1/2" as the minimum for concrete floor slabs on the ground; 4" is the usual thickness for patios, walks and shed floors.
Bag yield varies a little by brand and mix, so read the bag. Running a few bags short leaves a cold joint in the middle of the slab; a few left over can go into post holes or a step.
Frequently asked questions
12x12 slab, 4" thick: how many 80 lb bags?
80 bags with no waste (48.00 cubic feet ÷ 0.60), or 88 with 10% extra. In 60 lb bags it is 107, or 118 with 10% extra.
12x12 slab: how many cubic yards of concrete?
1.78 cubic yards at 4" thick and 2.67 at 6", before waste. With 10% extra, order 1.96 or 2.93 cubic yards, rounded up to what your supplier sells.
12x12 slab, 6" thick: how many bags?
120 bags of 80 lb or 160 bags of 60 lb for 72.00 cubic feet, before waste. With 10% extra, 132 bags of 80 lb or 176 of 60 lb.
Estimates only. Check measurements, product labels and local building codes before you buy or build.