A slab 8 × 8 ft poured 4" thick holds 21.33 cubic feet of concrete, which is 0.79 cubic yards. In premix that is 36 bags of 80 lb or 48 bags of 60 lb with nothing extra. Add 10% for waste, the same as the concrete calculator does by default, and buy 40 bags of 80 lb or 53 bags of 60 lb (0.87 cubic yards).
Pour it 6" thick for a driveway apron, a hot tub pad or heavier loads and the same slab takes 32.00 cubic feet (1.19 cubic yards): 54 bags of 80 lb or 72 bags of 60 lb, or 59 and 79 with 10% waste.
How the numbers are worked out
Concrete is bought by volume. Length × width gives the area: 8 × 8 = 64 square feet. Turn the thickness into feet (4 ÷ 12 = 0.333) and multiply: 64 × 0.333 = 21.33 cubic feet. A cubic yard is 27 cubic feet, so 21.33 ÷ 27 = 0.79 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. 21.33 ÷ 0.60 = 35.6, so 36 bags of 80 lb; 21.33 ÷ 0.45 = 47.4, so 48 bags of 60 lb. Bags are always rounded up. For 10% waste, multiply the volume by 1.1 first: 23.47 cubic feet.
Bags or ready-mix?
At 0.87 cubic yards with waste, this slab is a bag job. The 40 bags of 80 lb weigh 3,200 lb in all, so check what your truck or trailer can carry and split the trips if you have to. A rented drum mixer keeps every batch the same; for a small pad a mixing tub and a hoe do the job.
8x8 slab at other thicknesses
8x8 slab at other thicknesses| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 80 lb bags + 10% |
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| 3 1/2" | 0.69 | 32 | 42 | 35 |
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| 4" | 0.79 | 36 | 48 | 40 |
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| 5" | 0.99 | 45 | 60 | 49 |
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| 6" | 1.19 | 54 | 72 | 59 |
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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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| 4 × 4 | 0.20 | 9 | 12 | 18 |
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| 4 × 8 | 0.40 | 18 | 24 | 36 |
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| 8 × 8 | 0.79 | 36 | 48 | 72 |
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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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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
8x8 slab, 4" thick: how many 80 lb bags?
36 bags with no waste (21.33 cubic feet ÷ 0.60), or 40 with 10% extra. In 60 lb bags it is 48, or 53 with 10% extra.
8x8 slab: how many cubic yards of concrete?
0.79 cubic yards at 4" thick and 1.19 at 6", before waste. With 10% extra, order 0.87 or 1.30 cubic yards, rounded up to what your supplier sells.
8x8 slab, 6" thick: how many bags?
54 bags of 80 lb or 72 bags of 60 lb for 32.00 cubic feet, before waste. With 10% extra, 59 bags of 80 lb or 79 of 60 lb.
Estimates only. Check measurements, product labels and local building codes before you buy or build.