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How Much Concrete for a 12×12 Slab at 6 Inches?
A 12 ft × 12 ft slab 6 inches thick needs this much concrete — cubic yards and bag counts, calculated for you. Here is the answer, plus a calculator you can adjust if your measurements are slightly different.
Answer: 2.67 cubic yards
How it changes with thickness
Same area, different thickness:
| Thickness | 4 | 5 | 6 |
|---|---|---|---|
| Concrete needed | 1.78 cubic yards | 2.22 cubic yards | 2.67 cubic yards |
| 80 lb bags | 80 bags | 100 bags | 120 bags |
What a 12×12 slab at 6 inches actually involves
This is the awkward middle: too big to enjoy mixing by hand — 120 bags is roughly 5.3 tons to carry and open — but small enough that a ready-mix supplier will add a short-load surcharge for sending a truck. Most people at this size either hire a drum mixer and do it in a day with three or four people, or pay the surcharge and take the easy route. Decide before you order, because you cannot change your mind halfway through a pour.
The slab is 144 ft² and, at 6 inches thick, takes 72.00 ft³ of concrete — 2.67 yd³, weighing about 5.3 tons. Everything below is worked out from those figures.
Full material list
| Item | Quantity | Notes |
|---|---|---|
| Concrete | 2.67 yd³ | 120 × 80 lb bags, or 160 × 60 lb bags |
| Gravel base | 2.13 yd³ | order loose for 4 in compacted; see the gravel calculator |
| Form lumber | 53 lin ft | 2×4 or 2×6, includes 10% waste |
| Form stakes | 12 | roughly one every 4 ft of perimeter |
| Welded wire mesh | 4 sheets | 5×10 ft sheets of 6×6-W1.4 mesh, 10% overlap |
| Rebar | 216 lin ft | #4 (1/2 in) on a 18 in grid, tied and on chairs |
| Vapor barrier | 158 ft² | about 16 lin ft of 10 ft wide roll |
| Control joints | 0 joints | 0 lin ft of saw cut |
Control joints and cracking
At 6 in thick the conservative rule gives a maximum joint spacing of 12 ft, and this slab is only 12 ft on its longest side, so no control joint is strictly required. Many contractors still cut or tool one down the middle to be safe, particularly if the slab sits in full sun.
Reinforcement at 6 inches
At 6 in, welded wire mesh is no longer adequate — this slab is thick enough to carry loads, and mesh just stretches once a crack opens. The usual reinforcement is #4 (1/2 in) rebar on a 18 in grid: about 9 bars running the 12 ft direction and 9 running the 12 ft direction, roughly 216 linear ft of steel. Tie every intersection and set the grid on chairs so the bars finish at mid-depth with 2 to 3 in of cover. Bars lying on the ground do almost nothing.
Bagged concrete or a ready-mix truck?
| Option | Quantity | Estimated cost |
|---|---|---|
| Bagged 80 lb | 120 bags | $720 |
| Ready-mix delivered | 2.67 yd³ + short-load fee | $543 |
On these assumptions ready-mix is cheaper ($543 vs $720) as well as far less work. It only wins if the truck can actually reach the site and you can place the whole load without stopping. If access is a problem, bagged at $720 is the practical fallback and lets you pour in stages.
Costs assume $6 per 80 lb bag, $170 per cubic yard delivered, and a $90 short-load surcharge below 5 yd³. Ready-mix pricing moves with region, season and fuel, so treat these as a starting point for calling suppliers rather than a quote.
Pour day, in order
- Excavate to 10 in below finished grade: 6 in of slab plus 4 in of base.
- Spread the base in two lifts and compact each one. Rent a plate compactor; a hand tamper leaves soft spots that crack later.
- Set the forms to 6 in, level and square, staked every 4 ft.
- Lay the vapor barrier with 6 in laps, then the mesh on chairs so it ends up mid-depth rather than resting on the ground.
- Set up the mixing station before you open a bag: water, hose, wheelbarrow or drum mixer, and a place to dump. Mix only what you can place within about 20 minutes.
- Place, screed and float in one continuous run. A batch that sits while you fetch something is a batch you will be fighting later.
- Tool a single joint down the middle while the concrete is still plastic, or skip it and accept the risk of a random crack.
- Cover it and keep it damp for 7 days. Full design strength arrives at 28 days.
Local codes cover frost depth, reinforcement and permits, and they vary from place to place. Check with your local building department before you dig or pour.
Common questions
How many bags of concrete for a 12×12 slab at 6 inches?
120 bags of 80 lb mix, or 160 bags of 60 lb mix. A 12×12 slab at 6 in is 72.00 ft³, which is 2.67 yd³. An 80 lb bag yields about 0.60 ft³ and a 60 lb bag about 0.45 ft³, so divide the volume by the yield and round up.
How much does a 12×12 concrete slab cost?
Roughly $720 in 80 lb bags, or about $543 delivered as ready-mix, which includes a short-load surcharge because the order is under 5 cubic yards. Material only — it excludes excavation, base, reinforcement and labour.
How thick should a 12×12 slab be?
Four inches for a patio, shed floor or footpath. Five to six inches if vehicles will drive on it. This page is worked through at 6 inches, so switch the calculator above to compare thicknesses.
How much gravel goes under a 12×12 slab?
About 2.13 yd³ of loose crushed base for a 4 in compacted layer, covering 144 ft². Order it loose rather than already compacted — the material settles by roughly a fifth under a plate compactor.
Other sizes
- How Much Concrete for a 10×10 Slab at 4 Inches?
- How Much Concrete for a 10×10 Slab at 6 Inches?
- How Much Concrete for a 12×12 Slab at 4 Inches?
- How Much Concrete for a 10×12 Slab at 4 Inches?
- How Much Concrete for a 10×12 Slab at 6 Inches?
- How Much Concrete for a 12×16 Slab at 4 Inches?
- How Much Concrete for a 12×16 Slab at 6 Inches?
- How Much Concrete for a 16×16 Slab at 4 Inches?
- How Much Concrete for a 16×16 Slab at 6 Inches?
- How Much Concrete for a 20×20 Slab at 4 Inches?