BuildCalc Guides · Concrete
How Many Bags of Concrete for a 12×12 Slab?
A 12 by 12 foot pad is the most common backyard patio size — big enough for a small table and chairs, small enough to pour in one session. Here is the short answer and the math.
The math
For a 12 by 12 foot slab at 4 inches thick:
- Volume in cubic feet = length × width × depth. 12 × 12 × (4 ÷ 12) = 48 ft³.
- Cubic yards = ft³ ÷ 27. 48 ÷ 27 = 1.78 yd³ net.
- Add waste. 1.78 × 1.05 = 1.87 yd³.
- Convert to bags. At 0.6 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag, that is 84 bags of 80 lb or 112 bags of 60 lb.
By slab thickness
Pick thickness by load. A 12×12 paver underlayment is fine at 3 inches, a patio at 4, a driveway-grade slab at 6.
| Thickness | ft³ | yd³ | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3" | 36.0 | 1.33 | 63 | 84 |
| 4" | 48.0 | 1.78 | 84 | 112 |
| 6" | 72.0 | 2.67 | 126 | 168 |
Includes a 5 percent waste factor and rounds up to whole bags.
Why most 12×12 pours go ready-mix
Mixing 84 bags of 80 lb concrete — that is 6,720 lbs of dry mix — takes a full day with a wheelbarrow mixer and two people, and you cannot stop in the middle without a cold joint. A 2 yard ready-mix short load takes 20 minutes to pour, runs about the same money after the short-load fee, and gives you a single continuous slab.
Stick with bags only when truck access is impossible or the pour site is split into pieces small enough to do in stages.
Calculate for your own slab
The numbers above are for a 12′ × 12′ × 4″ pad. For any other size, depth or shape — including footings and round sonotube columns — BuildCalc's concrete calculator handles fractional input, waste factor and live material cost.
Open the concrete calculator →FAQ
How many cubic yards of concrete is a 12x12 slab?+
A 12 by 12 foot slab at 4 inches thick is about 1.78 cubic yards of net concrete. Add a 5 percent waste allowance and you would order 1.87 cubic yards — typically rounded up to 2 yd³ for a ready-mix order.
How thick should a 12x12 patio slab be?+
4 inches is the standard call for a 12×12 patio or shed pad. Drop to 3 inches only for a paver underlayment. Step up to 6 inches if a vehicle will park on it — and add a thickened edge if it stands free of an existing slab.
Should I do bags or ready-mix for a 12x12?+
At ~2 cubic yards, ready-mix is the easy call. Mixing 84 bags by hand takes most of a day and costs about the same as a short-load delivery. Stick with bags only if a truck cannot reach the site or you need to pour in stages.
How many 80 lb bags in a cubic yard?+
About 45 bags. An 80 lb bag yields 0.6 cubic feet and a cubic yard is 27 ft³, so 27 ÷ 0.6 = 45 bags per yard before waste. With a 5 percent waste allowance, plan on 48 per yard.
How much waste should I add for a 12x12 slab?+
Add at least 5 percent — that covers normal spillage and minor over-excavation. The bag counts in the table already include 5 percent. For rough or hand-dug subgrade, push to 10 percent (about 4 extra bags of 80 lb at 4 inch).
Do I need rebar in a 12x12 slab?+
For a 4 inch patio, welded wire mesh or #3 rebar at 16 to 24 inch on-center is standard — it controls cracking. For a 6 inch driveway-grade slab at 12×12, step up to #4 rebar at 12 to 16 inch on-center, both directions.
Related guides
- How many bags of concrete for a 10×10 slab? — one size down, with the bag-vs-truck trade-off.
- How many bags of concrete for a 16×20 slab? — typical garage-floor or detached-shop size.
- How many bags of concrete for a driveway? — why every driveway should be ready-mix.