How much concrete for a 12x12 slab?
A 12x12 patio 4" thick takes 1.8 cubic yards — the point where bags stop making sense and a truck starts. Yardage and bag counts for every common slab.
A 12' × 12' slab 4" thick takes 1.78 cubic yards.
That is 48 cubic feet, or 80 bags of 80 lb mix — 6,400 lb to carry, mix and place before any of it starts setting.
How that is worked out
- Volume first, all in feet: 12' × 12' × 0.333' = 48 ft³. The thickness is the one that catches people — 4" is 0.333', not 4.
- A cubic yard is 27 ft³, so 48 ÷ 27 = 1.78 yd³.
- In bags that is 80 at 80 lb, 107 at 60 lb or 160 at 40 lb. Past about a cubic yard, ordering ready-mix is cheaper, faster and better concrete.
Slab volume and bags, by size and thickness
| Slab | Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 8' × 8' | 4" | 21.3 | 0.79 | 36 | 48 |
| 10' × 10' | 4" | 33.3 | 1.23 | 56 | 75 |
| 12' × 12' | 4" | 48.0 | 1.78 | 80 | 107 |
| 12' × 16' | 4" | 64.0 | 2.37 | 107 | 143 |
| 12' × 12' | 6" | 72.0 | 2.67 | 120 | 160 |
| 20' × 20' | 4" | 133.3 | 4.94 | 223 | 297 |
Worth knowing
- Above about a cubic yard, order ready-mix. Forty-five bags of 80 lb mix is a ton and a half to move by hand, and a slab poured in batches over an afternoon has cold joints through it.
- Order 5–10% over. The subgrade is never perfectly flat, and the low spots take more than the arithmetic says — running short mid-pour is far worse than having a barrow left over.
- 4" is the usual thickness for a patio or a walkway; a driveway or anything a vehicle sits on wants 5" to 6" and reinforcement. What your slab actually needs depends on the soil and the load, and that is a local question.
- The sub-base does as much work as the concrete. A well-compacted gravel base is what stops a slab cracking; more concrete over soft ground just makes a heavier thing to crack.
Work out your own numbers
- Concrete footings and slabs
Cubic yards for a run of post footings or a slab, and the bags of ready-mix that buys.
- Fence post spacing
Post count and the actual on-centre spacing for any run and maximum span.
Related questions
- How many bags of concrete for a 12x12 slab?
- About 80 bags of 80 lb mix for a 4" slab — which is 6,400 lb and a genuinely bad afternoon. At that size order ready-mix: it is cheaper per yard, it arrives mixed consistently, and it goes in as one pour rather than eighty separate ones.
- How do I calculate cubic yards of concrete?
- Multiply length × width × thickness, all in feet, then divide by 27. The thickness is the one to watch — 4" is 0.333', not 4. A 12 × 12 × 0.333 slab is 48 ft³, which is 1.78 cubic yards.
- How thick should a concrete patio be?
- 4" over a compacted gravel base covers most patios and walkways. Go to 5" or 6" with reinforcement for a driveway or anywhere a vehicle parks. Soil conditions and frost depth change the answer locally, so it is worth a call to the building department before you dig.