A slab 12 × 20 ft poured 4" thick holds 80.00 cubic feet of concrete, which is 2.96 cubic yards. In premix that is 134 bags of 80 lb or 178 bags of 60 lb with nothing extra. Add 10% for waste, the same as the concrete calculator does by default, and buy 147 bags of 80 lb or 196 bags of 60 lb (3.26 cubic yards).
Pour it 6" thick for a driveway apron, a hot tub pad or heavier loads and the same slab takes 120.00 cubic feet (4.44 cubic yards): 200 bags of 80 lb or 267 bags of 60 lb, or 220 and 294 with 10% waste.
How the numbers are worked out
Concrete is bought by volume. Length × width gives the area: 12 × 20 = 240 square feet. Turn the thickness into feet (4 ÷ 12 = 0.333) and multiply: 240 × 0.333 = 80.00 cubic feet. A cubic yard is 27 cubic feet, so 80.00 ÷ 27 = 2.96 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. 80.00 ÷ 0.60 = 133.3, so 134 bags of 80 lb; 80.00 ÷ 0.45 = 177.8, so 178 bags of 60 lb. Bags are always rounded up. For 10% waste, multiply the volume by 1.1 first: 88.00 cubic feet.
Bags or ready-mix?
At 3.26 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 147 bags weighing 11,760 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.
12x20 slab at other thicknesses
12x20 slab at other thicknesses| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 80 lb bags + 10% |
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| 3 1/2" | 2.59 | 117 | 156 | 129 |
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| 4" | 2.96 | 134 | 178 | 147 |
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| 5" | 3.70 | 167 | 223 | 184 |
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| 6" | 4.44 | 200 | 267 | 220 |
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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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| 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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| 16 × 16 | 3.16 | 143 | 190 | 285 |
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| 20 × 20 | 4.94 | 223 | 297 | 445 |
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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
12x20 slab, 4" thick: how many 80 lb bags?
134 bags with no waste (80.00 cubic feet ÷ 0.60), or 147 with 10% extra. In 60 lb bags it is 178, or 196 with 10% extra.
12x20 slab: how many cubic yards of concrete?
2.96 cubic yards at 4" thick and 4.44 at 6", before waste. With 10% extra, order 3.26 or 4.89 cubic yards, rounded up to what your supplier sells.
12x20 slab, 6" thick: how many bags?
200 bags of 80 lb or 267 bags of 60 lb for 120.00 cubic feet, before waste. With 10% extra, 220 bags of 80 lb or 294 of 60 lb.
Estimates only. Check measurements, product labels and local building codes before you buy or build.