How Many Bags of Concrete Do I Need for a 12×12 Slab?
The 12 × 12 ft slab is the classic shed pad and hot tub pad size, usually poured 4 inches thick. Here is the short answer, with the full table for 4 in and 6 in below.
Exact count before waste is 80 × 80 lb bags. The 88-bag figure includes the standard 10% allowance for spillage, uneven ground, and forms that are not perfectly level. Running short mid-pour leaves a weak cold joint, so the cushion is worth it.
Bags to buy for a 12×12 slab, 4 in thick (with 10% waste)
12×12 Slab Bag Table: 4 in and 6 in, by Bag Size
Exact counts are the mathematical minimum, rounded up. The "with waste" counts multiply the volume by 1.10 before dividing, which is the number to actually buy. Yields: 80 lb bag = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
| Thickness | Volume | 80 lb exact | 80 lb +10% | 60 lb exact | 60 lb +10% | 40 lb exact | 40 lb +10% |
|---|---|---|---|---|---|---|---|
| 4 in | 48.0 ft³ (1.78 yd³) | 80 | 88 | 107 | 118 | 160 | 176 |
| 6 in | 72.0 ft³ (2.67 yd³) | 120 | 132 | 160 | 176 | 240 | 264 |
Which thickness? 4 inches is the standard for shed pads, patios, and walkways. Go to 6 inches for driveways, garage floors, and anywhere vehicles park. That jump takes the pour from 1.78 yd³ to 2.67 yd³, and from 88 bags to 132 bags of 80 lb mix with waste.
How the Math Works
A slab is a simple box: length × width × thickness. The only trap is mixed units, so convert the thickness from inches to feet first.
- Convert thickness: divide inches by 12. (4 in = 0.333 ft, 6 in = 0.5 ft.)
- Multiply: 12 ft × 12 ft × thickness in feet = cubic feet.
- Cubic yards: divide cubic feet by 27.
- Add waste: multiply cubic feet by 1.10 for a 10% cushion.
- Bags: divide by the yield printed on the bag and round up.
Volume (ft³) = L × W × (T ÷ 12) · Bags = ft³ ÷ yield, rounded up
Worked example at 4 in: 12 × 12 × (4 ÷ 12) = 48.0 ft³, which is 48.0 ÷ 27 = 1.78 yd³. Bags of 80 lb mix: 48.0 ÷ 0.60 = 80 exactly, so 80 bags. With waste: 48.0 × 1.10 = 52.8 ft³, and 52.8 ÷ 0.60 = 88 exactly, so 88 bags.
At 6 in: 12 × 12 × 0.5 = 72.0 ft³ (2.67 yd³). Bags: 72 ÷ 0.60 = 120 bags exact, or 132 with the 10% cushion.
When Bags Stop Making Sense: The 1 yd³ Crossover
Bags usually win under about 1 cubic yard. A 12×12 slab is past that line at any standard thickness: 1.78 yd³ at 4 inches, 2.67 yd³ at 6 inches. So for this particular pour, ready-mix is almost always the right call.
Why the crossover exists: ready-mix suppliers bill a 1 yd³ minimum. On a small pour you pay for a full yard you do not use, plus a delivery fee, and often a short-load surcharge on top. That padding is what makes bags cheaper under a yard. Once your pour is genuinely over a yard, you are using what you paid for and the truck's per-yard price beats the per-bag price.
At the example prices used on the main calculator ($6.50 per 80 lb bag, $160 per cubic yard, $75 delivery):
| 12×12 slab | Bags (with waste) | Ready-mix truck | Winner |
|---|---|---|---|
| 4 in thick | 88 bags = $572.00 | 1.96 yd³ billed = $387.89 | Ready-mix by ~$184 |
| 6 in thick | 132 bags = $858.00 | 2.93 yd³ billed = $544.33 | Ready-mix by ~$314 |
Then there is labor. 88 bags at 80 lb is 7,040 lb of material you lift, mix, and wheelbarrow; 132 bags is 10,560 lb — over five tons. That is a long day of mixing even with a rented mixer, and concrete does not wait while you catch up: a slow bag-by-bag pour risks cold joints across a slab this size. A truck places the same pour in well under an hour. Bags still make sense for a 12×12 when the truck cannot reach the pour (tight backyard access), when you want to work in stages over several days, or when local ready-mix quotes come in far above the example prices. Run your own quotes through the cost comparison on the calculator before deciding.
Pouring a Different Size?
These numbers are only exact for a 12 × 12 slab. For any other footprint, thickness, or shape, use the free calculator: it figures cubic yards, cubic feet, and bag counts live, and compares bag cost against a ready-mix quote with your local prices. Pouring a different standard size instead? See the 10×10 slab bag guide or the 10×20 slab bag guide.
12×12 Slab FAQ
How many bags of concrete do I need for a 12x12 slab 4 inches thick?
88 bags of 80 lb mix to buy (80 exact, plus 10% waste). In 60 lb bags: 118 to buy, 107 exact. The slab holds 48.0 ft³, or 1.78 yd³, of concrete.
How many bags do I need for a 12x12 slab 6 inches thick?
132 bags of 80 lb mix to buy (120 exact, plus 10% waste). In 60 lb bags: 176 to buy, 160 exact. At 6 inches the slab holds 72.0 ft³, or 2.67 yd³.
How many cubic yards is a 12x12 slab?
At the standard 4 in thick, a 12 × 12 slab is 1.78 yd³ exactly, or about 1.96 yd³ with the usual 10% waste allowance. At 6 in thick it is 2.67 yd³ exactly, about 2.93 yd³ with waste. Either way it is past the 1 yd³ point where a ready-mix truck starts beating bags on price.
For a 12x12 slab, is it cheaper to buy bags or order ready-mix?
Almost always the truck. A 12×12 slab is close to double the 1 cubic yard crossover at any standard thickness. At typical prices the 4 in pour runs about $572 in bags versus $388 delivered ready-mix, and the truck also saves you mixing three and a half tons of concrete by hand. Bags can still win if a truck cannot reach the site or your local ready-mix prices are unusually high. Check with your own quotes in the cost comparison.
Estimates Only
Results are planning estimates. Actual concrete needs vary with ground conditions, form accuracy, over-excavation, and product yield. Bag yields follow manufacturer figures for standard mixes; your product may differ slightly. Always order a little extra — running short mid-pour creates a weak cold joint. For structural work (foundations, load-bearing footings, retaining walls), consult a licensed professional and follow your local building code.